Abstract
Nanocrystalline N-doped SiO2/TiO2 visible-light photocatalyst thin films were synthesized using the sol-gel method on glass substrates. The synthesized catalysts were then characterized using several analytical techniques like x-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and UV–vis absorption spectroscopy (UV–vis). The experimental results revealed that the maximum optical response of the synthesized SiO2/TiO2 thin films shifted from the ultraviolet (UV) to the visible-light region (λ⩾420 nm). The photocatalytic activity of N-doped SiO2–TiO2 photocatalyst was considerably higher than that of SiO2–TiO2, and this result was obtained with an optimal concentration of 40 mol% of N. The enhanced photocatalytic activity was attributed to the increasing surface area and forming more hydroxyl groups in the doped catalyst.
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1. Introduction
Titanium dioxide (TiO 2) has been widely used as a photocatalyst for solar energy conversion and environmental applications because of its low cost, non-toxicity and good photoactivity. When TiO 2 is irradiated by sunlight with a wavelength of less than 387 nm (ultraviolet range), electrons are passed across the band gap into the conduction band, leaving holes in the valence band [1]. These holes have high oxidation power, thus can easily react with adsorbed hydroxide ions to produce hydroxyl radicals, the main oxidizing species which are responsible for the photooxidation of organic compounds [2]. The addition of a second metal oxide like SiO 2, ZrO 2 or Al 2 O 3, etc, was also found to be an effective route to improve the thermal stability and UV photocatalytic activity of TiO 2 [3]. Among them, SiO 2–TiO 2 materials were most widely investigated in the photocatalysis field because they exhibited higher photocatalytic activity than pure TiO 2. This could be explained by the addition of SiO 2 into TiO 2 retarding or inhibiting the crystallization of anatase phase. A contact angle of SiO 2–TiO 2 thin films with 15 mol% SiO 2 concentration is less than 2° and these films can maintain a super-hydrophilic property for a long time in dark conditions, thus exhibiting excellent antifogging capabilities [4]. The effects of texture, surface state and activity of the N-doped SiO 2–TiO 2 were investigated in detail.
The major problem that only about 4% of the solar spectrum falls in the useful ultraviolet (UV) range leads to low efficiency use of the catalyst TiO 2. Therefore, improving efficient use has become an appealing challenge [5]. One of the approaches to achieving this objective is to dope TiO 2 with nonmetal atoms such as S, C, and N. Among these anion-doped TiO 2 photocatalysts, N-doped TiO 2 has been the most extensively studied, and considerable success has been achieved in enhancing the visible-light-driven photocatalytic activity by decreasing the band gap of N-doped TiO 2 [5]. It has been discussed that doped N atoms help to improve the visible-light absorption capability of the TiO 2 catalyst. The synergetic effects of doped N on optical shift, crystallinity, surface areas, and the activity of TiO 2 have also been studied.
In the present study, nanocrystalline N-doped SiO 2–TiO 2 were prepared by the sol-gel method. The synthesized films were tested for their photocatalytic activities using a degrading Methylene Blue (MB), which is known to be difficult to degrade under irradiation and is often used as a model dye contaminant to evaluate the activity of a photocatalyst [6].
2. Experimental procedures
2.1. Preparation of N-SiO2–TiO2 thin films
All chemicals used in the study were procured from Aldrich, Germany and used as received. Titanium tetraisopropoxide (TTIP) and tetraethylorthosilicate (TEOS) were used as precursors for titania and silica, respectively. First, TEOS was hydrolyzed in an aqueous HCl solution, and a TTIP ethanol mixture (1 mol TTIP per 20 mol ethanol) was slowly introduced dropwise. The molar concentration of Si/Ti in the solutions was chosen at an optimal value of 15 mol% [4]. Urea was also added to the solution with various N/SiO 2+TiO 2 molar concentrations from 10 to 50 mol%. The detailed synthesis procedure is shown in figure 1. N/SiO 2–TiO 2 thin films were deposited on glass substrates with dimensions of 26×76 mm 2 by a dip coating process at room temperature. The substrates were immersed in the as-prepared N-SiO 2–TiO 2 solution for 1 min, and withdrawn from the solution at a velocity of 4 mm s −1. After each layer was deposited, the film was annealed at 300 °C on a hotplate for 5 min. The procedure from coating to drying was repeated three times to achieve a film thickness of about 150 nm. Afterward, the substrates were calcined at 500 °C for 2 h. In our work, N/SiO 2–TiO 2 powders were also prepared using the same calcination procedure.
Figure 1 Synthetic process of N/SiO 2–TiO 2 solution.
2.2. Characterization of film structure and morphology
X-ray diffraction (XRD) patterns of the calcined gels were obtained with Cu-Kα radiation (Siemens Kristalloflex diffractometer) to determine the crystal phase composition of the prepared photocatalysts. The average crystal size was estimated by applying the Scherrer equation to the apparent full-width-at-half-maximum intensity (FWHM) of the (101) peak of anatase TiO 2 [7], as follows:
where d denotes the average crystallite size, k=0.9, λ=0.15405 nm is the x-ray wavelength of Cu-Kα, β is the full-width of the peak measured at half-maximum intensity (FWHM) and θ is the Bragg angle of the peak. Synthesized samples were also studied by using UV–vis absorption spectra with Jasco UV–vis V530 double beam spectrophotometer in a wavelength range from 190 to 1100 nm. Transmission Electron Microscopy (TEM) was used to study the particle size. Samples for TEM measurement were suspended in ethanol and ultrasonically dispersed. Drops of the suspensions were placed on a copper grid coated with carbon. These were analyzed using a JEOL 1400 Field Emission Electron Microscope. In addition, the morphology of the as-prepared photocatalyst films was observed using a Scanning Electron Microscope (Jeol JMS-6480LV) and Atomic Force Microscope (Nanotech Electonica SL).
2.3. Photocatalytic activity measurements
The self-cleaning activities of synthesized films were evaluated by analyzing the decrease of MB concentration during exposure to visible-light irradiation. Glass samples (26×26 mm 2) were placed in a container filled with 10 ml of 10 ppm MB. This container was then exposed to the visible light provided by a compact Philips lamp (18 W). The MB solution was then taken out after being exposed for 2 h, and the concentration of MB was determined using UV–Visible spectrometry. The changes of MB concentration were estimated by the absorbance peak for MB at 664 nm. The obtained results showed that the concentration of MB decreased when the exposure time was increased. The efficiency of decomposition of MB can be calculated by applying the following formula [8]:
where R0 is the efficiency of decomposition, A0 is the absorbance of the peak for MB at 664 nm before exposure and At is the absorbance of the peak for MB at 664 nm after t exposing hours.
3. Results and discussions
3.1. X-ray diffraction
The powder XRD diagrams of samples with different N contents calcined at 500 °C are shown in figure 2. All samples showed only the anatase phase with high intensity and other crystal phases (rutile or brookite) weren't detected. Bragg reflections at angles of 25.4°, 38°, 48°, 54° and 55° corresponded to (101), (004), (200), (105) and (211) tetragonal crystal planes of anatase phase TiO 2, respectively. To obtain transparent self-cleaning TiO 2 films on glass substrates, a calcination procedure is carried out at 500 °C. The existence of anatase phase in our synthesized films is a very important result, because it is well known that anatase exhibits the highest photocatalytic activity. From the full-width-at-half-maximum (FWHM) of the strongest peak (101) anatase phase, crystallite sizes were calculated using Scherrer's equation (table 1).
Figure 2 XRD diagrams of N/SiO 2–TiO 2 powders with different N content.
Table 1. Crystallite sizes of N-SiO 2–TiO 2 powders with different N content.
| N0 | 15 mol% SiO 2–TiO 2 | 9 [9] |
| N10 | TiO 2–15 mol% SiO 2–10 mol% N | 9.25 |
| N20 | TiO 2–15 mol% SiO 2–20 mol% N | 10.13 |
| N30 | TiO 2–15 mol% SiO 2–30 mol% N | 10.36 |
| N40 | TiO 2–15 mol% SiO 2–40 mol% N | 15.59 |
| N50 | TiO 2–15 mol% SiO 2–50 mol% N | 13.07 |
The effects of N-doped concentration on the crystallite size of TiO 2 powders are demonstrated in table 1. It is revealed that the size of TiO 2 powders increases from 9.25 to 15.59 nm when the N-doped concentration increases from 10 to 40 mol%, and the size decreases to 13.07 nm at sample N50. It can be said that the larger the amount of N-doping, the better the crystallization that takes place, and the larger the grain size of the TiO 2 powders. But when the N-doped concentration increased more than 40%, the crystalline size decreased. This proves the retarding effect of N on the crystallinity of TiO 2 when the concentration of the dopant is too high.
3.2. Optical properties of as-prepared N/SiO2–TiO2 thin films
The optical absorbance spectra of the N/SiO 2–TiO 2 thin films with different N-doped concentrations measured in the region of 300–800 nm are shown in figure 3. The transmittance within the visible and near infrared region is higher than 70%, which reveals the superior optical properties of N/SiO 2–TiO 2 produced in this work. The spectra show shoulders near 350 nm and bases which approach zero at about 300 nm. The transmittance quickly decreases below 350 nm due to the absorption of light caused by the excitation of electrons from the valence band to the conduction band of TiO 2. The absorption edge shifted towards longer wavelengths (i.e. red shift) with the increase of N-doped concentration from 0 to 40 mol%. A shift towards shorter wavelengths (i.e. blue shift) is observed when the amount of N-doping is more than 40 mol%.
Figure 3 Optical transmission spectra of N/SiO 2–TiO 2 films with different N content.
3.3. Surface morphology observation
As far as the geometry of the surface is concerned, the hydrophilic properties are well known to be enhanced for films with fine roughness. Therefore, controlling the surface microstructure of the films is a solution to improve the hydrophilic property of the synthesized TiO 2 films [6]. Figure 4 shows SEM and TEM images of N40. We observed that the film surface is smooth and homogeneous without cracks. Moreover, the surface consists of little granular crystallization with a size of 15 nm. This result of SEM and TEM observation is similar to that obtained by XRD study. It can be said that synthesizing films without the agglomeration of large clusters results in an increase of specific surface area, and subsequently enhances the desired photocatalytic properties of the films.
Figure 4 SEM image (a) and TEM image (b) of N40 thin film.
The AFM images from figure 5 indicate that the film roughness changed according to the same rule in XRD or UV–Vis: the larger the amount of N-doping, the better the crystallization, the larger the grain size of TiO 2 powders, the smaller the grain boundary, and the smaller the surface roughness. But there is a threshold limit value of N-doped concentration at which the surface roughness (RMS) doesn't decrease, that is threshold N40. However, these RMS values are around 1 nm, so that the rate of the oxidation-reduction reaction substrate by e− and h+ is similar.
Figure 5 AFM image of (a) N20 film, (b) N30 film and (c) N40 film.
3.4. Hydrophilic properties of N40 thin films
Figure 6 illustrates the dependence of photo-induced change on the water contact angle of N40 films, which were treated after 2 h with visible-light irradiation and then kept overnight in a dark environment. The hydrophilic ability of the sample may be explained by the contact angle of water on the surface. The super-hydrophilic property of the surface allows water to spread completely across the surface rather than remaining as droplets. The observed result means that N40-coated glass is a good material for antifogging and self-cleaning purposes. Moreover, figure 6(b) shows a very low contact angle (<2°) of water on the N40-coated sample, while high values of water contact angles resulted when water was deposited on normal glass substrates. In addition, after storing overnight in ambient conditions, the contact angle of water on N40 film-coated glass slowly increased to about 6° (figure 6(c)). This result means that the coated sample could maintain the super-hydrophilic capability for a long time in a dark environment.
Figure 6 The contact angle of water on normal glass substrate (a), on N40 film (b) and on N40 film stored overnight (c).
An 18 W compact lamp is used to irradiate one N40 thin film coating and one normal glass substrate for 2 h. To investigate the antifogging ability of coating films, hot or cold water vapor is used. Figure 7 shows the result of this test. With the N40 film-coated glass sample (a, c), we can read the letter behind clearly. In contrast, the letters behind the glass substrate sample without coating (b, d) cannot be read. This result exhibits the excellent antifogging ability of N40 film produced in this work. These transparent self-cleaning TiO 2 films on glass substrates have great potential for practical applications such as mirrors, window glass and windshields of automobiles.
Figure 7 The antifogging ability of N40 film and normal glass substrate with cold water vapor (I), and hot water vapor (II). (a)–(c) are the results for N40 films, while (b)–(d) are the results for normal glass substrate, respectively.
3.5. Photocatalysis property of N/SiO2–TiO2 film on glass substrate
Photocatalytic performance capabilities of the N/SiO 2–TiO 2 thin films and without pores were evaluated by degrading MB under visible-light irradiation. It should be noted that MB may decompose itself under visible-light irradiation, and physical adsorption can also occur during the process. Figure 8 shows the variation of photo degradation of MB under visible-light irradiation. This result was obtained after taking into account both the physical adsorption and the self-degradation of MB under visible light. The obtained results show that the percentage of degradation increased with increasing visible-light exposure time. Moreover, the pure TiO 2 film showed about 60.44% degradation of MB after 6 h of visible-light exposure. The photocatalytic properties of the films increased to about 71.76, 82.29 and 75.8% with an addition of 30, 40 and 50 mol% of N, respectively. Therefore, it can be said that a significant improvement of photocatalytic performance can be achieved by choosing appropriate compositions for the synthesized films. It is also interesting to note that the addition of N further improved the photocatalytic properties of the TiO 2 films.
Figure 8 Photocatalysis decomposition of MB with N/SiO 2–TiO 2 films.
Finally, we qualitatively analyse the photocatalysis property of N/SiO 2–TiO 2 film on glass substrate by decoloration of MB, and figure 9 shows the photocatalysis decoloration of MB with N40-coated glass substrate. It can be seen that the concentration of MB on the experimental substrate decreased about 50% after a period of about 15 min and almost no MB was detected after a period of 34 min.
Figure 9 Photocatalysis decoloration of MB with N40 film on glass substrate. (a) N40 film/glass; (b) glass substrate.
4. Conclusion
Nanocrystalline N/SiO 2–TiO 2 thin films were prepared by a combination of the sol-gel and dipping coating technique. The N/SiO 2–TiO 2 film with 40 mol% N-doped concentration gives optimal results on crystalline structure, optical property, surface area, and photocatalysis property. The photocatalytic properties of the prepared thin films were evaluated by degrading MB under visible light. It is noted that photocatalytic performance can be improved by adding active N elements. In the present study, the N40-coated films exhibited the highest photocatalytic performance, demonstrated by the fact that the MB was nearly completely decomposed after only 6 h of visible-light exposure.
Acknowledgments
The authors appreciate the financial support of the Department of Science and Technology, Ho Chi Minh City, Vietnam.