Microstructure characterization and photocatalytic activity of mesoporous TiO2 films with ultrafine anatase nanocrystallites

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Surface & Coatings Technology 202 (2008) 1944 – 1950 www.elsevier.com/locate/surfcoat

Microstructure characterization and photocatalytic activity of mesoporous TiO2 films with ultrafine anatase nanocrystallites Yongjun Chen a , Elias Stathatos b , Dionysios D. Dionysiou a,⁎ a

Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH, 45221-0071, USA b Technological Institute of Patras, Electrical Engineering Department, 263 34 Patras, Greece Received 8 May 2007; accepted in revised form 16 August 2007 Available online 30 August 2007

Abstract A series of mesoporous TiO2 films on borosilicate glass with ultrafine anatase nanocrystallites were successfully synthesized using a non-acidic sol gel preparation route, which involves the use of nonionic surfactant Tween 20 as template through a self assembly pathway. The microstructure of these TiO2 films was characterized by XRD, SEM, HR-TEM, UV–Vis spectroscopy, and N2 adsorption–desorption isotherm analysis. Their photocatalytic activities were investigated by using creatinine as a model organic contaminate in water. It was found that all mesoporous TiO2 films prepared with Tween 20 exhibited a partially ordered mesoporous structure. The photocatalytic activity of the TiO2 films could be remarkably improved by increasing Tween 20 loading in the sol at the range of 50% (v/v), which yielded large amount of catalyst (anatase) on the glass support and enhanced specific surface area. The optimum Tween 20 loading was 50% (v/v) in the sol, above which good adhesion between TiO2 films and borosilicate glass could not be maintained. The final TiO2 film (Tween 20: final sol = 50%,v/v) exhibits high BET surface area (∼ 120 m2/g) and pore volume (0.1554 cm3/g), ultrafine anatase nanocrystallinity (7 nm), uniform and crack free surface morphology, and improved photocatalytic activity. © 2007 Elsevier B.V. All rights reserved. Keywords: Nonionic surfactant; Tween 20; Self-assembling; Sol gel; TiO2; Photocatalysis

1. Introduction Titanium dioxide (TiO2) is an important photocatalyst in environmental applications due to its high activity, no toxicity, low cost and excellent durability [1–3]. Anatase TiO2 nanocrystallites are considered to be the most active crystalline structure of TiO2 photocatalyst. Ultrafine anatase nanocrystallites (Q-sized particles, i.e., less than 10 nm) possess unique photochemical properties due to quantum size effects [4,5]. It has been reported that Q-sized TiO2 nanocrystallites can expedite the diffusion of electron and holes to the surfaces and inhibit electron-hole recombination through charge carrier trapping [4,5]; as a result, the photocatalytic activity can be remarkably enhanced. Several studies have dealt with the synthesis of ultrafine TiO2 nanoparticles and their application in water purification. However, powder type TiO2 photocatalysts ⁎ Corresponding author. E-mail address: [email protected] (D.D. Dionysiou). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.08.041

have to be separated after water treatment and the process for the separation of ultrafine nanoscale particle is in fact tedious and costly [6]. Therefore, development of film type TiO2 photocatalyst with ultrafine anatase nanocrystallites is more attractive in water treatment applications. The sol gel method has become an important approach for the synthesis of immobilized TiO2 films. In order to prepare TiO2 films with controlled crystal size and texture, it is necessary to search for effective ways to control the precursor reactivity because hydrolysis and condensation of titanium precursor (i.e., titanium alkoxide) is so exothermic and violent that it often leads to uncontrollable network and particle morphology [7–9]. Complexation of titanium alkoxide precursor by acetylacetone has been considered to be an effective way to avoid fast hydrolysis/condensation reaction in sol gel methods [9]. On the other hand, some acidic chelating agents such as organic acids (i.e., acetic acid) and inorganic acids (i.e., HCl) may cause acidic corrosion on sensitive metal substrates (i.e., steel), the advantage of employing acetylacetone to stabilize the titanium

Y. Chen et al. / Surface & Coatings Technology 202 (2008) 1944–1950 Table 1 Compositions of the final sol Films

1-Butanol sol Tween 20 1-Butanol Final Tween 20 volume/mL volume/mL volume/mL precursor sol volume/total volume/mL precursor sol volume

No T20 1.0 T20Bu 2.5 T20Bu 5.0 T20Bu

5.0 5.0 5.0 5.0

0 1.0 2.5 5.0

5.0 4.0 2.5 0

10.0 10.0 10.0 10.0

0 1:10 1:4 1:2

alkoxide is that the pH of the sol can be kept in a nearly neutral value so that it is suitable for coating to any substrate. Therefore, in this study, we employ such a non-acidic modified precursor in sol formation. In addition, we used the nonionic surfactant Tween 20 as pore-directing agent because Tween 20 is not only environmentally friendly but also highly viscous, which is beneficial to increase the film thickness per coating [7]. All films are prepared at high calcination temperature (500 °C), which is beneficial to eliminating organic composition and obtaining sufficient ceramic bond to get good adhesion between the films and substrate. Film crystalline structure, film thickness/film weight, surface morphology, optical properties, and pore structure were systematically investigated by XRD, Raman, HR-TEM, SEM, UV–vis spectroscopy, and isotherm analysis of nitrogen adsorption–desorption. The photocatalytic activity of the as-prepared TiO2 films was investigated using creatinine as a model organic contaminant in water. To the best of our knowledge, this is the first attempt to employ a non-acidic sol gel system modified by nonionic surfactant Tween 20 as template to synthesize high performance mesoporous TiO2 films with ultrafine anatase nanocrystallites after calcination at high temperature (500 °C). 2. Experimental 2.1. Preparation of mesoporous TiO2 films Compared with short alkyl chain alcohol such as ethanol, 1butanol solvent has low sensitivity to its surrounding environment (i.e., humidity). Therefore, employing 1-butanol as solvent is beneficial to preparing TiO2 films with good repeatability [10]. In addition, the critical micelle concentration of nonionic surfactants can be decreased by employing a long alkyl chain alcohol as solvent [10]. Therefore, in this study, we employed a sol gel system with 1-butanol as solvent. Firstly, 1butanol sol was made, which included 0.46 M commercial ultrapure titanium isopropoxide (TTIP, 97%, Aldrich), 0.28 M acetylacetone (99%, Fisher Scientific), 0.92 M H2O, and 1butanol (anhydrous, 99.8%, Aldrich). Subsequently, Tween 20 and 1-butanol solvent were added to the 1-butanol sol at various ratios but kept with the same total volume, so as to maintain the same concentration of TTIP, H2O and acetylacetone. Various ratios of Tween 20 and 1-butanol are summarized in Table 1. During the sol preparation, the alkoxide solution at each preparation step was vigorously stirred at room temperature, so as to keep a homogeneous mixture of the chemical compositions. At

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last, the final sol was kept stirring at least one hour before TiO2 samples were prepared. The dip coating apparatus was the same with that was explained in previous publication [7]. After dip coating (12.3 ± 0.5 cm min− 1 withdrawal velocity), the wet films were placed into a furnace (Paragon Model HT-22-D, Thermcraft Inc., Winston-Salem, North Carolina) for heat treatment. The furnace temperature was incremented at a ramp rate of 15 °C min− 1 until 100 °C; this temperature was held for 15 min. The temperature of the oven was subsequently increased at a ramp rate of 15 °C min− 1 to 500 °C and was held at this value for 15 min. Finally, the films were cooled naturally to room temperature. 2.2. Characterization of TiO2 films The crystal phase composition of the photocatalytic films coated on borosilicate glass was determined by X-ray diffraction (XRD) using a Siemens Kristalloflex D500 diffractometer with Cu Kα radiation. Film morphology and film thickness were investigated by Scanning Electron Microscopy (SEM; Hitachi S4000). Raman spectra were detected using a dispersive Raman scattering instrument (a model T 64000 Jobin Yvon triple monochromator system equipped with an optical multichannel detector, CCD array). Small-angle X-ray diffraction measurements were performed using Rigaku 12 kW rotating anode with Cu source and point focus with 2-d detector. Micromeritics TriStar 3000 Gas Adsorption Analyzer was employed for the analysis of BET surface area and pore structure of these photocatalytic films. Because it was difficult to obtain enough amount of TiO2 particle samples (i.e., 0.1 g) scraped from the photocatalytic films, TiO2 particle samples scraped from “thick” coating crystallized from enough amount of sol at the “same” conditions (one heating cycle) are characterized. Although TiO2 particles obtained using this procedure were in fact not completely the same as those of as-prepared films, characterization of these TiO2 particles were still considered to be valuable for comparing the structural properties of the final photocatalytic films [7,11]. The crystal size and crystal structure of TiO2 particles were determined by a JEM-2010F (JEOL) High Resolution-Transmission Electron Microscope (HR-TEM) with field emission gun at 200 kV. Samples were dispersed in methanol (HPLC grade, Pharmco) using an ultrasonic cleaner (2510R-DH, Bransonic) for 5 min and fixed on a carbon-coated copper grid (LC200-Cu, EMS). The strength of adhesion of the TiO2 films was measured by the crosshatch adhesion test (ASTM D3359B-02 [12]). The cut samples of all TiO2 films were also examined by a microscope (The Buehler VersaMet®3 metallograph, USA). 2.3. Evaluation of photocatalytic activity of the films The photocatalytic activity of the TiO2 films was evaluated using aqueous solution of creatinine (99.5%, Aldrich) with an initial concentration of approximately 19.5 mg/L. The total volume of creatinine solution was 8 mL. The initial pH of the solution was approximately 6.1. The photocatalytic reactor used was a round borosilicate dish with inside diameter of 4.7 cm. The dimensions of the coatings dipped into creatinine solution were 4.2 cm (length) by 2.5 cm (width). The UV source consisted of

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Fig. 1. XRD spectra of TiO2 films with different Tween 20 loading in the sol (eight dip coating layers, 500 °C calcination).

two 15W integrally filtered low-pressure mercury UV tubes (Spectronics Corp., Westbury, New York) emitting radiation with wavelength in the range 300–400 nm and a peak at 365 nm. The distance between the lamps and the films was less than 1 cm. On the film surface, the average intensity of the UV radiation for each tube was approximately 1025 μW/cm2, which was measured using a UV radiation meter (1L 1700 International Light, Serial No. 2547). The reactor was cooled by a fan (Duracraft Corporation, South Borough, Massachusetts) positioned near the reactor. Creatinine samples were analyzed using HPLC (Agilent 1100 series) equipped with a diode array detector (DAD) and an auto sampler. The stationery phase used was a 3.9 mm × 150 mm 5-μm NovaPak C18 column (Waters). The mobile phase composition was Ammonia sulfate (0.045 M) with a flow rate of 0.320 mL/min. This analysis method was originally developed by Dash and Sawhney [13] and later modified by Antoniou et al. [14]. 3. Results and discussion 3.1. Adhesion and crystalline structure of TiO2 films Good mechanical stability of immobilized TiO2 films is an important property for their application in environmental remediation and especially for water treatment. All photo-

Fig. 2. Raman spectra of TiO2 films with different Tween 20 loading in the sol.

catalytic films prepared at different Tween 20 loadings were examined by Tape test before further study was performed. It was found that increasing Tween 20 loading cannot cause an obvious decrease in adhesion between photocatalytic films and borosilicate glass until Tween 20 loading reached above 50% (v/v). It is believed that high calcination temperature (500 °C) is one of the important factors for obtaining such a high optimum Tween 20 loading (50%) for maintaining good adhesion between TiO2 films and borosilicate glass. Consequently, in this study, all photocatalytic films were prepared using Tween 20 at loading less or equal to 50% v/v. Fig. 1 shows X-ray diffraction spectra of the as-prepared TiO2 films. The film samples were prepared with eight dip coating layers, so as to obtain enough amount of crystalline materials for XRD analysis. The results show that all films present an obvious peak at 25.4°, which corresponds to the anatase phase (101). Enhanced peak intensity suggests an increased amount in the nanocrystallite phase immobilized on the support. This was proved by the increase in film weight (refer to Table 2). Considering that Raman spectroscopy is more sensitive in the analysis of the polarized Ti–O bond [15–18], these films were also analyzed by Raman spectroscopy to obtain further insights on the local surface structure of the nanophase TiO2 crystalline domain [16–18]. The results of Raman spectroscopy analysis are shown in Fig. 2. All TiO2 films exhibited well-resolved Raman peak at 147 cm− 1 and four weak

Table 2 Effect of Tween 20 loading in the sol on the BET surface area, pore volume, pose size, porosity, film thickness, film weight and crystal size of TiO2 films Film

T20 volume (mL) in 10 mL final sol a

BET (m2/g)

Pore volume b (cm3/g)

Pore size c (nm)

Porosity d (%)

Film thickness per layer e (nm)

Film weight f (μg/cm2)

Crystal size g (nm)

No T20 1.0 T20Bu 2.5 T20Bu 5.0 T20Bu

0 1.0 2.5 5.0

56.7 79.4 105.2 118.8

0.072041 0.097217 0.135375 0.155373

5.1 4.9 5.1 5.2

21.9% 27.5% 34.6% 37.7%

42 e 85 e 125 e 182 e

12.8 24.0 31.9 44.2

8–14 7–9 5–9 5–8

a b c d e f g

The total volume of the final sol is considered to be the sum of the volume of 1-butanol sol + the volume of 1-butanol solvent + the volume of Tween 20. Single point adsorption total pore volume. Adsorption average pore width (4 V/A by BET). Based on pore volume and 3.9 g/cm3 of anatase density. Film thickness obtained from SEM cross-sectional images. Calculation based on pore volume, film thickness and 3.9 g/cm3 of anatase TiO2 density. Based on TEM, samples scratched from the “thick” coatings (1 calcination cycle).

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Fig. 3. HR-TEM images of TiO2 films with different Tween 20 loading in the sol (inserted image is SAED).

peaks in the high frequency region located at 197, 399, 513 and 639 cm− 1, which is in agreement with the spectrum of pure anatase crystallites [19,20]. In addition, a much higher signal from Raman scattering of TiO2 film (5.0 T20Bu, 50%v/v) is observed, which is believed to be correlated to several factors, such as the larger amount of nanocrystallite phase and decreased transparency for the transmission of laser light. In order to obtain further information on the crystallite size and crystallite morphology of TiO2 films, the films (powder samples with one heating cycling) were analyzed by High Resolution TEM and Selected-area electron diffraction (SAED). The results are shown in Fig. 3(a) to (d) and summarized in Table 2. The average crystal size of TiO2 film without Tween 20 is approximately 11 nm. As the Tween 20 loading increases, the average crystal size gradually decreases. It has been reported that nonionic surfactant can inhibit crystallite growth in an acetic acid associate sol gel system [7,21]. Therefore, we believed that Tween 20 could inhibit crystal growth at some extent regardless of pH of the sol. It was observed that all TiO2 films prepared in the presence of Tween 20 addition were made of uniform and ultrafine nanocrystallites with less than 10 nm average crystal sizes. The TiO2 film (5.0 T20Bu) presented the smallest average crystallite size, which was approximately 7 nm. Inserted SAED results showed that there is no amorphous diffusion ring, which further proves that all TiO2 films have been completely crystallized into the anatase phase. In addition,

the crystallites in all TiO2 films exhibited a nearly round shape. Therefore, the crystallite shape cannot be significantly altered by incorporating Tween 20 into the sol. 3.2. Film morphology and textural structure During the drying process, the volatilization of 1-butanol solvent increases the concentration of Ti(O2C2H9)4 − x(acetyl)x precursor (x = 0 to 4, acetyl: acetylacetone). The inorganic Ti– O–Ti network is subsequently formed via a relatively slow hydrolysis and condensation of this new precursor. The final mesoporous crystalline framework is formed after removal of surfactant micelles and organic content and crystallization of the Ti–O–Ti network during calcination (i.e., 500 °C). The results of SEM analysis showed that all TiO2 films exhibit a smooth, uniform and glass-like surface morphology without any crack formation. As an example, Fig. 4(a) and (b) shows surface and cross-section images of TiO2 film (5.0 T20Bu), respectively. In order to investigate how Tween 20 affected the textural properties of the photocatalytic films, TiO2 particles prepared at the “same” conditions were analyzed by N2 adsorption. The results are shown in Fig. 5(a) and (b) and summarized in Table 2. All samples exhibit type IV (BDDT classification). Increasing Tween 20 loading in the sols can lead to a remarkable enhancement in BET surface area and pore volume/porosity of photocatalytic films. However, increasing Tween 20 loading did

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Fig. 5. (a) Pore size distribution of TiO2 films and (b) hysteresis loop of TiO2 films.

Fig. 4. (a) SEM image of TiO2 film with Tween 20:final sol = 1:2 (v/v) and (b) cross-section SEM image of TiO2 film with Tween 20:final sol = 1:2 (v/v).

not have an obvious effect on the average pore size and did not change the type of pore size distribution and the shape of hysteresis loop of the TiO2 films. All TiO2 films exhibited a narrow mono-modal pore size distribution (PSD) with an average pore size of approximately 5 nm. Such a small mesopore size is in fact associated with the formation of the primary particles (i.e., inter-particle pores) [22]. Fig. 6 shows small-angle XRD patterns of TiO2 films prepared from the sols with different Tween 20 loadings. No peak in small-angle XRD pattern is observed for TiO2 film prepared without Tween 20, which suggested that it has a non-ordered structure. On the other hand, a weak single peak at ∼ 0.6° (2θ) with a d-spacing of ∼ 14 nm is observed for TiO2 film prepared in the presence of Tween 20 in the sol. This result further proved that TiO2 films prepared with Tween 20 in the sol had mesoporous structure [23,24]. Because the appearance of a single peak at low angle XRD suggests a more or less regular diameter channel in the materials [23,24], it is believed that these TiO2 films prepared in the presence of Tween 20 in the sol have partially ordered mesoporous structure. In addition, the shape of small-angle XRD pattern of TiO2 film with Tween 20:final sol = 1:10 (v/v)

looks like that between TiO2 film without Tween 20 and TiO2 film with Tween 20:final sol = 1:4 (v/v) or 1:2 (v/v), which suggested a gradual transformation of pore structure induced by small amount of Tween 20. In the Ti(OC4H9)4–1-butanol– H2O–Acetylacetone–Tween 20 sol gel system, the pH of the sol is around 7. Among many typical forces, such as hydrogen

Fig. 6. Small-angle XRD patterns of TiO2 films. (a) TiO2 film without Tween 20, (b) TiO2 film with Tween 20:final sol = 1:10 (v/v), (c) TiO2 film with Tween 20: final sol = 1:4(v/v),and (d) TiO2 film with Tween 20:final sol = 1:2(v/v).

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bonds, electrostatic forces, van der Waals forces, capillary forces and so on, hydrogen bonding between Tween 20 and the inorganic Ti–O–Ti network is believed to be the main force for the assembly of mesoporous structure. Therefore, assembly of mesoporous TiO2 inorganic network via such a nonionic surfactant self-assembling sol gel process is most probably followed by SoIo assembly pathway [25]. It has been reported that SoIo assembly pathway can lead to the formation of a thicker framework walls, which is beneficial to improving structural stability of porous inorganic materials [25]. Therefore, it is reasonable to conclude that formation of thicker framework walls may be one of the main reasons for maintaining a relatively ordered mesoporous structure at such a high calcination temperature (i.e., 500 °C). 3.3. Film optical properties Optical properties of TiO2 films prepared with different Tween 20 loading were analyzed by UV–Vis spectroscopy. The results are shown in Fig. 7(a). During the UV–Vis spectroscopy measurements, the borosilicate glass was used as a blank. Our experimental results show that increase in Tween 20 loading

Fig. 8. Photocatalytic activity of TiO2 film with different Tween 20 loading in the sol.

increases film thickness and film weight (refer to Table 2). However, it can be noticed, from Fig. 7(a), that the transmittance of the films at the lower wavelength region (i.e., 365 nm) does not always decrease as the film thickness increases. This is due to porosity, another important parameter that affects UV absorbance [26], which is also increased by increasing Tween 20 loading. All TiO2 films exhibit some different features in their transmission spectra. TiO2 films including 5.0 T20Bu and 1.0 T20Bu have the highest transmittance at 340 nm, which is approximately 90% (2 layers). The difference in the features of transmission spectra are most probably due to several reasons such as different UV adsorption, different crystallite size and different thickness/interference effect between the TiO2 films and their borosilicate glass substrates [27]. In addition, the relatively ordered mesoporous structure may also affect UV transmittance of TiO2 films. In our studies, a blue shift induced by decreased crystal size could not be observed. This is most probably due to the existence of organic impurities in the films. Fig. 7(b) shows results of UV transmittance of TiO2 film (5.0 T20Bu). As the number of dip coatings increases, the transmittance decreases at the wavelength range from 300 to 375 nm, which obeys the Beer–Lambert law. 3.4. Photocatalytic activity of TiO2 films

Fig. 7. (a) Transmittance of TiO2 films with different Tween 20 loading, and (b) transmittance of TiO2 film with Tween 20:final sol = 1:2 (v/v) at different number of dip coating layers.

The photocatalytic activity of these TiO2 films was evaluated by employing creatinine, a waste product in urine. Fig. 8 shows the degradation rate of creatinine by the photocatalytic films prepared from sols with different Tween 20 loadings. In our photocatalytic reactor, an obvious decrease in the creatinine concentration cannot be observed, after 150 min of photolysis alone (absence of TiO2 films) or dark adsorption using mesoporous TiO2 film with multiple layers (i.e., 5 T20 Bu, 6 layers). Therefore, the decreased concentration of creatinine after 150 min of reaction was mainly attributed to the photocatalytic degradation of creatinine by the TiO2 films. As Tween 20 loading in the sol increases, the photocatalytic activity of TiO2 films increases. This can be easily explained by both enhanced active surface area/pore volume and increased

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amount of anatase nanocrystalline materials immobilized on the support. The degradation rate of creatinine by TiO2 film (5.0 T20Bu, 1 dip coating layer) is approximately ten times that of TiO2 film without Tween 20 (1 dip coating layer). Therefore, Tween 20 is a very effective template in such a non-acidic sol gel system for the synthesis of high performance mesoporous TiO2 films. It should also be emphasized that increasing film thickness can improve the photocatalytic activity of these TiO2 films, which is contributed to their porous structure. Our research results show that the optimum number of layers is six for TiO2 film (5.0 T20Bu), while the optimum number of layers is only four for TiO2 film without Tween 20. Considering that they have similar pore size, it is believed that a much higher porosity of TiO2 film (5.0 T20Bu) could cause more number of creatinine molecules to pass through the upper layers of the film. As a result, the optimum film thickness of TiO2 film (5.0 T20Bu) can be larger than that of the control TiO2 film prepared without Tween 20. 4. Conclusions A non-acidic sol gel system, containing titanium isoproxide, 1-butanol, acetylacetone, H2O and nonionic Tween 20, can yield partially ordered mesoporous TiO2 films with ultrafine anatase nanocrystallite after calcination at 500 °C. The optimum Tween 20 loading in the sol is 50% (v/v), under which good adhesion between TiO2 film and borosilicate glass can be maintained. The final mesoporous TiO2 film (50%) possesses several promising properties including highly optical clearance, high BET surface area, high porosity, pure anatase nanocrystalline phase with small crystal size, uniform and crack free morphology, mechanical robustness, good adhesion, and high photocatalytic activity. By employing such a non-acidic Tween 20-associated sol gel system, we provide a non-corrosive preparation route for the synthesis of mesoporous TiO2 films with controllable crystal size and textural structure, which can be coated on various supports for environmental and industrial applications. Acknowledgment This work was funded in whole by a grant from the Office of Biological and Physical Research of the National Aeronautics

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