Materials 2015, 8, 31-41; doi:10.3390/ma8010031
OPEN ACCESS
materials
ISSN 1996-1944
www.mdpi.com/journal/materials
Article
Enhanced Photocatalytic Efficiency of N–F-Co-Embedded
Titania under Visible Light Exposure for Removal of
Indoor-Level Pollutants
Seung-Ho Shin 1, Ho-Hwan Chun 2 and Wan-Kuen Jo 1,*
1
2
Department of Environmental Engineering, Kyungpook National University, Daegu 702-701,
Korea; E-Mail: ssho@knu.ac.kr
Department of Naval Architecture and Ocean Engineering, Pusan National University,
63 Jangjeon-dong, Geumjeong-gu, Busan 609-735, Korea; E-Mail: chunahh@pusan.ac.kr
* Author to whom correspondence should be addressed; E-Mail: wkjo@knu.ac.kr;
Tel./Fax: +82-53-950-6584.
Academic Editor: Klara Hernadi
Received: 13 September 2014 / Accepted: 1 December 2014 / Published: 24 December 2014
Abstract: N–F-co-embedded titania (N–F–TiO2) photocatalysts with varying N:F ratios
were synthesized and tested for their ability to photocatalyze the degradation of pollutants
present at indoor air levels using visible light. The synthesis was achieved using a
solvothermal process with tetrabutyl titanate, urea and ammonium fluoride as sources of
Ti, N and F, respectively. Three selected volatile organic compounds (toluene, ethyl
benzene and o-xylene) were selected as the test pollutants. The prepared composites were
characterized using X-ray diffraction, energy-dispersive X-ray spectroscopy, X-ray
photoelectron spectroscopy and Ultra-violet (UV)-visible spectroscopy. The photocatalytic
degradation efficiencies of N–F–TiO2 composites were higher than those obtained using
pure TiO2 and N–TiO2. Moreover, these efficiencies increased as the N:F ratio decreased
from sixteen to eight, then decreased as it dropped further to three, indicating the presence
of an optimal N:F ratio. Meanwhile, as retention time decreased from 12.4 to 0.62 s,
the average photocatalytic efficiencies decreased from 65.4% to 21.7%, 91.5% to 37.8%
and 95.8% to 44.7% for toluene, ethyl benzene and o-xylene, respectively. In contrast, the
photocatalytic reaction rates increased as retention time decreased. In consideration of all
of these factors, under optimized operational conditions, the prepared N–F–TiO2
composites could be utilized for the degradation of target pollutants at indoor air levels
using visible light.
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Keywords: N,F co-embedment; N:F ratio; retention time; indoor air levels
1. Introduction
Among semiconductors developed for photocatalytic applications, titanium dioxide (TiO2) is most
common because of its oxidizing potential, chemical inertness and high photo-resistance [1,2].
Nevertheless, practical applications for TiO2 are limited by its wide band gap, which requires
Ultra-violet (UV) radiation for photocatalytic activation [3]. As a result, much work has been directed
at bringing the band gap energy difference to within the visible range throughout the use of metal [4,5]
or non-metal [6–9] dopants. Certain metals, however, cause an increase in the number of
recombination centers for photo-produced charge carriers, which results in thermal instability and a
decrease in photocatalytic efficiency [10]. Furthermore, the toxic properties of heavy metals restrict
later disposal.
Embedding TiO2 with non-metal elements, including nitrogen (N), fluorine (F), carbon (C) and
sulfur (S), is therefore a promising alternative approach [6‒9]. N doping especially has been
extensively investigated, chiefly owing to the fact that the atomic size of N is comparable to that of
oxygen (O), in addition to it forming metastable centers, exhibiting low ionization energy and having
high thermal stability [2,10,11]. While the exact mechanism for the activation of N-doped TiO2
(N–TiO2) is uncertain, it is generally ascribed to the narrowed band gap resulting from the integration
of N 2p states that are higher in energy than the top of the valence band [12,13]. Additionally, N
doping alters the surface structure of TiO2 and controls the surface transfer of charge carriers, thereby
enhancing photocatalytic performance [12,14]. Consequently, N–TiO2 possesses superior photocatalytic
activity under visible light irradiation when compared to pure TiO2, allowing for the improved
degradation of various pollutants, including aqueous rhodamine B, gaseous acetaldehyde and aromatic
hydrocarbons [9,12,15,16].
In addition to N, F can increase the surface acidity of TiO2, thereby improving photocatalytic
performance [17]. Certain studies reported that F doping alone does not significantly shift the light
absorption into the visible spectral range [13], while other studies found that single modified titania
with F could induce enhanced visible light-driven photocatalytic activity for the degradation of
gas-phase acetone or acid orange 7 [18,19]. This difference is ascribed to different experimental
conditions, such as the synthesis method, the F-to-TiO2 ratio, the target compound and the media.
Regardless of this issue, in order to further take advantage of the benefits of N-doping, N–F-codoping
of TiO2 (N–F–TiO2) has been developed [13,18–20]. When compared to single element doping,
codoping strategies like this one that employed two elements, such as C and N [21] or Pt and N [22],
have yielded higher photocatalytic activity.
Several studies have explored N–F–TiO2 synthesis, relying on sol–gel [18‒20], solvothermal [13]
and single-step combustion [23] methods, employing many different conditions by varying the
calcination temperature and the ratio of the heteroatoms to Ti. However, the effect of the F to N ratio
remains unaddressed. Additionally, previous studies have focused only on the photocatalytic efficiency
of the degradation of specific aqueous pollutants; the reaction mechanisms for these degradations may
Materials 2015, 8
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be different outside of solution [24]. This study, therefore, addresses both of these issues, relying on a
solvothermal route and analyzing the degradation of volatile organic compounds (VOCs) present at
indoor levels under visible light irradiation. Pure TiO2 and N–TiO2 were also investigated for
comparison. Toluene, ethyl benzene and o-xylene were chosen as the aromatic pollutants to be
analyzed, because of their relatively high frequency in the selected environments [25] and the health
hazards that they pose [26].
2. Results and Discussion
2.1. Characteristics of Prepared Photocatalysts
The N–F–TiO2 composites employing varying N:F ratios, along with the pure TiO2 and N–TiO2
reference catalysts, were characterized by X-ray diffraction (XRD), energy dispersive X-ray analysis
(EDX), X-ray photoelectron spectroscopy (XPS) and UV-visible spectroscopy (UV-Vis).
The corresponding XRD patterns are shown in Figure 1. The N–F–TiO2 composites, as well as the two
reference photocatalysts, exhibited only anatase phase peaks at 2θ = 25.3°, 37.9°, 47.9°, 53.9°, 62.7°
and 70.3°, consistent with previous studies [13,18], for samples calcined at or below 600 °C. Notably,
both the N–F–TiO2 and N–TiO2 composites displayed a shift in the (101) crystal plane (2θ = 25.2°),
suggesting the presence of lattice distortion [18]. Based on the anatase (101) diffraction data, the
crystalline sizes of pure TiO2, N–TiO2 and N–F–TiO2 composites with N:F ratios of 16, 6, 6, 4 and 3
(referred to as N–F–TiO2-16, N–F–TiO2-8, N–F–TiO2-6, N–F–TiO2-4 and N–F–TiO2-3, respectively)
were estimated to be 14.1, 12.6, 13.2, 13.4, 13.7, 13.8 and 14.0 nm, respectively. The smaller crystal
sizes for the doped samples are in line with the proposal that doping might somewhat suppress TiO2
crystal growth [13].
Figure 1. X-ray diffraction patterns of N–F–TiO2 with different N:F ratios (N–F–TiO2-3,
N–F–TiO2-4, N–F–TiO2-6, N–F–TiO2-8 and N–F–TiO2-16), N–TiO2 and pure TiO2.
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The chemical compositions of the photocatalyst samples were investigated with the help of XPS
analyses. The XPS spectra confirmed the presence of both N and F in the N–F–TiO2 composites
(Figure 2). Table 1 shows the derived binding energies and the N and F concentrations for all tested
samples. F1s peaks appeared at 684.1, 683.1, 683.9, 683.1 and 683.9 eV in N–F–TiO2-16, N–F–TiO2-8,
N–F–TiO2-6, N–F–TiO2-4 and N–F–TiO2-3, respectively; these peaks likely result from F− ions
adsorbed onto the TiO2 surface [20]. In addition, the F1s peak at 688 eV was assigned to F atoms that
substituted for O sites within the TiO2 lattice (data not shown) [13]. Meanwhile, the N1s peaks were
observed at 399.4, 399.2, 399.4, 399.4 and 399.4 eV for N–F–TiO2-16, N–F–TiO2-8, N–F–TiO2-6,
N–F–TiO2-4 and N–F–TiO2-3, respectively; these were associated with molecularly chemisorbed N
atoms [12]. The XPS data also revealed Ti2s, Ti2p, Ti3s, Ti3p and O1s peaks at 565.5‒567.2,
459.1‒459.9, 61.5‒62.9, 37.7, and 529.2‒529.8 eV, respectively. Pelaez et al. [20] have reported,
based on XPS results, that N and F atoms can be successfully embedded into TiO2 using a
fluorosurfactant-based sol-gel process.
Figure 2. X-ray photoelectron spectroscopy of N–F–TiO2 with different N:F ratios
(N–F–TiO2-3, N–F–TiO2-4, N–F–TiO2-6, N–F–TiO2-8 and N–F–TiO2-16), N–TiO2 and
pure TiO2.
Table 1. Binding energy (eV) and amounts (%) of N and F for N–F–TiO2-3, N–F–TiO2-4,
N–F–TiO2-6, N–F–TiO2-8, N–F–TiO2-16, N–TiO2 and pure TiO2 *.
Photocatalyst
N–F–TiO2-16
N–F–TiO2-8
N–F–TiO2-6
N–F–TiO2-4
N–F–TiO2-3
N–TiO2
pure TiO2
Ti2p
459.9
459.9
459.9
459.7
459.1
459.1
459.7
Ti3p
37.7
37.7
37.7
37.7
37.7
37.7
37.7
Ti2s
566.1
567.2
565.5
566.2
566.1
567.1
566.1
Ti3s
62.6
62.6
61.5
62.6
62.9
62.6
62.8
O1s
529.8
529.4
529.2
529.2
529.4
529.8
529.8
N1s
399.4 (7.2)
399.2 (6.9)
399.4 (7.0)
399.4 (7.4)
399.4 (6.8)
NA
NA
* Numbers in parenthesis represent the amounts (%) of N or F; NA, not available.
F1s
684.1 (0.4)
683.1 (0.8)
683.9 (1.3)
683.1 (1.7)
683.9 (2.1)
NA
NA
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Figure 3 displays the UV-Vis absorption spectra of the tested samples. Pure TiO2 showed an
absorption edge at approximately 410 nm, a value that is in agreement with previous studies [15,27].
In contrast, the absorption spectra of both the N–TiO2 and N–F–TiO2 composites shifted toward the
visible region, with values of 449.2, 456.4, 461.5, 471.1, 477.5 and 483.9 nm for N–TiO2, N–F–TiO2-3,
N–F–TiO2-4, N–F–TiO2-6, N–F–TiO2-8 and N–F–TiO2-16, respectively. This effect was attributed to
the impurity states at the substitutional lattice sites resulting from N integration [12,13,28].
Additionally, visible light absorption intensity was greater for N–F–TiO2 than for N–TiO2, while also
increasing gradually as the N:F ratios decreased. Di Valentin et al. [19] also reported that the visible
light absorption increased gradually as the N:F ratios decreased from to 100 to 1.0 in N–F–TiO2
composites, which were prepared by a sol–gel process. Both of these effects were attributed to some
kind of synergistic effect, given that F alone usually does not derive efficient light absorption.
(a)
(b)
Figure 3. (a) UV-Vis spectra of N–F–TiO2 with different N:F ratios (N–F–TiO2-3,
N–F–TiO2-4, N–F–TiO2-6, N–F–TiO2-8 and N–F–TiO2-16), N–TiO2 and pure TiO2;
(b) The enlarged scale of the spectra is also provided.
2.2. Photocatalytic Activities of N–F–TiO2, N–TiO2 and Pure TiO2
The photocatalytic activities of the fabricated materials were investigated by exposure to visible
light after allowing for adsorption in the dark. A control test performed using an uncoated Pyrex tube
under visible light irradiation showed insignificant photolysis of the target compounds. Figure 4 shows
time series of the photocatalytic degradation efficiencies (PDEs) of toluene, ethyl benzene and
o-xylene for both reference photocatalysts and the assorted N–F–TiO2 composites under visible light
exposure. N–F–TiO2 showed the highest activity, with average PDEs of 29.1%, 49.6% and 60.2% for
toluene, ethyl benzene and o-xylene, respectively. N–TiO2 showed decreased activity of 17.4%,
25.3% and 34.2%, respectively, while pure TiO2 was the least active, with values of 15.7%, 18.7%
and 20.4%, respectively. Previous studies have compared N–F–TiO2 performance with that of
Degussa P25 TiO2, prepared TiO2, N–TiO2 and F–TiO2 and have demonstrated improved activity for
the degradation of acetic orange, methyl orange methylene blue and microcystin in aqueous
media [13,18‒20,23]; again, this enhanced activity was ascribed to synergistic effects.
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Figure 4. Time-series photocatalytic degradation efficiencies (PDEs, %) of (a) toluene,
(b) ethyl benzene and (c) o-xylene as determined using N–F–TiO2 with different N:F ratios
(N–F–TiO2-3, N–F–TiO2-4, N–F–TiO2-6, N–F–TiO2-8 and N–F–TiO2-16), N–TiO2 and
pure TiO2.
Figure 4 also outlines the performance dependence on N:F ratios, with PDEs increasing as the N:F
ratio decreased from sixteen to six. This pattern again suggests increasing synergy with increasing F
content. However, the value then proceeded to drop as the N:F ratio decreased from six to three;
this effect has been previously attributed to excess F species acting as an inhibitor by screening the
TiO2 surface or capturing photon-generated holes [20]. Notably, N–F–TiO2-6 exhibited the highest
PDEs, even though it absorbed less light than N–F–TiO2-4 and N–F–TiO2-3, suggesting that the
photocatalytic activity is not strictly dependent on visible light absorption.
Figure 5 shows time series of the PDEs for toluene, ethyl benzene and o-xylene obtained for
N–F–TiO2-6 under visible light exposure based on retention time, demonstrating a positive correlation.
Specifically, the average PDEs for toluene decreased from 65.4% to 21.7% as the retention time
decreased from 12.40 to 0.62 s. This agrees with previous research by Jo and Kang [29], who reported
that the PDEs of select aromatic vapors treated with polyacrylonitrile-supported TiO2 fibers decreased
gradually with retention time. Retention times were estimated by dividing the reactor volume by the air
flow rate. The low PDEs for low retention time conditions were ascribed to short reaction times inside
the continuous-flow Pyrex reactor.
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Figure 5. Time-series photocatalytic degradation efficiencies (PDEs, %) of (a) toluene,
(b) ethyl benzene and (c) o-xylene as determined using N–F–TiO2-6, according to
retention time.
The photocatalytic reaction rates were estimated by combining the retention times with the
following equation:
rR = fc·(Ci − Co)Qair/Ac
(1)
where rR represents the photocatalytic reaction rate (PRR) (μmol·m−2·s−1), Ci and Co represent the
upstream and downstream concentrations of each target chemical (ppm), respectively, Qair represents
the airstream flow rate (m3·s1), Ac represents the inner-wall area coated with the photocatalyst (m2)
and fc represents the conversion coefficient (40.9 μmol·m−3·ppm−1). Unlike the PDEs, the PRRs
increased as retention time decreased (Table 2), with values for toluene of 0.2 × 10−3 and 1.0 × 10−3 for
retention times of 12.4 and 0.62 s, respectively. Previous studies reported the same pattern, suggesting
that PRRs are affected by the mass transfer effect, a phenomenon that is closely associated with
heterogeneous reaction kinetics [29,30]. Consequently, the dependence of PRRs on retention time was
not assigned to photocatalyst surface reactions.
Table 2. Reaction rates (μmol·m−2·s−1) of three target compounds obtained using the
N–F–TiO2-6 according to retention time.
Compound
Toluene
Ethyl benzene
o-Xylene
0.62
1.0 × 10−3
1.8 × 10−3
2.1 × 10−3
1.24
0.7 × 10−3
1.2 × 10−3
1.4 × 10−3
Retention Time (s)
2.48
4.13
−3
0.4 × 10
0.3 × 10−3
0.7 × 10−3
0.5 × 10−3
0.8 × 10−3
0.5 × 10−3
12.4
0.2 × 10−3
0.2 × 10−3
0.2 × 10−3
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3. Experimental Section
3.1. Synthesis and Characterization of Photocatalysts
N–F–TiO2 photocatalysts with varying N:F ratios were synthesized by a solvothermal method,
using tetrabutyl titanate (TBT, Ti(OC4H9)4), urea (CO(NH2)2) and ammonium fluoride (NH4F) as
sources of Ti, N and F, respectively. TBT (9 mL, 97%, Sigma-Aldrich, St. Louis, MO, USA) was
added to ethyl alcohol (32 mL, 99.9%, Sigma-Aldrich) and concentrated nitric acid (0.4 mL, 69%,
Merck, Whitehouse Station, NJ, USA). In addition, urea (0.22 g, 99%, Sigma-Aldrich), ammonium
fluoride (98%, Sigma-Aldrich) and deionized water (2 mL) were added to ethyl alcohol (70 mL).
The synthesis of N–F–TiO2-16, N–F–TiO2-8, N–F–TiO2-6, N–F–TiO2-4 and N–F–TiO2-3 required the
use of 0.014, 0.028, 0.042, 0.056 and 0.084 g, respectively, of ammonium fluoride. Subsequently, the
former solution was slowly added to the latter under magnetic stirring. After further stirring of the
mixture at room temperature for 2 h, it was hydrothermally treated in an autoclave (150 mL) at 150 °C
for 20 h. Finally, the treated mixture was washed with deionized water, dried at 100 °C overnight and
treated at 400 °C for 3 h to obtain the desired N–F–TiO2 powder. Pure TiO2 and N–TiO2 were
prepared following the same procedure, but without the addition of the corresponding element sources.
It is worth noting that the so-called N:F ratio was only the ratio of precursors, which were highly
unlikely to be the same as the composition of the resulting photocatalysts. The prepared photocatalysts
were examined by XRD (Rigaku D/max-2500 diffractometer, Tokyo, Japan), XPS (PHI Quantera SXM,
Chanhassen, MN, USA) and UV-Vis (Varian CARY 5G, Santa Clara, CA, USA).
3.2. Tests for Photocatalytic Activity
The photocatalytic activities of the synthesized photocatalysts were tested using a plug-flow Pyrex
reactor (3.8 cm i.d. and 26.0 cm length) with an inner wall coated in a thin film of the appropriate
catalyst. To apply the coatings, titanium tetra-isopropoxide (50 mL, 97%, Sigma-Aldrich) was first
added to glacial acetic acid (10 mL, 99%, Sigma-Aldrich) under stirring. The resulting solution was
mixed with 1000 mL deionized water and then 10 mL nitric acid (98%, Sigma-Aldrich), stirred until a
white precipitate was obtained and then heated at 80 °C for 5 h in a bath to obtain a sol. The selected,
previously synthesized photocatalyst (2 g) was then added to the sol, after which the mixture was
sonicated for 30 min to afford a sol coating. The outer wall of the Pyrex reactor was wrapped with a
commercially-available vinyl sheet and dipped in the coating for 10 min, after which it was removed at
a rate of 2 cm min−1 and kept in a clean room for 3 h. The coating and drying process was performed
three times to maximize coating. A cylindrical lamp (F8T5DL, Youngwha Lamp Co., Seoul, Korea)
designed to simulate daylight was placed in the coated reactor. A pure dried air stream provided from a
compressed air tank was humidified by passing it through impingers, while the desired 0.1 ppm
standard gas concentration was achieved by mixing the humidified air with the target chemicals, which
were injected into a glass chamber via a syringe pump (Model Legato 100, KdScientific, Holliston,
MA, USA). The prepared gas was routed into an empty buffering bulb (1 L) to minimize fluctuations
in the supplied gas concentrations, after which it was fed into the reactor.
The photocatalytic decomposition efficiencies of the prepared photocatalysts were examined under
a fixed stream flow rate of 1 L min−1 and a relative humidity of 45%, representative of a comfortable
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humidity level. The intensity of supplied light was 0.5 mW cm−2 at a distance from the lamp to the
inner wall of the reactor. In addition, the PDE of N–F–TiO2-6, which was selected as representative of
the N–F–TiO2 photocatalysts, because it showed the highest activity, was examined under retention
times of 0.62, 1.24, 2.48, 4.13 and 12.4 s. All other parameters were adjusted to the values described
above. Each experiment was conducted in triplicate.
Gas concentration measurements were completed upstream and downstream from the reactor.
Samples were collected by drawing air from sampling ports fitted with Tenax adsorbent traps. Gases
that had been adsorbed on the Tenax were pretreated using a thermal desorbing system (Perkin Elmer
ATD 350, Llantrisant, UK) and analyzed by a gas chromatograph/mass spectrometer (Perkin Elmer
Clarus SQ 8) outfit with a capillary column (DB-5, Agilent, Santa Clara, CA, USA). The target
compounds were qualitatively determined on the basis of their retention times and mass spectra
(Wiley 275 software library). Quantification of gaseous compounds was carried out using calibration
curves, which were established using four concentrations normalized to an internal standard.
Laboratory blanks and spiked adsorbent traps were used for the quality control of these analyses, with
one blank trap analyzed on the day of the experimentation to check for any contamination.
The detection limits of the target pollutants ranged from 0.002 to 0.005 ppm, depending on the chemical.
4. Conclusions
In this study, N–F–TiO2 photocatalysts with varying N:F ratios were synthesized and analyzed for
their visible range photocatalytic performance in the degradation of VOCs present at standard indoor
air concentrations. XPS demonstrated the successful integration of N and F into the TiO2, while
UV-Vis spectra of both the N–F–TiO2 samples and the N–TiO2 control demonstrated improved visible
light absorption. The N–F–TiO2 composites displayed superior photocatalytic degradation of toluene,
ethyl benzene and o-xylene when compared to pure and N–TiO2, with precise activity dependent on
the N:F ratio. In addition, retention time was found to be a significant factor affecting performance.
Overall, these results indicate the utility of the prepared N–F–TiO2 composites under optimized
operational conditions.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the
Korean Ministry of Education, Science and Technology (MEST) (No. 2011-0027916) and through the
Global Core Research Center for Ships and Offshore Plants GCRC-SOP (No. 2011-0030013).
Author Contributions
Wan-Kuen Jo established the research protocol and analyzed the experimental data. Seung-Ho Shin
performed experimental works, and Ho-Hwan Chun assisted in data analysis.
Conflicts of Interest
The authors declare no conflict of interest.
Materials 2015, 8
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