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Materials Chemistry and Physics 143 (2013) 247e255

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Materials Chemistry and Physics

journal homepage: www.elsevier .com/locate/matchemphys

(Ratios: 5, 10, 50, 100, and 200) PolyanilineeTiO2 composites undervisible- or UV-light irradiation for decomposition of organic vapors

Wan-Kuen Jo*, Hyun-Jung Kang 1

Department of Environmental Engineering, Kyungpook National University, Daegu 702-701, Republic of Korea

h i g h l i g h t s

� PANI/TiO2 composites with different ratios of TiO2 to PANI were successfully prepared.� PANI/TiO2 composites could be functioned under visible- as well as UV-light exposure.� Photocatalytic performance of the composites depended on ratio of TiO2 to PANI.� Photocatalytic performance of the composites decreased as air flow rate increased.� PANI/TiO2 composites showed superior performance than reference photocatalysts.

a r t i c l e i n f o

Article history:Received 23 February 2013Received in revised form24 July 2013Accepted 30 August 2013

Keywords:Composite materialsPolymersHeat treatmentChemisorption

* Corresponding author. Tel.: þ82 53 950 6584; faxE-mail addresses: wkjo@knu.ac.kr (W.-K. Jo), khj4

1 Tel.: þ82 53 950 6584; fax: þ82 53 950 6579.

0254-0584/$ e see front matter � 2013 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2013.08.060

a b s t r a c t

Polyaniline (PANI)/TiO2 composites with different ratios of TiO2 to PANI were prepared using a hydro-thermalechemisorption process, and their photocatalytic activities for the decomposition of four organicvapors (benzene, toluene, ethyl benzene, and xylene (BTEX)) under both visible- and UV-light irradiationwere investigated. Thermal-gravimetric analysis indicated that PANI and TiO2 could be successfullycombined to form PANI/TiO2 composites. Both X-ray diffraction and Fourier transform infrared resultsconfirmed that the PANI/TiO2 composites included anatase phase, but not rutile phase TiO2. The UVevisible spectra demonstrated that the light absorption region extended well into the visible-light rangefor all prepared PANI/TiO2 composites. Under visible-light irradiation, the average photocatalyticdecomposition efficiencies of PANI/TiO2 composite increased over a 3-h period from 16 to 61, 28e92, 40e95, and 53e96% for BTEX, respectively, as their ratios of TiO2 to PANI increased from 5 to 200. Theaverage photocatalytic decomposition efficiencies of PANI/TiO2 composite over a 3-h period undervisible-light irradiation decreased from 58 to 16, 92e45, 94e62, and 94e69% for BTEX, respectively, asthe air flow rate increased from 1 to 4 L min�1. Moreover, the PANI/TiO2 composites showed higherdecomposition efficiency than as-prepared TiO2 and P25 TiO2 powders under both UV- and visible-lightirradiation, demonstrating improved photocatalytic performance of the PANI/TiO2 composites.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Titanium dioxide (TiO2) powders have been extensively appliedfor purification of environmental pollutants, because they canoxidize a range of water and air pollutants with high decompositionpotentials instead of simply accumulating them [1,2]. However, amajor drawback of TiO2 is its wide band energy gap (3.0 or 3.2 eV inthe rutile or anatase crystalline phase, respectively) which neces-sitates the use of UV light for its photocatalytic activation [3e5]. Toovercome this drawback, a variety of chemical modification

: þ82 53 950 6579.35@naver.com (H.-J. Kang).

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methods such as dye sensitization, metal ion doping, nonmetaldoping, and defect induced doping were developed to enhance thelight absorption and photocatalytic activity of TiO2 under visiblelight irradiation [6e9]. Conducting polymers have recently beensuggested as potential sensitizers that could extend the photo-absorbance of TiO2 to the visible region [10e14]. Polyaniline (PANI)is a particularly outstanding conducting polymer that has excellentconductivity and environmental stability [15]. Additionally, PANI isless expensive than other conducting polymers such as poly-thiophene and polypyrrole [10].

In several studies [10e12], PANI has been coupled to TiO2microbelts or powders to enhance the photoresponse of the TiO2

powder. The combination of PANI with TiO2 likely results in effec-tive charge separation of photogenerated carriers owing to thehetero-junction formed between these two materials, which

W.-K. Jo, H.-J. Kang / Materials Chemistry and Physics 143 (2013) 247e255248

reduces electronehole recombination and enhances photocatalyticactivity [12]. PANI/TiO2 composites had higher photocatalytic ac-tivities than pure TiO2 powder for decomposition of aqueous-phasepollutants, such as phenol, methylene orange, rhodamine B, and 4-chlorophenol [10e12]. Unlike these aqueous pollutant applications,reports of the application of PANI/TiO2 composite for decomposi-tion of gas-phase pollutants have rarely been found in scientificliterature. Photon energy absorbance and photocatalytic reactionmechanisms of chemical species differ between gasesolid andliquidesolid interfaces [16], indicating that the photocatalyticperformance determined for the wateresolid photocatalytic pro-cess might not be applicable to that for the airesolid photocatalyticprocess. Accordingly, it is necessary to investigate the application ofPANI/TiO2 composites to gas-phase pollutants. Moreover, PANI/TiO2composites have narrower band energy gap when compared tostand-alone TiO2 nanoparticles [11], which will allow PANI/TiO2composite to effectively adsorb visible-light and more photons.Therefore, it should be possible to activate PANI/TiO2 compositesand apply them for effective decomposition of gas- and liquid-phase pollutants under visible-light irradiation.

In the present study, PANI/TiO2 composites with different ratiosof TiO2 to PANI were prepared using a hydrothermalechemisorp-tion process and their surface and morphological characteristicsand photocatalytic activities for the decomposition of airbornepollutants under both visible- and UV-light irradiation wereexamined. Four aromatic organic vapors (benzene, toluene, ethylbenzene, and xylene (BTEX)) were selected as target compoundsbased on their toxicity and prevalence in residential environments.These compounds are typically present at high concentrations inindoor environments [17] and are known as toxic environmentalpollutants [18]. Specifically, benzene is a known human carcinogenthat induces leukemia, while other target compounds are closelyassociated with damage to the nervous system and liver [18].Toluene is also one of the most investigated model pollutants inphotocatalytic oxidation studies [19]. The photocatalytic activitiesof as-prepared TiO2 and commercially-available Degussa P25 TiO2were also examined for comparison under the same experimentalconditions as those used for PANI/TiO2 composite.

2. Experimental

2.1. Preparation of PANI/TiO2 composites

PANI/TiO2 composites with different weight ratios of TiO2 toPANI were prepared using a hydrothermalechemisorption process.Titanium (IV) chloride (TiCl4 98%, Aldrich Inc.) (1.8 mL) was addedto deionized water (21.9 mL) in a flask partially immersed in an ice-water bath, after which ammonium sulfate ((NH4)2SO4 99.5%,Aldrich Inc.) (2.2 g) and urea (CO(NH2)2 100%, Aldrich Inc.) (18 g)were added to this solution in turn. Next, this mixture was stirredfor 4 h, after which ethanol (C2H5OH 99.9%, Aldrich Inc.) (22.4 mL)was added. The resulting transparent solutionwas then transferredinto a 100 mL Teflon-lined stainless steel autoclave and heated at95 �C for 24 h. After thermal treatment was completed, the auto-clave was allowed to cool at a room temperature over 20 h. Thismixture was then filtered to obtain precipitates and washed withethanol and deionized water in turn. Finally, the filtered precipitatewas calcined at 460 �C for 2 h to give TiO2 powder.

A specified amount of as-prepared TiO2 powder (0.06, 0.12, 0.60,1.2, or 2.4 g) was added to PANI (12 mg) þ tetrahydrofuran (C4H8O99.9%, Aldrich Inc.) (100 mL) solution to prepare PANI/TiO2 com-posites with different weight ratios of TiO2 to PANI (5, 10, 50,100, or200, respectively). This mixture was sonicated for 30 min, stirredfor 24 h, and then filtered to obtain precipitate. The resultant pre-cipitate was then washed with ethanol and deionized water, after

which it was dried at 80 �C for 12 h to give the PANI/TiO2composites.

The surface and morphological properties of as-prepared PANIeTiO2 composite powders and films, as-prepared TiO2 film, andDegussa P25 TiO2 film were investigated by scanning electron mi-croscopy (SEM), thermo-gravimetric (TG) analysis, X-ray diffraction(XRD), UVevisible spectroscopy, and/or Fourier transform infrared(FTIR) spectroscopy. The particle morphology was investigatedusing a Hitachi FE-SEM S-4300 coupled with an energy-dispersiveX-ray (EDX) spectrometer (EDX-350 FE-SEM) at an accelerationvoltage of 15 kV. TG analysis was conducted using a TA InstrumentSDT Q600 TG/DTA with a heating rate of 10 �C min�1. The crystalstructures of the samples were obtained using a Rigaku D/max-2500 diffractometer with Cu Ka radiation, which was operated at40 kV and 100 mA in the range of 20e80� (2q) at a scanning rate of10� min�1. Light absorption properties were determined for the drypressed disk samples using a diffuse reflectance ultravioletevisibleenear infrared Varian CARY 5G spectrophotometer equippedwith an integrating sphere. The structural information was ob-tained from a Perkin Elmer Spectrum GX FTIR spectrophotometerat a resolution of 4 cm�1 in the spectral range of 400e4000 cm�1

using a KBR pellet as the reference sample.

2.2. Photocatalytic activity test

The photocatalytic activities of the as-prepared PANI/TiO2composites were evaluated using a continuous-flow cylindricalPyrex reactor (4.0 cm i.d. and 26.5 cm length) with an inner wallthat was coated with a PANI/TiO2 composite film, a reference as-prepared TiO2 film, or a reference Degussa P25 TiO2 film. A cylin-drical 8-W fluorescent black light lamp (F8T5BLB, Youngwha LampCo.) or 8-W fluorescent day light lamp (F8T5DL, Youngwha LampCo.) was inserted inside the Pyrex reactor, where it acted as theinside surface boundary layer of the cylindrical reactor. To preparethe coatings, ground photocatalyst powders were added to 0.1 Methylenediaminotetraacetic acid solution. This mixture was thendiluted by slowly adding deionized water and Triton X-100 in turn,after which the resultant sol was pasted onto the inner wall of thereactor. The treated reactor was then dried in an oven at 100 �C for0.5 h and calcined in a furnace at 350 �C for 0.5 h. A pure dried airstreamwas supplied at 1e4 Lmin�1 from a compressed air cylinder.Dried air was then humidified by passing it through a charcoal filterfollowed by water-containing impingers that were immersed in atemperature-adjusted water bath. A standard gas stream (0.1 ppmfor each compound) was prepared by injecting the target com-pounds into a mixing chamber via an auto-programmed syringepump (Model Legato 100, KDScientific Inc.). The mixed gas streamwas then transferred into an empty buffering chamber to minimizethe inlet concentration fluctuation and fed into the reactor. The airflow rate (AFR) and relative humidity (RH) were measured usingmass flow controllers (Defender 510, Bios International Co.) and ahumidity meter (TR-72S, T & D Co.), respectively.

The photocatalytic decomposition efficiency of the as-preparedPANI/TiO2 composites were investigated under different experi-mental conditions by varying the weight ratio of PANI:TiO2 and AFR.This study used four different weight ratios of TiO2 to PANI (5, 10, 50,100, and 200) and four different AFRs (1, 2, 3, and 4 Lmin�1). The RHfor this study was adjusted to 45% to represent a comfortable hu-midity level. For the parameter tests, all other parameters were fixedto the representative values at a weight ratio of TiO2 to PANI of 100and an AFR of 2 L min�1. For comparison, the decomposition effi-ciencies of as-prepared TiO2 and Degussa P-25 TiO2 photocatalystswith the same TiO2 weight that contained in the PANI/TiO2 com-posite (100weight ratio of TiO2 to PANI)were also determined underthe representative operational conditions described above. In

Fig. 1. Scanning electron microscopy of PANI/TiO2 composite powders prepared using different weight ratios of TiO2 to PANI (ratios, a, 5; b, 10; c, 50; d, 100; and e, 200), PANI/TiO2

composite film with a TiO2-to-PANI ratio of 100 (f), as-prepared TiO2 film (g), and Degussa P25 TiO2 film (h).

W.-K. Jo, H.-J. Kang / Materials Chemistry and Physics 143 (2013) 247e255 249

addition, the generation of gas-phase byproducts was examinedduring the photocatalytic process of the PANI/TiO2 composite with aTiO2-to-PANI ratio of 100 under visible-light irradiation.

Organic vapors in the air streamwere measured at the inlet andoutlet ports of the cylindrical Pyrex reactor. Volatile organic com-pounds (VOCs) including BTEX were collected by filling an evacu-ated Tedlar bag and then drawing air from this bag through a Tenaxtrap. Formaldehyde (HCHO) and acetaldehyde (CH3CHO) weresampled using Sep-Pak DNPH-silica cartridges (Waters, USA).Sampling times varied from one to 5 min depending on the sam-pling volume. VOCs collected on the trap were analyzed using an

automatic thermal desorber (ATD 400, Perkin Elmer Co.) coupled toa gas chromatograph (GC, 7890, Agilent Inc.) equipped with a flameionization detector and a capillary column (DB-1, Agilent Co.). Al-dehydes were analyzed using a high-performance liquid chro-matograph (HPLC) with a UV detector (Shimadzu LC-10A, Japan). Inthis study, organic vapors were qualitatively determined based ontheir retention times. Quantification of these compounds wasconducted using calibration curves. The quality control program forthese measurements was composed of laboratory blank traps andspiked traps. On each experimental day, a laboratory blank samplewas analyzed to check for any contamination and none was

Fig. 2. Energy-dispersive X-ray images of PANI/TiO2 composite powders prepared using different weight ratios of TiO2 to PANI (ratios, a, 5; b, 10; c, 50; d, 100; and e, 200), PANI/TiO2

composite film with a TiO2-to-PANI ratio of 100 (f), as-prepared TiO2 film (g), and Degussa P25 TiO2 film (h).

W.-K. Jo, H.-J. Kang / Materials Chemistry and Physics 143 (2013) 247e255250

observed. An external standard of the mixture of target compoundswas analyzed daily to confirm the quantitative response of the GC.The detection limits of organic vapors ranged from 0.0002 to0.007 ppm, depending on the compound. In addition, carbonmonoxide (CO) and carbon dioxide (CO2) measurements wereperformed using an on-line gas chromatograph/flame ionizationdetector (GC/FID) system coupled with a CH4 converting system(Model Alpha 12, Synspec B.V. Inc., Netherlands). The instrumentaldetection limit for both CO and CO2 was 0.05 ppm.

3. Results and discussion

3.1. Characteristics of PANIeTiO2 composites

The surface and morphological characteristics of PANI/TiO2composites prepared using different ratios of TiO2 to PANI wereobtained from SEM/EDX, TG, XRD, UVevisible, and FTIR analyses.The morphologies of the PANI/TiO2 composite powders with five

different ratios of TiO2 to PANI weight, PANI/TiO2 composite filmwith a TiO2-to-PANI ratio of 100, as-prepared TiO2 film, andDegussa P25 TiO2 film are shown in Fig. 1. The SEM images indicatethat the composite powders consisted of polydispersed micro-sphere particulates. According to Fig. 1aee, greater agglomerationof small particles was observed in the PANI/TiO2 composites withhigher PANI contents (lower ratios of TiO2 to PANI), which wasascribed to uncombined PANI. Nevertheless, the particle sizes of thePANI/TiO2 composites with TiO2-to-PANI ratios of 5, 10, 50, 100, and200, as determined using the Image Tool software (Version 3.00,The University of Texas Health Science Center in San Antonio), didnot differ significantly. Specifically, the sizes ranged from 1.8 to 5.4,1.9e5.5, 1.2e5.1, 1.3e6.1, and 1.1e5.2 mm, respectively. However,these sizes were larger than the matched anatase diameters thatwere determined by the XRD results, which were ascribed tocombination of TiO2 and PANI. Consequently, these SEM resultssuggested that PANI/TiO2 composites could be successfully pre-pared by the hydrothermalechemisorption method used in this

Fig. 3. Thermo-gravimetric analysis curves of PANI/TiO2 composite powders preparedusing different weight ratios of TiO2 to PANI (5, 10, 50, 100, and 200) and as-preparedTiO2 photocatalyst. Fig. 4. X-ray diffraction pattern of PANI/TiO2 composite powders prepared using

different weight ratios of TiO2 to PANI (ratios, a, 5; b, 10; c, 50; d, 100; and e, 200),PANI/TiO2 composite filmwith a TiO2-to-PANI ratio of 100 (f), as-prepared TiO2 film (g),and Degussa P25 TiO2 film (h).

Fig. 5. UVevisible absorption spectra of PANI/TiO2 composites prepared usingdifferent weight ratios of TiO2 to PANI (5, 10, 50, 100, and 200).

W.-K. Jo, H.-J. Kang / Materials Chemistry and Physics 143 (2013) 247e255 251

study. In addition, Fig. 1f and g shows that, similar to the PANI/TiO2

composite powders, the PANI/TiO2 composite film and as-preparedTiO2 film showed uniformly distributed microspheres, respectively.In contrast, Fig. 1h indicates that the Degussa P25 TiO2 photo-catalysts exhibited more aggregated shapes.

Fig. 2 displays the EDX images of PANI/TiO2 composite powderswith different weight ratios of TiO2 to PANI, PANI/TiO2 compositefilm with a TiO2-to-PANI ratio of 100, as-prepared TiO2 film, andDegussa P25 TiO2 film. The EDX spectra of all photocatalystsexhibited peaks of Ti and O atoms, which were attributed to TiO2.The peaks of C and Pt in the EDX spectra were assigned to thecarbon-containing tape used for fixation and Pt coating pretreat-ment of the samples, respectively. Meanwhile, no peaks associatedwith PANI were observed in the EDX images, which was likely dueto their being present in amounts below the instrumental detectionlimits.

The thermo-gravimetric analysis curves of PANI/TiO2 compositepowders with different weight ratios of TiO2 to PANI and as-prepared TiO2 photocatalyst are shown in Fig. 3. For both thePANI/TiO2 composite and pure TiO2 photocatalysts, the first weightloss with % appeared between 50 and 100 �C, which was likely dueto the volatilization of adsorbed water molecules from the photo-catalyst surface [20]. The second weight loss with % occurred be-tween 100 and 450 �C, which was ascribed to the partialdehydoxylation of the titanium surface [20]. However, the thirdweight loss was observed between 450 and 800 �C for the PANI/TiO2 composites, whereas it was not observed for the pure TiO2.Accordingly, the third weight loss observed in the TGA of the PANI/TiO2 composites was ascribed to the decomposition of PANI [10,21],suggesting that PANI molecules were successfully combined withTiO2 nanoparticles.

Fig. 4 shows the XRD patterns of PANI/TiO2 composite powderswith different weight ratios of TiO2 to PANI, PANI/TiO2 compositefilm with a TiO2-to-PANI ratio of 100, as-prepared TiO2 film, andDegussa P25 TiO2 film. The as-prepared TiO2 and all PANI/TiO2composites, regardless of powders or films, revealed anatase crystalphase TiO2 peaks, with the maximum peak occurring at 2q ¼ 25.2�

without exhibiting any rutile crystal phases. These XRD patternswere consistent with those of a PANI/TiO2 composite with a TiO2-to-PANI ratio of 17 reported by Liao et al. [12]. Additionally, theaverage anatase diameters of PANI/TiO2 composites with fivedifferent ratios of TiO2 to PANI (5, 10, 50, 100, and 200) determined

based on the diffraction peaks having a maximum intensity at2q ¼ 25.2�, were 13.9, 14.3, 13.9, 14.7, and 14.1 nm, respectively.Consequently, the anatase sizes of the PANI/TiO2 composites didnot differ significantly. Meanwhile, as shown in Fig. 4h, the XRDpatterns of Degussa P25 TiO2 photocatalysts showed an anatasecrystal with a major band at 2q ¼ 25.3� and a rutile crystal with amajor peak at 2q ¼ 27.3�, which were consistent with the resultsreported by Sano et al. [20].

Fig. 5 shows the UVevisible absorbance spectra of PANI/TiO2composites with different weight ratios of TiO2 to PANI. Accordingto previous studies [22e24], pure TiO2 photocatalysts generallyhave an absorption wavelength edge at around 410 nm. However,for all prepared PANI/TiO2 composites, the light absorption regionextended well into the visible-light range, up to 800 nm. Similarly,Liao et al. [12] reported that PANI/TiO2 composite prepared withdifferent weight ratios of TiO2 to PANI revealed a similar light ab-sorption shift. Moreover, the intensity of visible-light absorption

Fig. 6. Fourier transform infrared spectra of PANI/TiO2 composites prepared usingdifferent weight ratios of TiO2 to PANI (5, 10, 50, 100, and 200).

W.-K. Jo, H.-J. Kang / Materials Chemistry and Physics 143 (2013) 247e255252

decreased as the ratios of TiO2 to PANI increased. This pattern wasconsistent with that reported by Li et al. [11]. These results wereattributed to the lower visible-light absorption by TiO2 of PANI/TiO2composites with high ratios of TiO2 to PANI.

The FTIR spectra of the PANI/TiO2 composites with differentweight ratios of TiO2 to PANI are shown in Fig. 6. The FTIR spectra of

Fig. 7. Photocatalytic decomposition efficiencies (PDEs) of BTEX determined via PANI/TiO2

200), as-prepared TiO2, and Degussa P25 TiO2 under visible-light irradiation: a, benzene; b

the PANI/TiO2 composites were similar to each other, although thetransmittance intensity differed somewhat. Major absorption peaksappeared at bands of 3420, 1573, 1490, 1299, 1238, 1124, and510 cm�1. The band at 3420 cm�1 was ascribed to NeH stretching[11], while those at 1573 and 1489 cm�1 were likely associated withC]N and C]C stretching of PANI constituents, respectively [11,12].Both bands of 1299 and 1238 cm�1 were attributed to CeNstretching of PANI, while the band at 1124 cm�1 was attributed toplane bending of CeH of PANI [12]. In addition, the low frequencybands around 510 cm�1 were likely due to the TieOeTi vibration ofanatase [25]. These findings confirmed the presence of an anatasecrystal phase in the PANI/TiO2 composites prepared in the presentstudy, which supported the XRD results above. Additionally, nobands were observed around 708 cm�1,which was associated withthe TieOeTi vibration of rutile TiO2 [26,27], confirming the absenceof a rutile crystal phase in the PANI/TiO2 composites prepared inthis study.

3.2. Photocatalytic activity of PANIeTiO2 composites

Photocatalytic activities of PANI/TiO2 composites with differentweight ratios of TiO2 to PANI and reference TiO2 films (as-preparedand Degussa P25 TiO2 films) were determined for decomposition ofthe four organic vapors under visible-light irradiation. As shown inFig. 7, the photocatalytic decomposition efficiencies of the PANI/TiO2 composites increased as their ratios of TiO2 to PANI increasedwithin the ratio range tested in this study for all target organicvapors, suggesting that these ratios should be considered animportant parameter when PANI/TiO2 composites are prepared for

composites prepared using different weight ratios of TiO2 to PANI (5, 10, 50, 100, and, toluene; c, ethyl benzene; and d, xylene.

Fig. 8. Photocatalytic decomposition efficiencies (PDEs) of BTEX determined via a PANI/TiO2 composite with a PANI-to-TiO2 ratio of 100 under visible-light irradiation according toair flow rate: a, benzene; b, toluene; c, ethyl benzene; and d, xylene. The relative humidity was adjusted to 45%.

W.-K. Jo, H.-J. Kang / Materials Chemistry and Physics 143 (2013) 247e255 253

organic vapor treatments. Specifically, the average photocatalyticdecomposition efficiencies of PANI/TiO2 composite with a ratio of200 were 61, 92, 95, and 96% for BTEX, respectively, over a 3-hperiod, whereas those of PANI/TiO2 composite with a ratio of 5were 16, 28, 40, and 53%, respectively. Moreover, under visible-lightirradiation, those PANI/TiO2 composites showed higher decompo-sition efficiency than the as-prepared and Degussa P25 TiO2 films. Itshould be noted that the PANI/TiO2 composite with the lowest ratioof 5, the as-prepared, and Degussa P25 TiO2 film revealed a gradualdecrease over the 3-h photocatalytic process, whichwas ascribed todeactivation of TiO2. Accordingly, these results indicated that PANI/TiO2 composites prepared with high ratios of TiO2 to PANI couldminimize TiO2 deactivation. Apparent TiO2 deactivation wasattributed to the accumulation of partially oxidized reaction in-termediates, which were generated during photocatalytic decom-position processes of BTEX and then strongly bound to the TiO2surface [28]. Superior photocatalytic oxidation performance ofPANI/TiO2 composites would generate lower amounts of partiallyoxidized reaction intermediates, resulting in less catalyst deacti-vation. Liao et al. [12] also found that the photocatalytic activity ofPANI/TiO2 composites for decomposition of aqueous rhodamine Bunder visible-light irradiation increased as their ratios of TiO2 toPANI increased from 11 to 17 and that these PANI/TiO2 compositesexhibited higher decomposition efficiency than pure TiO2 powder.These findings were ascribed to agglomeration of excess PANImolecules, which would interfere with the transfer of photo-generated carriers, and therefore effective separation of electronehole pairs [29]. In contrast to the results of the present study, Liaoet al. [12] reported that the photocatalytic activity of PANI/TiO2composites for decomposition of aqueous rhodamine B decreasedas their ratios of TiO2 to PANI increased from 17 to 100. As describedearlier, this differencewas likely due to potential differences in light

absorbance and reaction kinetics of chemical species between va-poresolid and wateresolid interfaces [16].

The formation of byproducts including HCHO, CH3CHO, CO, andCO2 in the effluent air stream of the photocatalytic reactor undervisible-light irradiation was investigated during the photocatalyticprocess of the PANI/TiO2 composite with a TiO2-to-PANI ratio of100. Although certain peaks appeared on the GC chromatogram ofsamples collected at the outlet of the photocatalytic system, peakareas were not large enough to quantify. In addition, an experimentconducted using an HPLC system demonstrated no significant for-mation of formaldehyde or acetaldehyde during the photocatalyticprocess. Similarly, some researchers [30,31] reported photo-catalytic oxidation of BTEX at low concentrations (�80 ppm)without detection of aldehydes or other gas-phase intermediates.The CO levels, which were determined during the photocatalyticprocesses of BTEX increased to 0.1 ppm, suggesting the formation ofCO. These results are consistent with those of a previous study [32],inwhich a certain portion of BTEX was found to be converted to CO.However, the CO concentrations that were generated during thePCO process were considered to be a negligible contribution toindoor CO levels because this level was much lower than theOccupational Safety and Health Administration 8-h exposure limitfor CO levels in the workplace (35 ppm).

The mineralization ratio (MR, %) of VOCs was estimated usingthe following equation [33]:

MR ¼ ½CO2�out � 104=�NC � ½VOC�in � PDE

�(1)

where [CO2]out and [VOC]in represent CO2 concentration (ppm)measured at the outlet port of the photocatalytic reactor and theinitial VOC concentration (0.1 ppm) supplied to the reactor,respectively; NC represents the average number of carbons for the

Fig. 9. Photocatalytic decomposition efficiencies (PDEs) of BTEX determined via a PANI/TiO2 composite with a TiO2-to-PANI ratio of 100, as-prepared TiO2, and Degussa P25 TiO2

under UV-light irradiation: a, benzene; b, toluene; c, ethyl benzene; and d, xylene. The relative humidity was adjusted to 45%.

W.-K. Jo, H.-J. Kang / Materials Chemistry and Physics 143 (2013) 247e255254

target compounds; and PDE represents the average PDE (75%) ofthe target compounds. The CO2 concentration measured at theoutlet port of the reactor was 0.4 ppm, while the MRwas estimatedto be 73%, which was within a range of 40 and 80%, with respect totoluene, as reported by Bouzaza et al. [33].

Fig. 8 shows the photocatalytic activities of the PANI/TiO2composite with a TiO2-to-PANI ratio of 100 for decomposition ofBTEX under visible-light irradiation according to AFR. For all targetcompounds, the photocatalytic decomposition efficiencies of thePANI/TiO2 composite decreased with increasing AFR. Specifically,the average photocatalytic decomposition efficiencies of PANI/TiO2composite over a 3-h period decreased from 58 to 16, 92e45, 94e62, and 94e69% for BTEX, respectively, as the AFR increased from 1to 4 L min�1. Similarly, Yu and Brouwers [34] reported that thephotocatalytic decomposition efficiencies of gaseous nitrogen ox-ide determined using a pure TiO2 powder under UV-light irradia-tion decreased from 62 to 13% as the AFR increased from 1 to5 L min�1. The residence times (defined as the average amount oftime that BTEX in the air stream spends in a photocatalytic reactor)for AFRs of 1, 2, 3, and 4 L min�1, which were estimated by dividingthe reactor volume by the AFR, were 0.2, 0.1, 0.07, and 0.05 min,respectively. In addition, the photocatalytic decomposition effi-ciencies of gaseous species could increase as the AFR increasedowing to increased mass transfer [35,36]. Consequently, the lowdecomposition efficiencies of BTEX for high AFRs obtained in thepresent study suggest that the effects of an insufficient residencetime in the photocatalytic reactor would outweigh the effects ofmass transfer on photocatalytic decomposition efficiencies oforganic vapors.

Fig. 9 presents the photocatalytic activities of PANI/TiO2 com-positewith a TiO2-to-PANI ratio of 100 and reference TiO2 films (as-

prepared and Degussa P25 TiO2 films) with the sameweight as thatused for this PANI/TiO2 composite for decomposition of the fourorganic vapors under UV-light irradiation. Similar to the visible-light irradiation conditions, the PANI/TiO2 composite revealedhigher photocatalytic decomposition efficiencies than the referenceTiO2 films (as-prepared TiO2 and Degussa P25 TiO2 films) for BTEXunder UV-light irradiation. Specifically, the average photocatalyticdecomposition efficiencies of the PANI/TiO2 composite over a 3-hphotocatalytic process were well above 90% for all target com-pounds. Meanwhile, the average photocatalytic decomposition ef-ficiencies of Degussa P25 TiO2 film were 83, 90, 90, and 91%,respectively, while those of the as-prepared TiO2 film were 79, 84,83, and 85%. As described earlier, these results were attributed tothe effective charge separation of photo-generated carriers for thePANI/TiO2 composite, which would lower electronehole recombi-nation and consequently improve photocatalytic performance [12].

4. Conclusions

In this study, PANI/TiO2 composites with different ratios of TiO2

to PANI were successfully prepared for application for the photo-catalytic decomposition of organic vapors under visible- or UV-light irradiation. Spectral investigations on the surface andmorphological characteristics of the PANI/TiO2 composites indi-cated that PANI and TiO2 could be successfully combined to formPANI/TiO2 composites, that the as-prepared PANI/TiO2 compositesincluded anatase phase but not rutile phase TiO2, and that thesecomposites could be activated under visible-light irradiation. Undervisible-light irradiation, the photocatalytic decomposition effi-ciencies of the PANI/TiO2 composites increased as their weight ra-tios of TiO2 to PANI increased, whereas the average photocatalytic

W.-K. Jo, H.-J. Kang / Materials Chemistry and Physics 143 (2013) 247e255 255

decomposition efficiencies of PANI/TiO2 composite decreased asthe AFR increased. In addition, the PANI/TiO2 composites showedhigher decomposition efficiency than as-prepared TiO2 and P25TiO2 powders under both UV- and visible-light irradiation,demonstrating the improved photocatalytic performance of PANI/TiO2 composites.

Disclosure statement

The authors have no actual or potential conflicts of interest todeclare.

Acknowledgment

This work was supported by the National Research Foundationof Korea (NRF) grant funded by the Korean Government (MEST)(2011-0027916). The authors thank two graduate students (JoonYeob Lee and Kun-Hwan Kim) in the Department of EnvironmentalEngineering, Kyungpook National University, for collecting samplesand conducting analyses.

References

[1] U.I. Gaya, A.H. Abdullah, J. Photochem. Photobiol. C 9 (2008) 1e12.[2] Y. Paz, Appl. Catal. B 99 (2010) 448e460.[3] R. Leary, A. Westwood, Carbon 49 (2011) 741e772.[4] A.D. Paola, E. García-López, G. Marcì, L. Palmisano, J. Hazard. Mater. 211�212

(2012) 3e29.[5] A. Stankovi�c, Z. Stojanovi�c, L. Veselinovi�c, S.D. �Skapin, I. Bra�cko, S. Markovi�c,

D. Uskokovi&cacute, Mater. Sci. Eng. B 177 (2012) 1038e1045.[6] R. Vinu, S. Polisetti, G. Madras, Chem. Eng. J. 165 (2010) 784e797.[7] K. Li, Z. Sun, Z. Xue, Mater. Chem. Phys. 137 (2012) 246e251.[8] I. Ganesh, A.K. Gupta, P.P. Kumar, P.S. Chandra Sekhar, K. Radha,

G. Padmanabham, G. Sundararajan, Mater. Chem. Phys. 135 (2012) 220e234.

[9] M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos,P.S.M. Dunlop, J.W.J. Hamilton, J.A. Byrne, K. O’Shea, M.H. Entezari,D.D. Dionysiou, Appl. Catal. B 125 (2012) 331e349.

[10] X. Li, D. Wang, Q. Luo, J. An, Y. Wang, G. Cheng, J. Chem. Technol. Biotechnol.83 (2008) 1558e1564.

[11] Q. Li, C. Zhang, J. Li, Appl. Surf. Sci. 257 (2010) 944e948.[12] G. Liao, S. Chen, X. Quan, Y. Zhang, H. Zhao, Appl. Catal. B 102 (2011) 126e131.[13] Q. Li, D.J.G. Satur, H. Kim, H.G. Kim, Mater. Lett. 76 (2012) 169e172.[14] Y. Wang, J. Zhang, L. Liu, C. Zhu, X. Liu, Q. Su, Mater. Lett. 75 (2012) 95e98.[15] N.-R. Chiou, A.J. Epstein, Adv. Mater. 17 (2005) 1679e1683.[16] A. Fujishima, X. Zhang, D.A. Tryk, Surf. Sci. Rep. 63 (2008) 515e582.[17] M. de Blas, M. Navazo, L. Alonso, N. Durana, M.C. Gomez, J. Iza, Sci. Total

Environ. 426 (2012) 327e335.[18] USEPA (United States of Environmental Agency), Health Effects Information

Used in Cancer and Noncancer Risk Characterization for the 1999 National-scale Assessment, 2005 (accessed in December 2012), http://www.epa.gov/ttn/atw/nata1999/99pdfs/healtheffectsinfo.pdf.

[19] AFNOR, XP-B44-013 Standard, in Photocatalysis: Test & Analysis Method forDetermining the Efficiency of Photocatalytic Systems for Eliminating VOC/Odours in Recirculating Indoor Air-confined Chamber Test, 2009.

[20] T. Sano, E. Puzenat, C. Guillard, C. Geantet, S. Matsuzawa, J. Mol. Catal. A 284(2008) 127e133.

[21] N. Chandrakanthi, M.A. Careem, Polym. Bull. 44 (2000) 101e108.[22] T.A. Egerton, J.A. Mattinson, J. Photochem. Photobiol. A 194 (2008) 283e289.[23] W.K. Jo, J.T. Kim, J. Hazard. Mater. 164 (2009) 360e366.[24] H. Znad, Y. Kawase, J. Mol. Catal. A 314 (2009) 55e62.[25] S.S. Madaeni, N. Ghaemi, A. Alizadeh, M. Joshaghani, Appl. Surf. Sci. 257 (2011)

6175e6180.[26] S. Nagarajan, N. Rajendran, Appl. Surf. Sci. 255 (2009) 3927e3932.[27] R.K. Keswani, H. Ghodke, D. Sarkar, K.C. Khilar, R.S. Srinivasa, Colloid Surf. A

369 (2010) 75e81.[28] M. Lewandowski, D.F. Ollis, Appl. Catal. B 45 (2003) 223e238.[29] H. Zhang, R. Zong, J. Zhao, Y. Zhu, Environ. Sci. Technol. 42 (2008) 3803e3807.[30] H. Einaga, S. Futamura, T. Ibusuki, Environ. Sci. Technol. 35 (2001) 1880e1884.[31] C.H. Ao, S.C. Lee, S.C. Zou, C.L. Mak, Appl. Catal. B 49 (2003) 187e193.[32] W.K. Jo, K.H. Park, Chemosphere 57 (2004) 555e565.[33] A. Bouzaza, C. Vallet, A. Laplanche, J. Photochem. Photobiol. A 177 (2006)

212e217.[34] Q.L. Yu, H.J.H. Brouwers, Appl. Catal. B 92 (2009) 454.[35] L. Yang, Z. Liu, J. Shi, H. Hu, W. Shangguan, Catal. Today 126 (2007) 359e368.[36] C. Akly, P.A. Chadik, D.W. Mazyck, Appl. Catal. B 99 (2010) 329e335.