Accepted Manuscript Positive Role of Incorporating P-25 TiO2 to Mesoporous-Assembled TiO2 Thin Films for Improving Photocatalytic Dye Degradation Efficiency Thammanoon Sreethawong, Supachai Ngamsinlapasathian, Susumu Yoshikawa PII: DOI: Reference:
S0021-9797(14)00338-5 http://dx.doi.org/10.1016/j.jcis.2014.05.032 YJCIS 19584
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
9 March 2014 21 May 2014
Please cite this article as: T. Sreethawong, S. Ngamsinlapasathian, S. Yoshikawa, Positive Role of Incorporating P-25 TiO2 to Mesoporous-Assembled TiO2 Thin Films for Improving Photocatalytic Dye Degradation Efficiency, Journal of Colloid and Interface Science (2014), doi: http://dx.doi.org/10.1016/j.jcis.2014.05.032
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Positive Role of Incorporating P-25 TiO2 to Mesoporous-Assembled TiO2 Thin Films for Improving Photocatalytic Dye Degradation Efficiency Thammanoon Sreethawonga,*, Supachai Ngamsinlapasathianb, and Susumu Yoshikawab a
Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 3 Research Link, Singapore 117602, Singapore b
Institute of Advanced Energy, Kyoto University, Uji, Kyoto 611-0011, Japan
*Corresponding author. E-mail address:
[email protected],
[email protected] Abstract In this work, a simple and effective strategy to improve the photocatalytic dye degradation efficiency of the mesoporous-assembled TiO2 nanoparticle thin films by incorporating small contents of commercial P-25 TiO2 during the thin film preparation was developed. The mesoporous-assembled TiO2 nanoparticles were synthesized by a sol-gel method with the aid of a mesopore-directing surfactant, followed by homogeneously mixing with P-25 TiO2 prior to the thin film coating on glass substrate. The mesoporous-assembled TiO2 film with 5 wt.% P-25 TiO2 incorporation and calcined at 400 °C provided an improved photocatalytic Acid Black (AB) dye degradation efficiency. The increase in number of coated layers to the optimum four layers of the aforementioned film was found to further improve the degradation efficiency.
The
recyclability
test
of
this
5
wt.%
P-25
TiO2-incorporated
mesoporous-assembled TiO2 film with four coated layers revealed that it can be reused for multiple cycles without a requirement of post-treatment while the degradation efficiency was retained. Keywords: Mesoporous-assembled TiO2; P-25 TiO2; Thin film; Photocatalysis; Dye degradation
1
1.
Introduction TiO2-based photocatalytic degradation of organic pollutants in wastewater has
received extensive research interest as TiO2 is a semiconductor oxide photocatalyst that is highly efficient for decomposing organic pollutant molecules, relatively cost-effective, and non-toxic as compared with other photocatalytically active materials [1-4]. Two modes of TiO2 utilization are unsupported powder suspension and thin film immobilization on substrate. In view of practical application for wastewater treatment, the immobilized TiO2 film on an appropriate substrate is preferentially desired as the costly filtering process is not required to separate and reuse the TiO2 powder as in the suspension system [5-7]. In a worse situation that the filtering process is insufficiently effective, the turbidity of the treated wastewater causes another secondary problem. This clearly makes the suspension mode much less attractive for practical uses than the immobilization mode. To date, the immobilization of TiO2 thin film on various substrates has been extensively investigated [8-10], in order to evaluate the photocatalytic performance and application feasibility, as well as to eliminate some drawbacks of the immobilized TiO2 film, such as limited mass transfer of pollutant molecules to surface active sites of a thick film and detachment of the film from substrate during the photocatalytic reaction for multiple cycles of use. The mesoporous-structured TiO2 thin films have been employed to improve the mass transfer of pollutant molecules across a thicker film layer, resulting in the enhanced photocatalytic activity [11-29]. Sol-gel methods with the aid of mesopore-directing high-molecular-weight polymer molecules have been widely used for the fabrication of mesoporous-structured TiO2 films by coating (e.g. dip coating, spray coating, and spin coating) either TiO2 sol or gel on substrates followed by heat treatment to form TiO2 thin films. In previous works [30-36], the incorporation of commercial Degussa P-25 TiO2 powder into self-synthesized TiO2 sols prior to the
2
coating step has been proposed to prepare thicker TiO2 thin films with good adherability on substrates. It has been reported therein that the P-25 TiO2 incorporation into the self-synthesized TiO2 sols largely affected the size, number, and physicochemical properties of the TiO2 crystallites as the sols further underwent the condensation to form gels and then crystallized during the heat treatment on the surface of incorporated P-25 TiO2 powder. This indicates that the P-25 TiO2 surface served as the sites for nucleation and growth of the self-synthesized TiO2 particles [30,33,35]; therefore, physicochemical properties of the TiO2 in the obtained P-25 TiO2/TiO2 mixture were significantly influenced by the content of P-25 TiO2. In order to circumvent the variation in nucleation and growth of a self-synthesized TiO2 in the resulting thin films, the incorporation of P-25 TiO2 into a sol-derived gel (instead of direct incorporation into sol)
was
proposed
in
this
work
to
prepare
P-25
TiO2-incorporated
mesoporous-assembled TiO2 thin films with a good adherability on glass substrate and an improved photocatalytic activity. Herein, the mesoporous-assembled TiO2 nanoparticle photocatalyst was initially synthesized by a sol-gel method with the aid of the mesopore-directing low-molecular-weight laurylamine hydrochloride surfactant. The resulting gel was incorporated with different contents of P-25 TiO2, followed by immobilizing on glass plates by the doctor-blading method and calcining at various temperatures to yield the P-25 TiO2-incorporated mesoporous-assembled TiO2 thin films. Multilayer coating of the P-25 TiO2-incorporated mesoporous-assembled TiO2 thin films was also performed. The
photocatalytic
activity
of
all
the
prepared
P-25
TiO2-incorporated
mesoporous-assembled TiO2 thin films was investigated comparatively via the degradation of Acid Black (AB) dye used as a model pollutant in textile wastewater. The recyclability of the P-25 TiO2-incorporated mesoporous-assembled TiO2 thin film prepared using optimum conditions for the photocatalytic AB dye degradation was also
3
evaluated.
2.
Experimental
2.1 Materials Tetraisopropyl orthotitanate (TIPT, Ti(OCH(CH3)2)4, Merck), acetylacetone (ACA,
CH3COCH2COCH3,
Carlo
Erba),
laurylamine
hydrochloride
(LAHC,
CH3(CH2)11NH2⋅HCl, Merck), Acid Black dye (AB, C22H14N6Na2O9S2, Nacalai Tesque), sodium hydroxide (NaOH, Lab Scan Asia), Triton X-100 (C14H22O(C2H4O)n, n = 9-10, Merck), ethanol (C2H5OH, Ajax Finechem), and propanol (CH3CH2CH2OH, Ajax Finechem) were used as in this work. All chemicals were of analytical reagent grade and used without further purification. Commercial P-25 TiO2 powder (J.J. Degussa Hüls Co., Ltd., specific surface area of 65 m2/g, crystallite sizes of 22 nm for its 74% anatase phase and 29 nm for its 26% rutile phase) was used to incorporate to the synthesized mesoporous-assembled TiO2 photocatalyst. 2.2 Synthesis of mesoporous-assembled TiO2 gel The mesoporous-assembled TiO2 gel was synthesized by a sol-gel method with the aid of the mesopore-directing LAHC surfactant [37,38], as follows. A predetermined amount of ACA was firstly introduced to TIPT with a controlled molar ratio of 1:1. The mixed solution was gently shaken until homogeneous mixing. A 0.1 M LAHC aqueous solution was separately prepared and then added to the TIPT/ACA solution, in which a molar ratio of TIPT to LAHC was controlled at 4:1. The resulting mixture was continuously stirred at 40 °C for 8 h to obtain a transparent yellow sol. Then, the sol was placed in an oven at 80 °C for a week to achieve a complete gel formation. The as-synthesized TiO2 gel was washed by mixing with propanol and continuously stirring at room temperature for 2 h. Finally, the mixture was centrifuged to separate the wet TiO2 gel for further use in the thin film immobilization step.
4
2.3 Immobilization of P-25 TiO2-incorporated mesoporous-assembled TiO2 thin films on glass plates To prepare a TiO2 paste, a specified amount of the obtained wet TiO2 gel was mixed with P-25 TiO2 powder, ethanol, and Triton X-100. The P-25 TiO2 contents were controlled in the range of 3-7 wt.%. The mixture was homogenized by continuously stirring at room temperature for 1 d. To prepare glass plates (2.4 cm × 7.6 cm × 2 mm), they were cleaned by soaking in a NaOH aqueous solution for 1 d to remove any impurities adsorbed on their surface. After that, they were sonicated in deionized water for removing residual NaOH, rinsed repeatedly with deionized water, and dried in an oven at 80 °C prior to use. The as-prepared TiO2 paste (2 g for each coated layer) was coated on the pretreated glass plates by the doctor-blading method, and the glass plates with TiO2 film (with film area dimensions of 1.9 cm × 6.6 cm) were placed in an oven at 80 °C for drying. The resulting TiO2 film-coated glass plates were finally calcined at 400-500 °C for 2 h in order to remove the LAHC surfactant. As a result, the P25 TiO2-incorporated mesoporous-assembled TiO2 thin films immobilized on glass plates were obtained. The coating process was repeated in the same manner for multilayer coating. It should be noted that the minimum calcination temperature of 400 °C was required
for
the
complete
LAHC
surfactant
removal,
based
on
the
thermogravimetric/derivative thermogravimetric analysis (TG-DTA) result. 2.4 Characterization techniques The BET specific surface area, mean mesopore diameter, and total pore volume of the TiO2 photocatalysts were measured by a N2 adsorption-desorption analyzer (Quantachrome, Autosorb I). Each TiO2 sample was degassed to remove humidity and volatile species adsorbed on its surface under vacuum at 150 °C for 4 h prior to the analysis. The sample morphology of the TiO2 photocatalysts immobilized on glass plates was observed by a scanning electron microscope (SEM, Hitachi, S-4800) and a
5
transmission electron microscope (TEM, JEOL, JEM 2100). The SEM was also used to determine the thickness of the TiO2 films. The SEM and TEM analyses were performed at accelerating voltages of 2 and 200 kV, respectively. An atomic force microscope (AFM, Park Systems, XE-100) was used to analyze the root-mean-square (RMS) surface roughness and surface topology of the TiO2 films with the scan size of 5 μm × 5 μm. The X-ray diffraction (XRD) patterns of the TiO2 film-coated glass plates were recorded by using a Bruker AXS system (D8 Advance) with a copper tube for generating CuKα radiation (1.54056 Å) at 40 kV and 30 mA with a nickel filter. A UV-visible spectrophotometer (Shimadzu, UV-2550) was used to identify the light absorption ability of the TiO2 film-coated glass plates with BaSO4 as the reference. The energy band gap of the TiO2 photocatalysts was determined based on their absorption onset wavelength obtained from the UV-visible absorbance spectra. 2.5 Photocatalytic activity tests The photocatalytic AB dye degradation experiments were performed in a Pyrex glass reactor at room temperature. An AB dye aqueous solution (total volume of 500 ml, initial concentration of 10 ppm, and initial solution pH of ~5) was freshly prepared and used for the photocatalytic activity tests. The prepared TiO2 film-coated glass plates (8 plates) were statically placed in the reactor containing the dye solution. Prior to each photocatalytic activity test, the dye solution continuously stirred by using a magnetic stirrer was left in a dark environment for 30 min to establish the adsorption equilibrium on the TiO2 film. The photocatalytic reaction was started by exposing the system to UV light irradiation (4 lamps, 11 W low-pressure Hg lamp, Philips). The dye solution was periodically withdrawn from the reactor. The samples were then analyzed for the AB dye concentrations by the UV-visible spectrophotometer at the maximum absorbance wavelength of the AB dye at 619 nm. The AB dye degradation efficiency (%) was determined by using the following equation:
6
⎛C − C⎞ ⎟⎟ × 100 , Degradation efficiency (%) = ⎜⎜ 0 ⎝ C0 ⎠ where C0 and C denote the AB dye concentrations at irradiation time (t) = 0 and t = t, respectively. The degradation efficiency was used as the indicator to evaluate and compare the photocatalytic AB dye degradation performance of all the prepared TiO2 thin films. For the recyclability experiments of a selected P-25 TiO2-incorporated mesoporous-assembled TiO2 film for photocatalytic AB dye degradation, the TiO2 film-coated glass plates (8 plates) were removed from the reaction mixture of the first cycle, washed repeatedly with deionized water, and dried in an oven at 80 °C overnight prior to the use in the next cycle. In the second cycle, the used TiO2 film-coated glass plates from the first cycle were statically placed in the reactor containing a freshly prepared AB dye aqueous solution (total volume of 500 ml, initial concentration of 10 ppm, and initial solution pH of ~5), and the remaining steps of the photocatalytic activity test were the same as described above. In this work, the recyclability of TiO2 film-coated glass plates were tested for three consecutive cycles, without any chemical or thermal post-treatment of the TiO2 film after each cycle.
3.
Results and discussion
3.1 Characterization results 3.1.1 Porous structure and textural properties of TiO2 photocatalysts Figure 1 comparatively illustrates the N2 adsorption-desorption isotherms and pore size distributions (insets) of the synthesized TiO2 photocatalysts without and with 5 wt.% P-25 TiO2 incorporation, both scraped from the TiO2 film-coated glass plates calcined at 400 °C. The isotherms of these and the other photocatalysts similarly exhibited the typical IUPAC type-IV pattern with the type-H2 hysteresis loop [39]. This
7
hysteresis loop behavior can be attributed to the existence of mesoporous structure (with mesoporous size between 2 and 50 nm) in the samples. A sharp increase in volume of adsorbed N2 can be clearly observed and located in the high relative pressure (P/P0) range of 0.70-0.95. This sharp increase can be ascribed to the capillary condensation of N2 molecules into the mesopores, indicating the small pore size of the samples. As illustrated in the insets of Figure 1, the pore size distributions obtained from the desorption branch of the isotherms were also very narrow and monomodal, revealing the homogenous pore size of the samples. The results of textural properties of all investigated photocatalysts, including specific surface area, mean mesopore diameter, and total pore volume, are summarized in Table 1. In the case of varying P-25 TiO2 content incorporated to the synthesized mesoporous-assembled TiO2 photocatalyst calcined at 400 °C, the specific surface area of the P-25 TiO2-incorporated mesoporous-assembled TiO2 photocatalyst tended to decrease with increasing the incorporated P-25 TiO2 content, whereas the mean mesopore diameter tended to conversely increase. The lower specific surface area of the P-25 TiO2-incorporated mesoporous-assembled TiO2 photocatalysts as compared with the one without P-25 TiO2 incorporation (~127 m2/g) is due to the fact that the P-25 TiO2 itself has a much lower specific surface area (~65 m2/g), resulting in the decreased specific surface area of the mesoporous-assembled TiO2 photocatalyst after the P-25 TiO2 incorporation. It is unexpectedly found that the total pore volume of the P-25 TiO2-incorporated mesoporous-assembled TiO2 photocatalyst tended to increase with increasing the incorporated P-25 TiO2 content. This may be attributed to the expansion of the mesoporous volume due to the incorporated P-25 TiO2 powders (possessing bigger particle size, as revealed in the next section), which are inserted among the synthesized TiO2 nanoparticles. In comparison, in the case of increasing calcination temperature for the mesoporous-assembled TiO2 photocatalyst with 5 wt.% P-25 TiO2 incorporation, a
8
higher calcination temperature caused the decreases in specific surface area and total pore volume and the increase in mean mesopore diameter due to the mesopore coalescence and collapse under a more severe calcination condition. 3.1.2 Morphology and particle size of TiO2 photocatalysts The SEM images (top and cross-sectional views) of the mesoporous-assembled TiO2 film with 5 wt.% P-25 TiO2 incorporation and calcined at 400 °C are exemplified in Figure 2. The top-viewed SEM image (Figure 2a) reveals the presence of aggregated clusters formed by agglomeration of the P-25 TiO2 nanoparticles (larger particle size) with the synthesized uniform-sized TiO2 nanoparticles (smaller particle size), with an insertion of the aggregated P-25 TiO2 clusters among the synthesized TiO2 nanoparticles. This nanoparticle aggregation can be considered as the main cause of the mesoporous-assembled structure formation in the prepared TiO2 film. The particle sizes of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 photocatalyst were also investigated by TEM analysis, as shown in Figure 3. The average particle sizes of the mesoporous-assembled TiO2 and P-25 TiO2 photocatalysts were in the range of 5-10 and 15-25 nm, respectively. The particle size of the P-25 TiO2 photocatalyst agrees well with its original crystallite size of 22 nm for its main anatase phase, indicating that the calcination temperature used in this work did not affect the P-25 TiO2 particle growth/transformation. Moreover, the cross-sectional view of the SEM image (Figure 2b) reveals that the coated smooth TiO2 film with a uniform thickness adhered very well on the glass substrate. 3.1.3 Thickness of TiO2 films and amount of TiO2 coated on glass substrate The results of film thickness obtained from the SEM images and amount of the mesoporous-assembled TiO2 photocatalysts without and with P-25 TiO2 incorporation scraped from the TiO2 film-coated glass plates are presented in Table 1. It was found that at the calcination temperature of 400 °C, when a higher content of the P-25 TiO2 in
9
the investigated range of 3-7 wt.% was incorporated to the mesoporous-assembled TiO2 film, a thicker film was achieved as compared to the film without P-25 TiO2 incorporation at the same single coated layer. This accordingly led to the increase in amount of coated TiO2 powders on glass plates with increasing the incorporated P-25 TiO2 content. These results reveal an important and effective role of the P-25 TiO2, with the small contents, to assist in increasing the thickness of the mesoporous-assembled TiO2 film. In contrast, the increase in calcination temperature in the investigated range of 400-500 °C for the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film only slightly affected the film thickness and amount of coated TiO2 powders (Table 1). In addition, a simple way to further increase the film thickness and amount of coated TiO2 powders of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film is to employ multilayer coating. As shown in Table 2, the increase in number of coated layers from one to four layers resulted in the increases in film thickness and amount of coated TiO2 powders of the TiO2 film. 3.1.4 Surface topology and roughness of TiO2 films Figure 4 shows comparatively the three-dimensional AFM images of the mesoporous-assembled TiO2 films without and with P-25 TiO2 incorporation at different P-25 TiO2 contents and calcined at different temperatures, while their corresponding surface roughness from the surface topology analysis is summarized in Table 1. It is evidenced that the surface topology of all the investigated TiO2 films consisted of aggregated clusters derived from nanoparticle agglomeration. The surface roughness results show that the incorporation of P-25 TiO2 to the mesoporous-assembled TiO2 film resulted in the increase in surface roughness, which is caused by the presence of high mountains and deep valleys on the film surface induced by the aggregated P-25 TiO2 clusters situated among the aggregated clusters of the mesoporous-assembled TiO2. It was also observed that the surface roughness of the film increased with increasing the
10
calcination temperature from 400 to 500 °C, possibly due to a non-uniform heat-induced film expansion over the entire film under a more severe calcination condition. 3.1.5 Crystalline structure and crystallite size of TiO2 photocatalysts The XRD patterns of the P-25 TiO2-incorporated mesoporous-assembled TiO2 films with different P-25 TiO2 contents coated on glass plates and calcined at 400 °C are shown in Figure 5a, while those of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 films coated on glass plates and calcined at different temperatures are shown in Figure 5b. The XRD patterns of all the investigated TiO2 films reveal the crystalline structure of the anatase TiO2 phase due to the presence of a distinct diffraction peak at 2θ of 25.2°, which represents an index of the anatase (101) plane (JCPDS Card No. 21-1272) [40]. It should be noted that several small diffraction peaks observed in Figure 5 belong to the glass plate, as comparatively shown by its XRD pattern therein. It can be observed that the anatase peak intensity increased with increasing both P-25 TiO2 content and calcination temperature. Since the P-25 TiO2 possesses both the anatase and rutile TiO2 phases (with the anatase and rutile contents of 74% and 26%, respectively), the contribution of the incorporated P-25 TiO2 on the anatase phase becomes more dominant with steadily increasing the P-25 TiO2 content. In contrast, the increase in calcination temperature led to the gradual growth of TiO2 crystallites with a higher crystallization degree, resulting in the observed increase in peak intensity. It can also be noticed that although the P-25 TiO2 possessing both the anatase and rutile TiO2 phases was incorporated to mesoporous-assembled TiO2 films, the crystalline structure of the films still exhibited only the anatase TiO2 phase. There was no main diffraction peak of the rutile TiO2 phase observed at 2θ of 27.4° for the rutile (110) plane (JCPDS Card No. 21-1276) [40], and this is plausibly due to the low P-25 TiO2 contents of 3-5 wt.% employed in this work. Hence, the incorporated P-25 TiO2 at such low contents did not significantly affect the crystalline structure of the P-25
11
TiO2-incorporated mesoporous-assembled TiO2 films. By further analyzing the XRD patterns, the crystallite size of the TiO2 photocatalysts could be calculated from the line broadening of the anatase (101) diffraction peak by using the Scherrer equation [41], as follows:
L=
kλ , βcos(θ)
where L is the crystallite size, k is the Scherrer constant usually taken as 0.89, λ is the wavelength of the X-ray radiation (0.15418 nm for CuKα), β is the full width at half maximum of the diffraction peak measured at 2θ, and θ is the diffraction angle. The results of TiO2 crystallite size of the mesoporous-assembled TiO2 films without and with P-25 TiO2 incorporation at different P-25 TiO2 contents and calcined at different temperatures are summarized in Table 1. The increase in P-25 TiO2 content caused the increase in TiO2 crystallite size due to the contribution from the incorporated P-25 TiO2 with a higher crystallite size (i.e. 22 nm for P-25 TiO2 as compared with 8.59 nm for the mesoporous-assembled TiO2). On the contrary, the increase in calcination temperature in the investigated range of 400-500 °C exerted less effect on the TiO2 crystallite size of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 films, as observed to be slightly increased when comparing with the case of increasing the P-25 TiO2 content. As demonstrated for the case of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 photocatalyst, it was also found that its TiO2 crystallite size calculated from the XRD pattern (Table 1) agrees well with the average particle size observed from the TEM image (Figure 3). 3.1.6 Light absorption ability and energy band gap of TiO2 photocatalysts The UV-visible absorbance spectra of the mesoporous-assembled TiO2 films without and with P-25 TiO2 incorporation at different P-25 TiO2 contents and calcined at different temperatures are shown in Figure 6. The strong absorption band in the wavelength lower than ~375-385 nm indicates the presence of Ti species as tetrahedral 12
Ti4+ and is generally associated with the electronic excitation of the valence band O2p electron to the conduction band Ti3d level [42]. The absorption onset wavelength of ~375 nm for the mesoporous-assembled TiO2 photocatalyst originates from the inherent property of the anatase TiO2 phase, while the absorption onset wavelength of the P-25 TiO2-incorporated mesoporous-assembled TiO2 photocatalysts is extended to ~410-415 nm due to the inherent property of the rutile TiO2 phase. The energy band gap (Eg, eV) of the investigated TiO2 photocatalysts was further estimated by determining the absorption onset wavelength (λg, nm) of each UV-visible absorbance spectrum, as shown by a dashed line in Figure 6, and calculating by the following equation [43]:
Eg = The
results
of
absorption
1240 . λg
onset
and
band
gap
energy
of
the
mesoporous-assembled TiO2 photocatalysts without and with P-25 TiO2 incorporation and calcined at different temperatures are summarized in Table 1. As the absorption onset wavelength of the mesoporous-assembled TiO2 photocatalyst was observed to be ~375 nm, its band gap energy was calculated to be ~3.31 eV, which approximately concurs with the anatase TiO2 band gap energy of ~3.20 eV [1-4]. In contrast, the UV-visible absorbance spectra of the P-25 TiO2-incorporated mesoporous-assembled TiO2 photocatalysts, regardless of the incorporated P-25 TiO2 content and calcination temperature, showed two absorption onsets at the wavelengths of ~380-385 and ~410-415 nm, where the latter can be correlated to the rutile TiO2 band gap energy of ~3.02 eV [1-4]. This clearly confirms the successful incorporation of the P-25 TiO2 to the mesoporous-assembled TiO2 films. 3.2 Photocatalytic AB dye degradation results 3.2.1 Effect of P-25 TiO2 content incorporated to the mesoporous-assembled TiO2 film The effect of P-25 TiO2 content incorporated to the mesoporous-assembled TiO2 film on the photocatalytic AB dye degradation efficiency was first investigated. The 13
degradation efficiency results of the P-25 TiO2-incorporated mesoporous-assembled TiO2 film with single coated layer and calcined at 400 °C are summarized in Table 1. It can be seen that the degradation efficiency initially increased with increasing the incorporated P-25 TiO2 content to reach the maximum value at 5 wt.% P-25 TiO2, but it adversely decreased with further increasing the incorporated P-25 TiO2 content to 7 wt.%. The optimum P-25 TiO2 content incorporated was therefore 5 wt.% for the maximum degradation efficiency. It can be seen from Table 1 that with increasing the incorporated P-25 TiO2 content to 5 wt.%, the increases in film thickness and amount of coated TiO2 powders accompanying by the increases in total pore volume and film surface roughness imparted positive effects on the degradation efficiency by enhancing the possibility of AB dye accessibility to active sites available on the TiO2 surface. Even though the slight decrease in specific surface area and the gradual increase in TiO2 crystallite size were observed, their negative effects were less dominant. Nevertheless, with further increasing the incorporated P-25 TiO2 content from 5 to 7 wt.%, the drastically decreased specific surface area led to the significantly reduced number of active sites available on the TiO2 surface, while the relatively too large TiO2 crystallite size resulted in the increased probability of photoinduced charge carrier recombination at the bulk traps, both leading to the observed decrease in the degradation efficiency. Even though the film thickness and amount of coated TiO2 powders were increased, the negative effects from the above-mentioned unfavorable properties of the photocatalyst itself became much more obvious. In addition, since the rutile TiO2 phase has been reported in previous works to be less photocatalytically active than the anatase TiO2 phase [44,45], the incorporation of rutile phase-comprising P-25 TiO2 with excessive content (i.e. 7 wt.%) may be another cause of the decreased degradation efficiency. Therefore, in this present work, the incorporated P-25 TiO2 content was optimized to be 5 wt.%, providing the highest photocatalytic activity of the P-25 TiO2-incorporated
14
mesoporous-assembled TiO2 film with single coated layer. 3.2.2
Effect of calcination temperature of the 5 wt.% P-25 TiO2-incorporated
mesoporous-assembled TiO2 film The effect of calcination temperature of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film on the photocatalytic AB dye degradation efficiency was further examined. The degradation efficiency results of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film with single coated layer and calcined at different temperatures in the range of 400-500 °C are included in Table 1. It was found that the degradation efficiency gradually decreased with increasing calcination temperature. This is mainly because of the decreased specific surface area and total pore volume and the increased TiO2 crystallite size, which are detrimental to the photocatalytic activity by reducing the AB dye accessibility to the surface active sites and increasing the probability of photoinduced charge carrier recombination at the bulk traps, respectively. Nevertheless, it could be observed that the degradation efficiency of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film calcined at different temperatures was still higher than that of the film without P-25 TiO2 incorporation. The results suggest that even though the decrease in specific surface area and the increase in TiO2 crystallite size were observed when the P-25 TiO2 was incorporated to the mesoporous-assembled TiO2 film (Table 1), the incorporated P-25 TiO2 at a suitable content (i.e. 5 wt.% in this work) improved the degradation efficiency of the mesoporous-assembled TiO2 film by increasing the adherability of the TiO2 film to achieve a higher film thickness and a higher amount of coated TiO2 photocatalyst. However, due to the limitation of active sites available for single coated layer, the increase in number of coated layers (i.e. implying the increases in film thickness and amount of the coated TiO2 photocatalyst) was expected to further improve the photocatalytic activity.
15
3.2.3
Effect of number of coated layers of the 5 wt.% P-25 TiO2-incorporated
mesoporous-assembled TiO2 film The effect of number of coated layers of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film on the photocatalytic AB dye degradation efficiency was next investigated. It was experimentally found that the second layer of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film calcined at the temperatures of 450 °C or higher easily peeled off from the glass plates during the photocatalytic activity tests; therefore, a calcination temperature higher than 400 °C could not be used to investigate the effect of increasing coated layers. For this reason, the calcination temperature of 400 °C was used for the multilayer coating. The degradation efficiency results of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film with various coated layers and calcined at 400 °C are summarized in Table 2. It could be clearly observed that the degradation efficiency increased with increasing the number of coated layers. This can be explained in that the increase in number of coated layers accordingly resulted in the increases in film thickness and amount of coated TiO2 photocatalyst (Table 2), which led to a higher available surface active sites and more frequent light scattering probability on TiO2 nanoparticles to generate more photoinduced charge carriers for photocatalytic reaction. Hence, the improved photocatalytic activity was observed. This reveals no mass transfer limitation of the thick multilayer TiO2 film, possibly due to the facilitated accessibility of the AB dye through the mesoporous structure across the film layer. Therefore, the mesoporous structure is very beneficial for the surface active sites of a thick TiO2 film to be exposed by the reactant molecules. However, even if the 5 wt.% P-25 TiO2 was incorporated to increase the TiO2 film thickness and to improve the adherability between TiO2 layers for the multilayer coating, the number of coated layers higher than four layers could not be achieved due to the easy peel-off of the TiO2 film from the glass plates during the
16
photocatalytic reaction. This might result from the too thick multilayer TiO2 films, and the sufficiently strong adherability between TiO2 layers could no longer be obtained. Currently, effective ways to improve the adherability between TiO2 layers and to solve this peel-off limitation are being investigated, and the results will be presented in our future work. 3.2.4 Recyclability of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film The recyclability of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film with four coated layers and calcined at 400 °C on the photocatalytic AB dye degradation was tested, without any chemical or thermal post-treatment of the film after each cycle. The degradation efficiency results as a function of irradiation time for three consecutive cycles are shown comparatively in Figure 7. It can be clearly seen that the investigated film exhibited the comparable degradation performance for the three consecutive cycles, and no detachment of the film from the glass plates was observed during the experiments. The capability of the film for multiple reuse with its acceptably high stability and adherability under mobile circumstance, as employed in this work, makes it potentially useful for practical application.
4.
Conclusions
This work focused on a simple and effective strategy to improve the photocatalytic
Acid
Black
(AB)
dye
degradation
efficiency
of
the
mesoporous-assembled TiO2 thin films by incorporating small contents of commercial P-25 TiO2 during the thin film preparation. The effects of incorporated P-25 TiO2 content, calcination temperature, and number of coated layers were investigated on the degradation efficiency of the P-25 TiO2-incorporated mesoporous-assembled TiO2 films. The optimum P-25 TiO2 content incorporated to the mesoporous-assembled TiO2 film
17
was found to be 5 wt.%, exhibiting an improved degradation efficiency as compared with the film without P-25 TiO2 incorporation. The degradation efficiency of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film was found to decrease with increasing calcination temperature, and the higher calcination temperature was not suitable for the preparation of thick multilayer TiO2 films. The suitable calcination temperature was observed at 400 °C. The increase in number of coated layers of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film to the optimum four layers was found to further improve the degradation efficiency. In addition, the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 film with four coated layers could be reused for multiple cycles without an involvement of chemical or thermal post-treatment, while the degradation efficiency was maintained at the same level as that observed for the freshly prepared TiO2 film.
Acknowledgments
The authors would like to thank the 21COE Program and the Nanotechnology Support Project, Japan; and Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore for their supports.
18
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24
Table 1 Results of N2 adsorption-desorption analysis, film thickness, amount of TiO2 on glass plates, film surface roughness, crystallite size,
absorption onset wavelength, band gap energy, and photocatalytic AB dye degradation efficiency of the P-25 TiO2-incorporated mesoporous-assembled TiO2 films with different P-25 TiO2 contents coated on glass plate for one layer and calcined at different temperatures (Photocatalytic reaction conditions: total reaction mixture volume, 500 ml; initial AB dye concentration, 10 ppm; initial solution pH, ~5; number of TiO2 film-coated glass plate, 8 plates; and irradiation time, 5 h)
P-25 TiO2 content (wt.%)
Calcination
Specific
temperature
surface area
(°C)
(m2·g-1)
0 (Pure
Mean mesopore diameter (nm)
Total pore
TiO2 film
volume
thickness
(cm3·g-1)
(μm)
Amount of
Surface
TiO2 on 8
roughness of
glass plates
TiO2 film
(mg)
(nm)
TiO2 crystallite size (nm)
Absorption onset wavelength, λg (nm)
Band gap
Degradation
energy, Eg
efficiency
(eV)
(%)
126.8
9.6
0.363
1.37
27
58
8.59
375
3.31
53.2
108.2
12.5
0.469
1.24
92
63
9.57
380, 410
3.26, 3.02
60.0
5
114.2
12.4
0.546
4.96
110
67
14.68
380, 410
3.26, 3.02
67.8
7
87.2
12.4
0.518
5.83
163
97
17.06
385, 415
3.22, 2.99
56.0
400
114.2
12.4
0.546
4.96
110
67
14.68
380, 410
3.26, 3.02
67.8
450
78.8
17.5
0.485
4.75
89
99
14.71
380, 410
3.26, 3.02
58.3
500
58.9
31.5
0.388
4.51
103
156
14.73
380, 415
3.26, 2.99
53.4
synthesized TiO2) 3 400
5
25
Table 2 Results of film thickness, amount of TiO2 on glass plates, and photocatalytic
AB
dye
degradation
efficiency
of
the
5
wt.%
P-25
TiO2-incorporated
mesoporous-assembled TiO2 films at various coated layers and calcined at 400 °C (Photocatalytic reaction conditions: total reaction mixture volume, 500 ml; initial AB dye concentration, 10 ppm; initial solution pH, ~5; number of TiO2 film-coated glass plate, 8 plates; and irradiation time, 5 h)
Number of coated
TiO2 film thickness
Amount of TiO2 on 8
Degradation
layers
(µm)
glass plates (mg)
efficiency (%)
1
4.96
110
67.8
2
21.4
255
68.4
3
33.7
496
71.3
4
46.4
796
78.8
26
List of Figure Captions Figure 1 N2 adsorption-desorption isotherms of (a) the mesoporous-assembled TiO2
photocatalyst and (b) the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 photocatalyst, both scraped from the TiO2 film-coated glass plates calcined at 400 °C (Insets: Pore size distributions). Figure 2 SEM images of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled
TiO2 film calcined at 400 °C with single coated layer: (a) top view and (b) cross-sectional view. Figure 3 TEM image of 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2
scraped from the TiO2 film-coated glass plats calcined at 400 °C. Figure
4
Three-dimensional
AFM
images
of
the
P-25
TiO2-incorporated
mesoporous-assembled TiO2 films with different P-25 TiO2 contents coated on glass plates and calcined at different temperatures. Figure 5 XRD patterns of (a) the P-25 TiO2-incorporated mesoporous-assembled TiO2
films with different P-25 TiO2 contents coated on glass plates and calcined at 400 °C and (b) the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 films coated on glass plates and calcined at different temperatures (A = Anatase TiO2). Figure 6 UV-visible absorbance spectra of (a) the P-25 TiO2-incorporated
mesoporous-assembled TiO2 films with different P-25 TiO2 contents coated on glass plates and calcined at 400 °C and (b) the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 films coated on glass plates and calcined at different temperatures. Figure
7
Recyclability
results
of
the
5
wt.%
P-25
TiO2-incorporated
mesoporous-assembled TiO2 film with four coated layers and calcined at 400 °C on photocatalytic AB dye degradation efficiency.
27
Figure 1 N2 adsorption-desorption isotherms of (a) the mesoporous-assembled TiO2
photocatalyst and (b) the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 photocatalyst, both scraped from the TiO2 film-coated glass plates calcined at 400 °C (Insets: Pore size distributions).
450
120 Dv (mm3 nm-1 g-1 )
Adsorbed amount (cm3(STP)g-1)
(a)
400 350 300 250
Adsorption Desorption
90 60 30 0 0
200
20 40 60 80 100 Pore diameter (nm)
150 100 50 0 0
0.2
0.4
0.6
0.8
1
Relative pressure (P/P0)
(b)
40
Dv (mm3 nm-1 g-1 )
Adsorbed amount (cm3(STP)g-1)
450 400 350 300 250
Adsorption Desorption
30 20 10 0 0
200
20 40 60 80 100 Pore diameter (nm)
150 100 50 0 0
0.2
0.4
0.6
Relative pressure (P/P0)
28
0.8
1
Figure 2 SEM images of the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled
TiO2 film calcined at 400 °C with single coated layer: (a) top view and (b) cross-sectional view.
(a)
P-25 TiO2
Mesoporous-assembled TiO2
(b)
TiO2 film
5 µm
Glass plate
29
Figure 3 TEM image of 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2
scraped from the TiO2 film-coated glass plats calcined at 400 °C.
P-25 TiO2
Mesoporous-assembled TiO2
30
Figure
4
Three-dimensional
AFM
images
of
the
P-25
TiO2-incorporated
mesoporous-assembled TiO2 films with different P-25 TiO2 contents coated on glass plates and calcined at different temperatures.
0 wt.% P-25, 400 °C
3 wt.% P-25, 400 °C
5 wt.% P-25, 400 °C
5 wt.% P-25, 450 °C
31
Figure 5 XRD patterns of (a) the P-25 TiO2-incorporated mesoporous-assembled TiO2
films with different P-25 TiO2 contents coated on glass plates and calcined at 400 °C and (b) the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 films coated on glass plates and calcined at different temperatures (A = Anatase TiO2).
(a)
Intensity (a.u.)
A(101)
7 wt.% P-25 5 wt.% P-25 3 wt.% P-25 Glass plate 10
20
30
40
50
60
70
60
70
2Theta (degree)
(b)
Intensity (a.u.)
A(101)
500 °C 450 °C 400 °C Glass plate 10
20
30
40
50
2Theta (degree)
32
Figure 6 UV-visible absorbance spectra of (a) the P-25 TiO2-incorporated
mesoporous-assembled TiO2 films with different P-25 TiO2 contents coated on glass plates and calcined at 400 °C and (b) the 5 wt.% P-25 TiO2-incorporated mesoporous-assembled TiO2 films coated on glass plates and calcined at different temperatures.
5
(a)
(a) (b) (c) (d)
Absorbance (a.u.)
4
0 wt.% P-25 3 wt.% P-25 5 wt.% P-25 7 wt.% P-25
3 (a) (b) (c) (d)
2 1 Absorption onset (λg)
0 250
300
350
400
450
500
Wavelength (nm)
5
(b)
(a) 400 °C (b) 450 °C (c) 500 °C
Absorbance (a.u.)
4 3 2
(a) (b) (c)
1 0 250
300
350
400
Wavelength (nm)
33
450
500
Figure
Recyclability
7
results
of
the
5
wt.%
P-25
TiO2-incorporated
mesoporous-assembled TiO2 film with four coated layers and calcined at 400 °C on photocatalytic AB dye degradation efficiency.
70 1st cycle
2nd cycle
3rd cycle
Degradation efficiency (%)
60 50 40 30 20 10 0 0
1
2
3
4
5
6
7
8
9
Irradiation time (h)
34
10
11
12
13
14
15
Graphical Abstract
35
Research Highlights
•
Incorporation of P-25 TiO2 to mesoporous-assembled TiO2 nanoparticle thin
•
Thickness of TiO2 thin films was simply enhanced by P-25 TiO2
films.
incorporation. •
P-25 TiO2 incorporation of 5 wt.% exhibited an improved dye degradation
efficiency. •
Multilayer coating of P-25-incorporated TiO2 thin films was demonstrated at
400 °C. •
Optimum 4-layer coating of TiO2 thin films gave the highest degradation
efficiency.
36