2016 3rd International Conference on Smart Materials and Nanotechnology in Engineering (SMNE 2016)

ISBN: 978-1-60595-338-0

Photocatalytic Activity of TiO2 Synthesized by Sol-gel Method with Different Titanium Precursor: Impact of Doping Elements and Doping Ratio

Manni Li, Qianyuan He, Dongxing Zhang, Shaojia Dong Materials Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang, China

*Corresponding author: limanni1120@163.com

ABSTRACT: In this paper, with tetrabutyl titanate and isopropyl titanate as the titanium precursors, TiO2 nanoparticles doped with different amounts of N and V were synthesized using sol-gel method. The nanoparticles were mainly characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Ultraviolet-visible diffuse-reflectance spectroscopy (DRS) and the vis-photocatalytic activity was determined by measuring the rate of methylene blue decolorization. The results show that the grain size of TiO2 prepared by isopropyl titanate is smaller and the rutile transformation temperature is higher than that of tetrabutyl titanate. Furthermore, N-doping or V-doping greatly enhance the visible-light photocatalytic ability of TiO2 nanoparticles and when the mole fraction of Nitrogen is in the range of 15%-20%, the photocatalytic activity of the N-doped samples is the highest, for V-doped samples, the optimal mole fraction is 0.05%. Overall, we have identified that the photocatalytic ability of N-TiO2 is stronger than V-TiO2. Eventually, we draw the conclusion that modified TiO2 visible light catalytic ability is positively correlated with the visible absorption capacity and negatively correlated with grain size.

1 INTRODUCTION

Although many photocatalyst have been researched, anatase phase TiO2 is the most appropriate not only in its high photozatalytic activity and chemical stability, but also in its non toxicity of its degradation products. However, the light absorption edge of pure titania is less than 387.5 nm due to its wide band gap(3.2eV), that is, the photocatalytic activity of pure TiO2 is limited only to the UV region, and hence solar energy cannot be effectively harnessed. Therefore, a visible-light response photocatalyst has become a recent trend in order to take full advantage of visible light. There are many reports to heighten the vis-photocatalytic capacity by chemical doping, either by inorganic metal ions or nonmetals

[1-3]. Among all the studies, V and N

doping seem to be more attractive [4-9]

. To achieve appropriate V and N doping, there are several techniques already in use: Thermal Vapor Condensation

[10], Liquid Phase Deposition

[11],

micro-emulsion method [12]

, ion sputtering [13]

, sol-gel process

[14,15] etc. Among these, the sol-gel

process is the most adoptable method for the preparation of V-doped and N-doped TiO2 nanoparticles because the particle size of samples

and doping levels can be easily commanded relying on the reaction conditions such as the synthesis temperature, solvent choice, hydrolysis rate etc. Although V and N doping titania catalysts have been extensively studied, which one is better on photocatalytic effect between V-doped and N-doped TiO2 and what is the best doping amount are still of great interesting.

In this work, several sets of experiments were carried out to test the visible catalytic activity of all the samples. Details on experimental process and testing result analysis are discussed in later sections. Moreover, tetrabutyl titanate and isopropyl titanate are the most common titanium precursors used in preparing TiO2.In this study, scores of experiments have been done to test and analysis the impact of titanium precursor on the microstructure of the samples.

2 EXPERIMENTAL PROCEDURES

2.1 Sample preparation

One of the titanium precursors was made by mixing 10mL of tetrabutyl titanate

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{[CH3(CH2)3O]4,Ti98%,Tianjin} with 15mL anhydrous ethanol (CH3CH2OH, 99.7%,Tianjin) and 0.5mL glacial acetic acid (CH3COOH,99%, Harbin) which aims at inhibiting the hydrolysis of the tetrabutyl titanate in the beaker. Another titanium precursor was made by 10mL isopropyl titanate {Ti[OCH(CH3)2]4,27.83%(TiO2),Tianjin)} with 15mL isopropanol [(CH3)2CHOH,99.7%,Tianjin)] and 1.5mL glacial acetic acid in the beaker. In order to prepare pure TiO2, in the dropping funnel 1.5mL deionized water was mixed with 10mL anhydrous ethanol and 0.2mL of 1M hydrochloric acid to adjust the PH to below 2.Aiming at preparing N-doped TiO2, a certain amount of ammonium chloride (NH4Cl,Harbin) was added to the same constituent solution used in the preparation of undoped TiO2. By changing the quality of ammonium chloride, four different mole fraction of N-doped TiO2 (5%, 10%, 15%, 20%) have been prepared. The precursor solution of V-doped TiO2 was prepared in the identical ingredient solution used in the preparation of undoped TiO2 by addition of a certain amount of ammonium metavanadate (NH4VO3, 99%,Tianjin). By changing the quality of ammonium metavanadate, four different mole fraction of V-doped TiO2 (0.05%.0.1%, 0.2%, 0.5%) have been prepared. All the doped ions solution was prepared in the dropping funnels. Each of the beakers with titanium precursor was placed in an ace bath and cooled to 0°C. Dropwise (~1drop/4s) addition of the doped ions solution to the beaker (under vigorous stirring) produced a sol solution. The gel was obtained after leaving the sol for 12 hours at room temperature and subsequently dried at 80°C for 12 hours. The dried compound was heated at 500°Cand 600°C respectively for 2 hours.

2.2 Measurements and characterization

The samples were characterized by scanning electron microscope (SEM), X-ray diffraction (XRD), Ultraviolet-visible diffuse-reflectance spectroscopy (DRS), and the photocatalytic activity was determined by measuring the rate of methylene blue decolorization by UV-vis spectrometer and the multipoint photochemical reaction apparatus. The SEM measurements were done with a Quanta 200FEG made by the FEI company of America. The crystalline phase was identified by XRD (Rigaku D/max-2500B2+/PCX, Japan).The DRS (HITACHI, U3410) was employed to study the composition and structure of the materials via the UV-Vis absorption spectra. The UV-vis spectrometer (Shimadzu, UV-2550) is suitable for the determination of substance concentration or transmittance. The multipoint photochemical reaction apparatus (CEL-LAB500) is mainly used to research the photochemistry reaction. The visible light source is from a 500W Xe lamp and the distance between the light source and quartz cuvette is 10cm.

3 RESULTS AND DISCUSSION

The samples were observed by SEM in order to research the nanostructure morphology and agglomeration state of TiO2 particles.Figure1 and Figure2 show SEM micrographs of (a) undoped TiO2 (b) N-doped TiO2 (c) V-doped TiO2.The scale is 10µm in Figure 1 and is 20nm in Figure 2.As is shown in the images, the grain sizes of pure TiO2, N-TiO2 or V-TiO2 are all around 20nm, which shows negligible distinction vary from each other. The particle diameter after reuniting is in the range of 1 to 20µm, where in the particle diameter which is less than 10µm can form more agglomerated particles.

Figure 1. Low magnification SEM micrograph of nanoparticles (a)pure TiO2 (b)N-TiO2 (c)V-TiO2.

Figure 2. High magnification SEM micrograph of nanoparticles. (a)pure TiO2 (b)N-TiO2 (c)V-TiO2

The XRD is applied to studying the effect of calcination temperature, titanium precursors and doping elements on microstructure of TiO2

nanoparticles. As are shown in three XRD images, the main anatase peaks are observed at 25.28°,37.80°,48.05°,53.89°,and62.69°,corresponding to the (110), (004), (105), (220) and (204) planes, respectively. The XRD peaks are seen to get sharper with the increase in annealing temperature that

a b

c

a b

c

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means the crystallinity of samples increase, and the crystalline phase will change to rutile when the temperature rises to a certain value. In Figure 3, for the pure TiO2 particles prepared by tetrabutyl titanate, only anatase form exist after sintering at 500°C,For annealing temperature at 600°C, some rutile whose diffraction angles are 27.48°, 36.06°, 41.12°, 43.68°, 56.11° appear. For the pure TiO2

particles prepared by isopropyl titanate, both the samples sintered respectively at 500°Cand 600°C are only of anatase form. The figure indicates that TiO2 grain grows up and the crystallinity increases as the temperature go up. There is some anatase change into rutile at 600°C when we use tetrabutyl titanate as the titanium precursors, however, there is no transformation when we use isopropyl titanate. It is concluded from theoretical analysis that the TiO2 grain size prepared by the isopropyl titanate is smaller (estimates by the Schrrer formula).The bigger grains change first when the anatase transform into rutile, and the smaller ones change later. It manifests that the rutile transformation temperature is related to grain size, and the transformation temperature decrease with the growth of grain size.

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tive in

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Figure 3. XRD spectra of pure TiO2: (a)tetrabutyl titanate as the titanium precursor, after calcining at 500°C (b)tetrabutyl titanate as the titanium precursors, after calcining at 600°C(c)isopropyl titanate as the titanium precursor, after calcining at 500°C (d)isopropyl titanate as the titanium precursor,after calcining at 600°C

Figure 4 shows that all the N-doped TiO2 particle samples are only of anatase and the crystallinity is higher after calcining at 600°C than that of 500°C.Compared with the samples without doping, the characteristic peaks of N-TiO2 are sharper than undoped TiO2.We can analyze that the N-TiO2 grain is smaller than pure TiO2.It demonstrates that Nitrogen doping can refine the grain size of N-doped TiO2.

Figure 5 shows the XRD micrography of the V-doped TiO2.As we can see from the picture, part of the anatase transform into rutile after calcining at

600°C when we use tetrabutyl titanate as the precursor. Known by the Scherrer formula, doping with Vanadium can refine the grain size, however, the extend of refinement is lower than doping with Nitrogen. Compared with undoped TiO2, the characteristic peaks of V-doped TiO2 are lower and with sleek shape, suggesting that the crystallinity is not high. We can infer that doping with Vanadium result in failure-deformation of the TiO2 grain.

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.)

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Figure 4. XRD spectra of N-TiO2:(a)tetrabutyl titanate as the titanium precursor, after calcining at 500°C (b)tetrabutyl titanate as the titanium precursors, after calcining at 600°C (c)isopropyl titanate as the titanium precursor, after calcining at 500°C (d)isopropyl titanate as the titanium precursor, after calcining at 600°C.

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Figure 5. XRD spectra of V-TiO2 :(a)tetrabutyl titanate as the titanium precursor, after calcination at 500°C (b) tetrabutyl titanate as the titanium precursors, after calcination at 600°C (c)isopropyl titanate as the titanium precursor, after calcination at 500°C (d)isopropyl titanate as the titanium precursor, after calcination at 600°C.

Figure 6 shows DRS data for undoped, N-doped and V-doped TiO2 prepared by different titanium

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precursors. As is shown in the image, undoped TiO2 can only absorb UV light with wavelength below 288nm, while doped TiO2 has some capacity for absorbing visible light. This means that doped TiO2 has a narrower band gap than undoped TiO2.These data reveal that doping in TiO2 nanoparticles transfers the absorption edge to higher wavelength, that is, V irons are incorporated into the TiO2 lattice replacing the position of the titanium atoms in the forms of V

4+ and V

3+ and N irons replaces the

position of oxygen atoms producing the local lattice distortion or make new oxygen vacancy. Oxygen vacancy and Ti

3+ redox center act as an active

position of reaction, the increase of the number of oxygen vacancy induces the red shift of absorption spectra. A conclusion can be obtained that the visible absorption capacity of V-doped TiO2 is the highest. And the samples prepared by isopropyl titanate are a little more capable of absorbing visible light than the tetrabutyl titanate one.

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Figure 6. UV-vis diffuse-reflection spectroscopy of annealed nanoparticles :(a)pure TiO2 prepared by tetrabutyl titanate; (b)pure TiO2 prepared by isopropyl titanate;(c)N-TiO2 prepared

by tetrabutyl titanate;(d)N-TiO2 prepared by isopropyl titanate;(e)V-TiO2 prepared by tetrabutyl titanate;(f)V-TiO2 prepared by isoproply titanate.

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Figure 7. Photocatalytic degradation of methylene blue dye in the presence of pure TiO2.

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Figure 8. Photocatalytic degradation of methlylene blue dye in the presence of N-TiO2 with different content:(a)5%N-TiO2

(b)10%N-TiO2 (c)15%N-TiO2 (d)20%N-TiO2.

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Figure 9. Photocatalytic degradation of methlylene blue dye in the presence of V-TiO2 with different content: (a) 0.05%V-TiO2 (b) 0.1%V-TiO2 (c) 0.2%V-TiO2 (d) 0.5%TiO2.

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Figure 10. Comparison of the photocatalytic decomposition of methylene blue under visible light irradiation, catalyzed by all the doped samples and the control sample.

The vis-photocatalytic activity of the TiO2

samples modified by Nitrogen and Vanadium with different content were evaluated by the de-colorization of methylene blue (MB) solutions. Figure 7 displays the absorption spectra of MB catalyzed by undoped TiO2 (the control group). Figure 8 and Figure 9 show the absorption spectra of MB catalyzed by N-doped TiO2 with different Nitrogen content (5%,10%15%20%) and V-doped TiO2 with different Vanadium content (0.05%,0.1%,0.2%,0.5%), respectively. From these figures, we can notice that the absorption of MB is 664nm. MB in the control group can also be slightly degraded as a result of the existence of thermal degradation and the UV light from fluorescent. The photocatalytic activity of the doped TiO2 samples increase obviously and MB can almost be entirely degraded by 20%N-doped TiO2.

Figure 10 shows the relative concentration of MB solution vs the irradiation time. The vis-photocatalytic activities of N-doped TiO2 and V-doped TiO2 have been widely reported in other

literature. It is more important to confirm the optimal doping amount. As is seen in Figure 10, the order of different doped TiO2 considering the vis-photocatalytic activity were 20%N-doped TiO2>15%N-doped TiO2>10%N-doped TiO2>0.05%V-doped TiO2>0.5%V-dopedTiO2>5%V-doped TiO2>0.1%V-doped TiO2>0.2%V-doped TiO2. The photocatalytic capacity between 15%N-doped TiO2 and 20%N-doped TiO2 is pretty much identical, that is, there is a limit for the catalytic capacity of N-doped TiO2. This conclusion is different from the one Susann Neubert

[16] reached. For the V-doped

TiO2, the maximum doping content can only reach to 5% because of the low solubility of ammonium vanadate and the small amount of water put in the process of experiment. It is noticeable that the photocatalysis ability is not proportional to the doping content, the reason for this result remains to be studied.

The data obtained from Figure 10 is that after catalyzing for 100 minutes, the degradation rate of the control group is 32.5%, and the degradation rate of 20%N-doped reach to 100%. Relying on obtained results, it can be assumed that the MB or other organic matter can be completely degraded by TiO2 photocatalyst.

Compared with Figure 6, the vis-photocatalytic ability of V-doped TiO2 which has the highest visible absorption ability is weaker than N-doped TiO2. It illustrates that the visible absorption ability is not the only factor that influences the vis-photocatalytic ability. The grain size of N-doped TiO2 is bigger than the V-doped TiO2; moreover, the vis-photocatalytic ability of N-doped TiO2 is stronger than V-doped TiO2.This is an implication that the grain size is another significant factor influencing the vis-photocatalytic ability of TiO2 nanoparticles.

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4 CONCLUSIONS

In this work, a multitude of experiments have been done to research the effect of doping elements and doping ratio and the precursor on the visible light photocatalytic activity of TiO2. On the basis of the above analysis, the following conclusions can be drawn:

(1) The grain size and rutile transformation temperature of modified TiO2 after calcining are related to the precursors material. Compared with tetrabutyl titanate, the grain size of TiO2 is smaller and the rutile transformation temperature is higher than prepared by isopropyl titanate.

(2) In this study, there are two factors that influence the grain size of TiO2, one is the precursor material, and the other is the doped element. The grain sizes were reduced by doping Nitrogen element and Vanadium element. The smaller the particle size, the higher the rutile transition temperature, it is conducive to enhancing the vis-photocatalytic ability of TiO2 photocatalyst.

(3) Pure TiO2 can hardly absorb visible light; the TiO2 nanopaticles doped with V and N elements have a certain ability to absorb the visible light.V-TiO2 visible absorption capacity which is around 3% is stronger than the N-TiO2 which is around 2%.

(4) Two crucial factors of the modified TiO2 vis-photocatalytic activity are demonstrated in this study. One is the grain size of the modified TiO2, the other is the visible absorption capacity. Modified TiO2 visible light catalytic ability is positively correlated with the visible absorption capacity and negatively correlated with grain size.

(5) Overall, we have identified that the visible catalytic ability of N-TiO2 is stronger than V-TiO2.For N-TiO2; the ability is the best when doped with 15-20% Nitrogen element. And for V-TiO2, the best doped content of Vanadium is 0.05%.

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