Trans. Nonferrous Met. Soc. China 26(2016) 464-471

Preparation of Zr-doped CaTiO₃ with enhanced charge separation efficiency and photocatalytic activity

Xiao-jun HUANG¹, Xin YAN¹, Hai-yan WU¹, Ying FANG¹, Ya-hong MIN¹, Wen-sheng LI¹, Shuang-yin WANG^1,2, Zhen-jun WU¹

1. College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China;

2. State Key Laboratory of Chem/Bio-Sensing and Chemometrics, Hunan University, Changsha 410082, China

Received 25 February 2015; accepted 10 October 2015

Abstract: A series of Zr-doped CaTiO₃ powders were prepared with the mild co-precipitation method and calcined at 850 °C for 3 h. The as-prepared Zr-doped CaTiO₃ samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-Vis diffuse reflectance spectra (DRS) and X-ray photoelectron spectra (XPS). XRD result revealed the presence of single perovskite phase of CaTiO₃. UV-Vis diffusive reflection spectra of Zr-doped CaTiO₃ indicated that the absorbance obviously increased in the visible light irradiation. XPS analysis showed that two types of oxygen existed on the photocatalyst surface, including lattice oxygen and absorbed oxygen. Their photocatalytic activity in the case of the degradation of methyl orange in water and photoelectrochemical activity were also tested. The 5%Zr-doped (mole fraction) CaTiO₃ sample showed the highest photocatalytic activity. The enhanced photocatalytic activity was ascribed to the change of the lattice structure, existence of oxygen vacancies and increase of the photogenerated charge separation efficiency.

Key words: CaTiO₃; Zr; doping; co-precipitation; photocatalysis; photoelectrochemistry

1 Introduction

Due to its potential applications to many environmental pollution problems, photocatalytic degradation of organic pollutants in wastewater over semiconductor has attracted much attention [1,2]. It is well known that, when a semiconductor absorbs a photon with energy equal to or higher than the band gap energy, a free electron will be excited to the empty conduction band, leaving a positive hole in the valence band. The photoexcited electron-hole pairs can be used for the degradation of organic pollutants. The holes can react with water (H₂O) or hydroxyl groups (OH^-) absorbed on the surface to generate hydroxyl radicals (OH·^-), and the electrons may be trapped by the dissolved oxygen species to form superoxide anion radicals (O₂·^-). These active radicals are powerful and indiscriminate oxidants for mineralizing in a wide range of organic pollutant [3]. The photocatalytic efficiency strongly depends on the photoinduced electron-hole recombination rate and solar energy utilization [4]. It is generally considered that the recombination of photogenerated electrons and holes is one of the most detrimental factors on the photocatalytic activity [5,6].

Perovskite-type catalysts (ABO₃) have attracted great interest for the unique structural features, such as variety of structural phase transitions and the variable composition [7-9]. The cations at both A and B sites can be substituted by a foreign one without destroying the matrix structure, which allows controlled alternation of the oxidation state of cations or the creation of oxygen vacancies [10-14].

CaTiO₃ has a large band gap value (~3.5 eV) [15] and thus the material is not useful in the solar spectrum for photocatalytic degradation. Nevertheless, due to its unique properties, CaTiO₃ is considered as a good host material to modify the microstructure and to enhance the photocatalytic efficiency [16-19]. Titanium and zirconium belong to the IV B group and have the similar atom radius (Ti: 2 ; Zr: 2.16 ) and the same valence state (+4). Therefore, it is possible for Zr atom to be incorporated into the CaTiO₃ lattice by substitution for Ti atom. Many researchers have demonstrated that incorporating Zr⁴⁺ into TiO₂ leads to higher photocatalytic activity than that of pure TiO₂ [20-22], because substituted Zr⁴⁺ for Ti⁴⁺ might create some structural defects, which could suppress the recombination of electrons and positive holes by their trapping [23]. What’s more, Zr⁴⁺ exhibits preferential electron trapping ability related to Ti⁴⁺ [24]. In those researches, Zr⁴⁺ was mainly doped in the surface layer of TiO₂, the higher surface acidity and the creation of surface defects were proposed as the possible reason for the enhanced photocatalytic performance.

In this work, we systematically reported Zr substitution at site B in CaTiO₃ and studied its effect on the photocatalytic properties of obtained CaTi_1-xZr_xO₃. The photocatalytic activities of the Zr-doped CaTiO₃ catalysts were evaluated by degrading methyl orange under the irradiation of simulated solar light. The influence of Zr concentration on the photocatalytic performance was investigated.

2 Experimental

2.1 Materials preparation

All reagents used in this work were of analytical grade and used without further purification. Distilled water was used in all experiments. CaTi_1-xZr_xO₃ samples (x=0, 0.02, 0.05, 0.08) were synthesized by the coprecipitation method described as follows. Firstly, 0.5 mL ritonx-100 was dissolved into 200 mL distilled water to form solution A. Secondly, 0.05 mol of tetra-n-butyl titanate, calcium nitrate and zirconium oxychloride were dissolved into 100 mL ethanol in the stoichiometric proportions corresponding to different values of x in the final oxides. This mixture was defined as solution B. Solution C was formed by dissolving excessive amount of ammonium oxalate into 100 mL distilled water. Thirdly, solutions B and C were simultaneously added into solution A drop by drop. This process was carried out at ambient temperature for 4 h to make the metal ions precipitate completely. The final precipitates were filtrated, washed with distilled water and dried. After being calcinated at 850 °C for 3 h, the catalyst powder was obtained.

To investigate the photoelectrochemical performance of the samples, the electrodes were prepared as follows: 5 mg sample and 3 mg polyethylene glycol 6000 (PEG-6000) were suspended in 3 mL ethanol to produce a slurry, and then dip-coated onto an ITO (indium tin oxide) glass substrate (3 cm × 2 cm) with a sheet resistance of 15 Ω. After being dried, the as-prepared electrodes were calcinated at 400 °C for 0.5 h to remove the PEG-6000.

2.2 Characterization of samples

XRD patterns were acquired on an X-ray diffractometer with Cu K_α radiation (XRD, Rigaku D/Max 2500). Diffusion reflection spectra (DRS) collection was carried out on a Cary 100 UV-Vis spectrophotometer with an integrating sphere using BaSO₄ as the reference sample. X-ray photoelectric spectroscopy (XPS) measurements were carried out with a K_α 1063 spectrometer by using a monochromated Al K_α X-ray source. The morphology of the samples was detected by scanning electron microscopy (SEM, Hitachi S4800) and transmission electron microscopy (TEM, FEI Tecnai 20). The photoelectrochemical experiment was carried out on an electrochemical system (CHI-D66H, China).

2.3 Photoelectrochemical measurements

Photoelectrochemical tests were carried out in a conventional three-electrode single-compartment quartz cell filled with 0.5 mol/L Na₂SO₄ electrolyte, using a potentiostat. The ITO/CaTi_1-xZr_xO₃ electrode was served as the working electrode, the counter and reference electrodes were a platinum black and a saturated calomel electrode (SCE), respectively. A 500 W xenon arc lamp with a photon flux of 100 mW/cm² (CHF-XQ-500W, Beijing Changtuo Co., Ltd.) with a bias of 0 V (vs SCE) was used as the excitation light source.

2.4 Photocatalytic experiments

The photocatalytic activities were evaluated by the degradation of methyl orange under a 500 W xenon lamp. An aqueous solution of methyl orange (50 mL, 10 mg/L) was placed in a quartz beaker and 15 mg photocatalyst was added. Prior to irradiation, the suspensions were ultrasonicated for 10 min and magnetically stirred in the dark for 30 min to reach the absorption-desorption equilibrium. In order to distinguish the performance of different catalysts, we shortened the illumination time to make sure that the methyl orange could not be degraded totally. After reaction for 30 min, 0.5 mL of the aliquots were sampled and analyzed by recording variations in the absorption band (465 nm) in the UV-Vis spectra of methyl orange using a Cary 300 spectrophotometer.

3 Results and discussion

Figure 1 shows SEM images of the CaTi_1-xZr_xO₃ samples (0≤x≤0.08) at an annealing temperature of 850 °C for 3 h. The morphology of Zr-doped CaTiO₃ had no changes when compared with pure CaTiO₃. The particle size decreased with increasing the doping content. When Zr doping content is 5% (mole fraction), the CaTi_1-xZr_xO₃ samples (0≤x≤0.08) showed the smallest particle size. The slow rate grain growth for the doped CaTiO₃ might be attributed to the presence of interstitial Zr⁴⁺ ions because the Zr⁴⁺ ions inhibited the grain growth by providing dissimilar boundaries [25].

Fig. 1 SEM images of sintered CaTi_1-xZr_x O₃

TEM images of Zr-doped CaTiO₃ are shown in Fig. 2. The images of all samples revealed the uneven size with a few aggregations. The 5%Zr-doped CaTiO₃ particles presented the smallest particle size. The average particle size of the 5%Zr-doped CaTiO₃ calcined at 850 °C for 3 h was approximately 50 nm. The results were in agreement with the SEM images. Moreover, their particle sizes were larger than their crystallite ones, which were attributed to the reunion of nano-sized crystallite. The elemental composition of the 5%Zr- doped CaTiO₃ sample was determined using energy dispersive spectroscopy (EDS) analysis and the result is shown in Fig. 2(e). Zr doping in CaTiO₃ was clearly identified from the EDS pattern of 5% Zr-doped CaTiO₃. The theoretical mole fraction of the sample was close to the experimental value. Thus, by this co-precipitation reaction it is possible to get desired doping of the metals into CaTiO₃ lattice.

The XRD patterns of CaTi_1-xZr_xO₃ samples (0≤x≤0.08) presented sharp diffraction peaks, and all the diffraction peaks of both as-prepared samples were in a good agreement with the orthorhombic structure of CaTiO₃ (JCPDS file No. 42-0423), as shown in Fig. 3(a). In addition, no impurity peaks were observed in the XRD patterns, which indicated that the as-synthesized samples were of a well-crystallized perovskite phase. It was observed that the peak at 2θ=33.1° for CaTi_1-xZr_xO₃ samples shifted to lower angles with the increase of Zr content, which showed that the presence of large radius of Zr⁴⁺ (0.072 nm) might interstitially substitute in CaTiO₃ lattice rather substitute for relatively small radius Ti⁴⁺ (0.065 nm), and this caused lattice distortion in CaTiO₃ [23, 26]. When the Zr content was 2%, the peak at 2θ=33.1° had a slight shift, indicating that the lattice parameters of CaTiO₃ had a minor change due to a small quantity of Zr doping. When the Zr contents were 5% and 8%, respectively, the peak at 2θ=33.1° had a large shift and the same level of shift, indicating that Zr could be doped into CaTiO₃ up to 5%. Whereas impurity peaks were not observed on the XRD pattern of CaTi_0.92Zr_0.08O₃ sample, the possible reason for this result might be that the XRD is not sensitive enough to detect the minor Zr which is present in other forms. Combined with the XRD patterns of CaTi_1-xZr_xO₃ samples, we calculated the average grain sizes of CaTi_1-xZr_xO₃ samples according to the Scherrer equation. The average grain sizes for 0, 2%, 5% and 8%Zr-doped CaTiO₃ were 33.3, 24.6, 21.5 and 23.4 nm, respectively.

Fig. 2 TEM images (a-d) and EDS pattern (e) of sintered CaTi_1-xZr_xO₃ samples

Fig. 3 XRD patterns of CaTi_1-xZr_xO₃ (a) and refined peaks near 2θ of 33.1° (b)

Fig. 4 UV-Vis diffuse reflectance spectra of CaTi_1-xZr_xO₃

UV-Vis absorption spectral analysis was used to probe the band structure of the as-obtained materials. UV-Vis absorption spectra of CaTiO₃ and Zr-doped CaTiO₃ are shown in Fig. 4. The modification of CaTiO₃ with Zr significantly affected the light absorption property of the photocatalysts. The absorbance of Zr-doped CaTiO₃ shifted toward shorter wavelength with the increase of Zr content. An optical band gap energy was obtained by plotting (Ahυ)² versus hυ, where A is the absorbance and hυ is photon energy. Extrapolation of the linear portion at (Ahυ)²=0 gives the band gap energy. The band gap energies for 0, 2%, 5% and 8%Zr-doped CaTiO₃ were 3.47, 3.49, 3.52 and 3.51 eV, respectively. All of the prepared Zr-doped CaTiO₃ showed higher band gap energy than CaTiO₃. The large band gap was important in preventing the electron-hole recombination and ultimately enhanced the photocatalytic activity [27].

The XPS patterns of Ti, Zr and O elements on the surface of CaTiO₃ with different Zr doping contents are shown in Fig. 5. Figure 5(a) showed Ti 2p state, Fig. 5(b) showed Zr 3d state and Fig. 5(c) showed the O 1s state of the perovskite. In Fig. 5(a), the measured binding energy of Ti 2p_3/2 electrons was within the experimental error, which was consistent with values reported in Ref. [28], showing the presence of Ti⁴⁺. This meant that the binding energy did not change regardless of the doped Zr content. The Zr 3d XPS patterns of the doped samples are presented in Fig. 5(b). The core level patterns of Zr 3d for the Zr-doped samples showed an increasing intensity along with a slight shift toward lower binding energy with increasing Zr doping up to 5%. This indicated that Zr doping concentration reaches a maximum at 5% Zr doping content. The binding energy of Zr 3d_5/2 electrons is lower than that of Zr⁴⁺ ions in ZrO₂, 183.5 eV [29]. The observed shift towards lower binding energies can be attributed either to the presence of Zr³⁺ ions or the presence of oxygen vacancies around Zr⁴⁺ ions. This defect structure may be responsible for higher photocatalytic activity of Zr-doped CaTiO₃ in comparison with the undoped CaTiO₃ [23]. The photoelectron spectra of O 1s electrons showed the presence of the two components corresponding to the two different chemical states of oxygen, lattice oxygen and adsorbed oxygen with increasing binding energy [29-31]. It could be found that the amount of crystal lattice oxygen decreased with increasing Zr doping content, while the amount of oxygen vacancies increased. The oxygen vacancies are a very active group, which are easily combined with other atoms or groups to become stable. This is responsible for the existence of a certain amount of adsorbed oxygen.

Fig. 5 XPS patterns of Ti (a), Zr (b) and O (c) elements on surface of CaTi_1-xZr_xO₃

The photogenerated charge separation efficiency could be investigated by the photocurrent spectra under irradiation of simulated solar light. Figure 6 shows the influence of Zr doping content on the photo- electrochemical activity of CaTiO₃ samples. It was shown that the photoelectrochemical activity of all of the Zr-doped CaTiO₃ samples was higher than that of the undoped CaTiO₃ sample. When the Zr doping content was 5%, the sample showed the highest photo- electrochemical activity, which was 13 times higher than that of the undoped CaTiO₃ sample. This clearly indicated that the Zr dopant improved the separation efficiency of photoinduced electrons and holes, and 5%Zr-doped CaTiO₃ owned the highest separation efficiency of photoinduced electrons and holes. The introduction of Zr⁴⁺ into the CaTiO₃ lattice could stabilize the current signal. We could suppose that surface oxygen vacancies and defects states by Zr doping could be favorable to capturing the photoinduced electrons during the process of photocatalytic reactions. When the Zr content was 8%, excess amounts of zirconium oxide covering the surface of CaTiO₃ would increase the number of recombination centers and resulted in low photocatalytic activity.

Fig. 6 Photoresponses of CaTi_1-xZr_xO₃

The photocatalytic activities of different Zr⁴⁺ doping contents samples calcined at 850 °C for 3 h were evaluated with the degradation rate of methyl orange. Figure 7 shows the degradation rates of various samples after 30 min irradiation. The photocatalytic activity increased with increasing the doping content. When Zr doping content was 5%, the sample showed the highest photocatalytic activity, 91.44% of methyl orange could be degraded after 30 min irradiation. A further increase in doping content reduced the photocatalytic activity. This change is similar to the photogenerated charge separation efficiency change with the increase in Zr content. At the top-right corner of Fig. 7, the kinetics curves of Zr-doped CaTiO₃ are shown. As we can see, the photocatalytic reaction processes of all samples were in line with the first order kinetics reaction. The rate constants increased with increasing doping content. When Zr doping content was 5%, the sample showed the largest rate constant. Combining the UV-Vis diffuse reflectance spectra of different Zr-doped CaTiO₃ samples, we speculate that the photogenerated charge separation efficiency is the primary cause that leads to different photocatalytic activities of different Zr doping contents.

Fig. 7 Comparison of photocatalytic activity of CaTiO₃with different Zr doping contents

4 Conclusions

1) CaTi_1-xZr_x O₃ (x=0, 0.02, 0.05, 0.08) perovskites were successfully synthesized by using a mild co-precipitation method.

2) A similar microstructure with fine grain size was observed in all perovskites. The XRD results indicated that Zr⁴⁺ addition induced a lattice symmetry change. The XPS data confirmed that two types of oxygen existed on the photocatalyst surface, including lattice oxygen and adsorbed oxygen, and more adsorbed oxygen was on the surface of CaTi_0.95Zr_0.05O₃, which resulted in the formation of more oxygen vacancies.

3) CaTi_0.95Zr_0.05O₃ showed the highest photo- electrochemical activity. The improved photocatalytic activity was ascribed to the change of the lattice structure, existence of oxygen vacancies and increase of photogenerated charge separation efficiency.

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具有高电荷分离效率和光催化活性的锆掺杂钛酸钙的制备

黄晓俊¹，严 欣¹，吴海燕¹，方 莹¹，闵亚红¹，李文生¹，王双印^1,2，吴振军¹

1. 湖南大学 化学化工学院，长沙 410082；

2. 湖南大学 化学生物传感与计量学国家重点实验室，长沙 410082

摘  要：通过温和共沉淀方法合成一系列锆掺杂的钛酸钙，并在850 °C下焙烧3 h。采用扫描电镜(SEM)、X射线衍射(XRD)、紫外-可见漫反射光谱(DRS)和X射线光电子能谱(XPS)等手段对合成的锆掺杂的钛酸钙进行表征。XRD结果表明，锆掺杂钛酸钙以单一的钙钛矿相存在；紫外-可见漫反射光谱分析表明，锆掺杂钛酸钙在可见光区的吸收明显增强；XPS分析显示，在光催化剂表面存在2种类型的氧，即晶格氧和吸附氧。此外，对其光催化降解甲基橙的性能以及光电化学性能进行测试。结果表明，5% Zr(摩尔分数)掺杂钛酸钙具有最高光催化活性。晶格结构的改变、氧空穴的存在、光生电荷分离效率的提高都有利于提高锆掺杂的钛酸钙的光催化活性。

关键词：钛酸钙；锆；掺杂；共沉淀；光催化；光电化学

(Edited by Wei-ping CHEN)

Foundation item: Projects (50702020, 51402100, 81171461) supported by the National Natural Science Foundation of China; Project (11JJ4013) supported by the Natural Science Foundation of Hunan Province, China; Project (2013GK3155) supported by Science & Technology Project of Hunan Province, China; Project supported by the Youth 1000 Talent Program of China; Project supported by the Interdisciplinary Research Program of Hunan University, China; Project supported by the Young Teacher Promotion Fund by Hunan University, China

Corresponding author: Zhen-jun WU; Tel/Fax: +86-731-85956121; E-mail: wooawt@163.com

DOI: 10.1016/S1003-6326(16)64097-9