Effect of Er³⁺ dopant on microstructure and photocatalytic property of nano-TiO₂

FAN Cai-mei(樊彩梅), TANG Qi(唐  琦), WANG Yun-fang(王韵芳),

HAO Xiao-gang(郝晓刚), LIANG Zhen-hai(梁镇海), SUN Yan-ping(孙彦平)

College of Chemistry & Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China

Received 15 July 2007; accepted 10 September 2007

Abstract: The nano-TiO₂ doped with Er³⁺ were prepared from Ti(OC₄H₉)₄ by sol-gel method, and the effect of Er³⁺ dopant on microstructure and photocatalytic activity of nano-TiO₂ was studied. The phase composition and crystallite sizes of Er³⁺-doped TiO₂ samples were analyzed by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The photocatalytic activity of Er³⁺-doped TiO₂ was investigated at different doping concentrations and different heat treatment temperatures in the photocatalytic degradation of phenol with 365 nm wavelength ultraviolet light irradiation. The results show that both the anatase phase and rutile phase are formed in doped TiO₂. Er³⁺ doping hinders the crystal transformation and makes the TiO₂ crystallite size change smaller as well as increases the photocatalytic activity of TiO₂ greatly. When Er³⁺ doping concentration is 1.2%(mass fraction) and the heat treatment temperature is 700 ℃, the photocatalytic activity of Er³⁺-doped TiO₂ is favorite in the experimental range. The photocatalytic activity is enhanced by about 18% compared with that of the pure TiO₂ and almost approaches the photocatalytic activity of P₂₅-TiO₂.

Key words: photocatalysis; nano-TiO₂; Er³⁺ doping; phenol

1 Introduction

Photocatalytic technology using TiO₂ as catalyst has potential application in treatment of wastewater containing organics because it can degrade organic substance into CO₂, water and inorganic acid[1]. But the wide band gap of TiO₂(3.2 eV) and the invalid recombination of photo-hole and photo-electron produces a lower reaction rate in the photocatalytic process and inhibits the practice utilization of this technology. Thus, to improve the photocatalytic activity of TiO₂, doping other suitable ions into nano-TiO₂ is an effective way[2-5].

Rare earth element has f electron, easy to form various kinds of compounds, and its oxide also has crystalline structure, good stability, the function of influencing the conversion of crystal-phase state and changing the crystal grain size when it exists in other main oxide as dopant, therefore it has been used in lots of field[6-8]. Many results have revealed that doping rare earth ions into nano-TiO₂ can improve the photocatalytical activity of TiO₂[9-11]. The previous study results about doping rare earth Y³⁺ and Ce³⁺ into TiO₂ demonstrated that suitable doping could make the absorption band of TiO₂ move toward the visible light region and increased the ability of absorbing light as well as improved the photocatalytic activity of TiO₂[12]. This work was designed to prepare Er³⁺/TiO₂ nano-powders by sol-gel method, and characterize their XRD, TEM patterns, then applied this doped TiO₂ in photocatalytic degradation of phenol in water and tried to give a clear relationship between Er³⁺/TiO₂ nano-powders and their photocatalytic activity.

2 Experimental

2.1 Reagents and apparatus

Anhydrous ethanol was purchased from Taiyuan Chemical Factory and Er(NO₃)₃?6H₂O was purchased from Ruiker Rare Earth Metallurgy and Functional Material State Engineering Research Institution. Phenol chemical was in analytic grade and was purchased from Beijing Chemical Plant. Ti(OC₄H₉)₄ was purchased from Tianjin Chemical Reagent Factory. All the other reagents were in analytic grade.

2.2 Preparation of catalyst

Water, Ti(OC₄H₉)₄ and anhydrous ethanol were mixed in terms of mass ratio of 1?3?12. First Ti(Obu)₄ were dissolved in Er(NO₃)₃?6H₂O anhydrous ethanol solution, then nitric acid was used to adjust the pH value of solution to 2 and the distilled water was added into the solution. The solution was vigorously stirred for 1 h in order to form sols. After aging for 5 h, the sols transformed into gels. The gels was dried under 100 ℃ in order to evaporate water and organic material as much as possible. Then the dry gels were ground in a ball mill and calcined at certain temperature in oven. By this procedure the dopant Er³⁺ nano-TiO₂ powders were obtained.

2.3 Characterization of photocatalysts

In order to determine the crystal phase composition of the doped photocatalyst powders, X-ray powder  diffraction(XRD) patterns were recorded at 2θ=10?-80? on a Rigaku D/max2500 with Cu K radiation using the Debye-Scherrer technique. Transmission electron microscopy(TEM) was carried out on JEM-200FXⅡ.

2.4 Photocatalytic activity experiments

The activity experiments of photocatalysts were performed in a batch slurry cylindrical quartz photoreactor with effective reactive volume of 50 mL. The photodegraded solution was phenol solution (20 mL/L) prepared with distilled water and analytic grade phenol. The slurry suspension for photocatalytic reactions was prepared by adding 1 g/L TiO₂ or doping TiO₂ nano-powders into phenol solution through stirring. Before the light is turned on, the solution was stirred for 30 min to ensure a good adsorption equilibrium between the catalyst and the solution. During the photocatalytic reaction, the solution was aerated by bubbling air and irradiated using a 250 W UV light(λ=365 nm) positioned by the reactor. At different irradiation time intervals, the samples of phenol solution were drawn and analyzed to determine the concentration of phenol and TOC(total organic carbon). The concentration of phenol in the solution was analyzed by UV-VIS spectrophotometer (Shanghai Third Analytic Instrument Plant) at 510 nm by 4-amionantipyrin method. TOC concentration in water samples was measured by TOC instrument Multi N/C   3 000 (Germany Yena Company) with results expressed in mg/L.

3.1 XRD patterns and TEM images of Er³⁺doped TiO₂

3.1.1 Phase structure of TiO₂ doped with Er³⁺

Fig.1 shows XRD patterns of doped TiO₂ powder calcined at 700 ℃ with different Er³⁺ contents(mass fraction) of 0, 0.8%, 1.2%, 1.5% (Fig.1(a) shows  anatase phase diffraction apex and Fig.1(b) shows rutile phase diffraction apex). These patterns show that the TiO₂ compound nano-powders are mixture of rutile and anatase phase. Compared with pure TiO₂ nano-powders, the apexes of doped nano-powders have a little movement, which indicates that the doping may leads to the change of crystal lattice parameters.

Fig.1 XRD patterns of Er³⁺/TiO₂ powders

Using the quantitative equation: X_A=1/(1+1.26I_A/I_B), where I_A and I_B are X-ray integrated intensities of 2θ=25.3? reflection of anatase and 2θ=27.4? reflection of rutile, respectively. The content of anatase in calcined TiO₂ at 700 ℃ with different doping levels can be calculated. The results are given in Table 1. Table 1 displays that the mass fractions of anatase and rutile in pure TiO₂ are different from those of doped TiO₂. The doping restrains the conversion of anatase to rutile in some extent.

Table 1 Effect of Er³⁺ contents on anatase contents of TiO₂

Using the Scherrer estimation: D=Kλ/(βcosθ), the average anatase grain size of TiO₂ powders was determined by the broadening of the anatase peak (2θ=25.3?). The results are given in Table 2. In comparison with the pure TiO₂ samples, the doped ones have relatively small particle size, indicating that the doping can improve the particle morphology, and retard the grain growth of TiO₂. The reason may be that some Ti⁴⁺ atoms in crystal lattice are replaced by Er³⁺ during the heat-treatment process, leading to the local lattice distortion, which prevents the movement of crystal interphase and the growth of TiO₂ crystal grain[13-14].

Table 2 Effect of Er³⁺ contents on particle sizes of Er³⁺/TiO₂

3.1.2 TEM micrographs of pure TiO₂ and doped TiO₂ with w(Er³⁺)=1.2%

Fig.2 shows TEM micrographs of pure TiO₂ and doped TiO₂ with w(Er³⁺)=1.2% calcined at 700 ℃. The results show that doping can retard the development of grain size of TiO₂ and decrease the diameter of TiO₂. The mean diameter of Er³⁺/TiO₂ is about 40-70 nm, which is smaller than that of pure TiO₂. This is correlated with the calculation results with Scherrer estimation.

Fig.2 TEM micrographs of TiO₂ nano-powder: (a) Pure TiO₂; (b) Er³⁺/TiO₂

3.2 Photocatalytic activity of doped TiO₂

3.2.1 Effect of doped concentration of Er³⁺ on photocatalytic activity

In order to study the effect of different Er³⁺ doped concentrations on the photocatalytic activity of TiO₂, four phenol degradation experimental processes were conducted under 10.5 mW/cm² light intensity irradiation with different Er³⁺ concentrations of 0, 0.8%, 1.2% and 1.5%, and the results are shown in Fig.3.

The experimental results indicate that the phenol

Fig.3 Effect of Er³⁺ doping concentrations on photocatalytic activities of TiO₂: (a) Remaining rate of phenol; (b) -lnc/c₀ vs time

remaining rates in solution are 15%, 6.3%, 0, 10.6%, respectively, for different Er³⁺ concentration of 0%, 0.8%, 1.2% and 1.5%, which demonstrates that there exists a optimal Er³⁺concentration for the photocatalytical activity of TiO₂ in degradation of phenol, when the content of Er³⁺ is among 0-1.2%. Doping can significantly improve the photocatalytic activity of TiO₂, but when the dopant concentration is more than 1.2%, the photocatalytic activity decreases, which means that more doping can convert the dopant from the trap center to the combination center of electron and hole[15] and makes the photocatalytic ability of TiO₂ decrease. On the other hand the band gap of semiconductor changes wider with the crystal grain decrease in the nano-dimension, then the photo-electron and photo-hole possess stronger oxidation and reduction[16]. In this study, due to the doping of Er³⁺ in TiO₂, the grain size and the anatase phase are changed, which is also favorable for the increase of photoactivity. Compared with pure TiO₂, the doped TiO₂ with 1.2% Er³⁺ makes the degradation rate of phenol enhanced by about 18%.

As shown in Fig.3(b), the relationship between –lnc/c₀ and irradiation time is linear, which means that the degradation of phenol follows pseudo first-order kinetics. Generally speaking, the larger the slope of pseudo first-order linear equation, the stronger the photoactivity of the photocatalyst[17]. The results in Fig.3(b) further demonstrates that 1.2% Er³⁺ dopant is suitable for the photocatalytic application of TiO₂, therefore in the following experiments, this doped TiO₂ was used as photocatalyst.

3.2.2 Effect of heat treatment temperature on photocatalytic activity

Fig.4(a) shows the degradation of phenol in solution using Er³⁺ doped TiO₂ photocatalysts calcinated at different temperatures of 500, 600, 700 and 800 ℃. The photocatalyst calcinated at 700 ℃ expresses a better photoactivity. The phenol in solution is removed completely after 30 min reaction. TOC concentration obtained at contained time reveals analogous trends, as seen in Fig.4(b). These results clearly demonstrate that the degradation rate increases with the increase of heat treatment temperature of Er³⁺/ TiO₂ up to 700 ℃,

Fig.4 Effect of different calcination temperatures on photocatalytic activity of Er³⁺/ TiO₂: (a) Remaining rate of phenol; (b) Remaining rate of TOC

and further increasing heat treatment temperature leads to a obvious decrease in degradation. This feature indicates that the heat treatment temperature has an important influence on photoactivity of TiO₂.

3.2.3 Comparison of photoactivity between Er³⁺/TiO₂ and P₂₅-TiO₂

In this study a degradation test was designed for comparing the photoactivity of pure P₂₅-TiO₂ (Germany Degussa) with nano-Er³⁺/TiO₂ lab-made under the favorite experimental conditions of Er³⁺ concentration of 1.2%, Er³⁺/TiO₂ calcinated temperature of 700 ℃ and the light intensity of 10.5 mW/cm². The results of phenol remaining rate are shown in Fig.5. From Fig.5 it can be seen that the phenol in solution is degraded completely in both reaction processes after 30 min reaction, but the P₂₅-TiO₂ photoactivity is still higher than that of Er³⁺/TiO₂. The degradation rate of the former is 96.1% and the latter is 78.6% after reaction lasts for 10 min. This different photoactivity maybe attributes to the different preparation method.

Fig.5 Comparison of photocatalytic activities of Er³⁺/TiO₂ and P₂₅-TiO₂

1) Nano-TiO₂ doped with Er³⁺ is prepared by gel-sol method. Compared with pure TiO₂, Er³⁺ doping hinders the crystal transformation and makes the TiO₂ crystallite size change smaller as well as increases the photocatalytic activity of TiO₂ greatly.

2) The Er³⁺ doping concentration of 1.2%(mass fraction) is favorite for the photocatalytic degradation, and the grain diameter of doped nano-TiO₂ is the smallest in experimental range. The photocatalytic activity is enhanced by about 18% compared with that of the pure TiO₂.

3) Heat treatment temperature has effect on the crystal state and grain diameter, and has further effect on the photocatalytic activity. When the heat treatment temperature is 700 ℃, the doped nano-TiO₂ expresses a higher photoactivity in the degradation process of phenol.

4) The comparison experiments of photoactivity between Er³⁺/TiO₂ and P₂₅-TiO₂ show that the degradation velocity of P₂₅-TiO₂ are faster than that of the lab-made Er³⁺/TiO₂, but the difference is very little.

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(Edited by CHEN Can-hua)

Foundation item: Project (20476070) supported by the National Natural Science Foundation of China; project (20041020) supported by Natural Science Foundation of Shanxi Province, China

Corresponding author: FAN Cai-mei; Tel: +86-351-6018193; E-mail: Fancm@163.com