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Synthesis and characterization of mesoporous ZnTiO3 rods via a polyvinylpyrrolidone assisted sol–gel method
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 5094–5099 www.elsevier.com/locate/ceramint
Synthesis and characterization of mesoporous ZnTiO3 rods via a polyvinylpyrrolidone assisted sol–gel method Yue Chia,n, Qing Yuanb, Shushan Houa, Zhankui Zhaoa,n a
College of Material Science and Engineering, Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, China b Institute of Nano-Photonics, School of Physics and Materials Engineering, Dalian Nationalities University, Dalian 116600, China Received 28 October 2015; received in revised form 2 December 2015; accepted 3 December 2015 Available online 12 December 2015
Abstract Mesoporous ZnTiO3 rods were fabricated via a polyvinylpyrrolidone assisted sol–gel method. In this method, the control of nanostructure growth was achieved by the cooperative assembly among precursors and polyvinylpyrrolidone, through which well-designed one-dimensional morphology and mesoporosity could be obtained. The regularity of rod-like morphologies was sensitive to cooperative assembly temperature. Furthermore, the mesoporous ZnTiO3 rods were used for photodegradation of organic dyes and proved to be useful photocatalysts with excellent reusability thanks to the well-designed nanostructure and one-dimensional structure. Hence mesoporous ZnTiO3 rods with good photocatalytic activity and low cost could offer broad opportunities for environmental remediation. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Mesoporous ZnTiO3 rods; Sol–gel method; Photocatalysis; Recyclability.
1. Introduction Semiconductors have been fervently researched over the past few decades due to their potential applications such as hydrogen production through photocatalytic water splitting, dye-sensitized solar cells, and photocatalytic remediation of harmful organics from air and water [1,2]. Among numerous investigations, semiconductors such as TiO2, ZnO have received extensive attention owing to the wide application as photoelectrochemical devices, photocatalysts, gas sensors, and the high activity involved in fabricating delicate composites [3,4]. Recently, ZnO–TiO2 composites have show promise in area such as paint pigments, gas sensor and catalytic sorbents. It has been reported that three compounds are known to exist in ZnO–TiO2 system, including Zn2TiO4 (cubic, tetragonal), ZnTiO3 (cubic, hexagonal), and Zn2Ti3O8 (cubic) [4–9]. n
Corresponding authors. Tel./fax: þ 86 431 85716644. E-mail addresses: [email protected] (Y. Chi), [email protected] (Z. Zhao). http://dx.doi.org/10.1016/j.ceramint.2015.12.024 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Among them, ZnTiO3 with high reduction potential and low oxidation potential has attracted particular attention due to the outstanding potential in photocatalysis [10,11]. Similar to other photocatalysts, rapid recombination of photogenerated electrons and holes is unfavorable to photocatalytic efficiency of ZnTiO3. As we know, large surface-to-volume ratio of materials can significantly increase the surface reaction sites and might even modulate the catalytic activity. Therefore, a useful strategy to improve decomposition efficiency of ZnTiO3 may rely on the development of well-engineered morphologies, nanostructures, geometry, and integration. One-dimensional (1D) structures, such as nanorods, nanowires and nanobelts, have attracted tremendous attention within the last decade. Similar to nanoparticles and nanosheets or thin films, 1D materials exhibit large surface area and quantum confinement effects. Beside, geometrical characteristics offer 1D materials unique properties over the other two categories, such as 1D confined transport of electrons or photons and excellent mechanical properties [12]. Therefore, the interest towards exploiting diverse strategies for controllable construction 1D materials has
Y. Chi et al. / Ceramics International 42 (2016) 5094–5099
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Fig. 1. (A) SEM image of mesoporous ZnTiO3 rods (S1), (B) schematic illustration of formation process for mesoporous ZnTiO3 rods, (C) EDX spectrum of S1, and (D) XRD pattern of S1.
continuously increased. During the past decades, many strategies including vapor–liquid–solid processes, vapor–solid processes, electrochemical deposition and solution growth have been developed to fabricate 1D materials [13,14]. Especially, because of excellent compositional control and homogeneity at molecular level, the sol–gel method has been successfully applied in the synthesis of 1D materials with well-designed nanostructures [15– 17]. To get enhanced performances, a further step beyond preparation of 1D materials is the construction of mesoporosity into 1D architecture, due to the high surface area, large pore size and pore volume of mesoporous materials [18–20]. In addition, the candidates with 1D structure have been proved beneficial to separate nanostructural photocatalysts from solution [21,22]. Based on above analysis, 1D mesoporous ZnTiO3 may be a good candidate for enhanced photocatalytic activity. However, a crucial challenge to this kind of materials is the ability to control the nanostructures growth by a facile synthesis method that is critical to the control of dimensions, mesoporosity and function. In this paper, we describe the synthesis of mesoporous ZnTiO3 rods via a polyvinylpyrrolidone (PVP) assisted sol–gel process, aiming to obtain an efficient, recoverable, stable, and cost-effective photocatalyst. The photocatalytic activity was investigated by measuring the decomposition of dye RhB as a test substance. Well-designed 1D morphology and mesoporosity could lead to high light-harvesting efficiency, facilitating
the high photocatalytic activity. Moreover, ZnTiO3 rods could be easily recycled without a significant decrease of photocatalytic activity due to large length to diameter ratio of 1D structure and well-designed nanostructure. 2. Experimental section 2.1. Materials and reagents PVP (K30, analytical grade) was purchased from Sinopharm Chemical Reagent Co. Ltd. Other chemicals were purchased from Beijing Chemical Corp. All chemicals were used as received without any further purification. 2.2. Materials synthesis In a typical synthesis, 0.32 g Zn(NO3)2 6H2O and 0.37 g Ti (OC4H9)4 were dissolved in 20 mL glycol under magnetic stirring, respectively. The two solutions were mixed together, and then slowly added into a mixture of glycol (60 mL), PVP (1.0 g) and concentrated ammonia aqueous solution (28 wt%, 300 μL) by stirring at 50 1C for 20 h. The product was collected by centrifugation, washed with ethanol for several times, and dried at 60 1C for 10 h. Finally, to remove PVP and obtain meosoporous ZnTiO3 rods, the as-prepared powder was
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Fig. 2. (A, B) SEM images of S2 and S3.
calcined at 600 1C for 3 h with a heating rate of 2 1C min 1. For simplicity, meosoporous ZnTiO3 rods were denoted as S1. The same procedures were carried out for the preparation of samples S2 and S3, except that the reaction temperature was 40 1C and 75 1C for S2 and S3 respectively.
Fig. 3. (A) FTIR spectra of (a) S1, (b) S2, and (c) S3. (B) XRD patterns of (b) S2 and (c) S3.
The degradation rate (D%) can be expressed as follows: C0 C D% ¼ C0
2.3. Photocatalytic test 2.4. Characterization Photocatalytic activity testing on the degradation of RhB was carried out in a 200 mL beaker containing 100 mL of 15 mg/L RhB solution and 15 mg of solid catalyst under 300 rpm stirring at room temperature. The photoreactor was designed with an internal light source surrounded by a quartz jacket (50 W high pressure mercury lamp with main emission wavelength 313 nm). Before the photocatalytic reaction was initiated, the solution was stirred in dark for 30 min to obtain a good dispersion and reach an adsorption–desorption equilibrium between the organic molecules and the catalyst surface. Subsequently, all the photocatalytic performances were carried out at the same place with the same UV intensity to make the data reasonably comparable. At given intervals of illumination, the samples (3 mL) of the reaction solution were taken out and centrifuged for investigation. For the evaluation of photocatalytic activities, C is the concentration of the RhB molecule at a real time t, and C0 stands for the initial concentration of RhB.
Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) were measured on JSM-5600. Transmission electron microscopy (TEM) was performed on JEM 3010. A few droplets of a suspension of the sample in ethanol were put on a microgrid carbon polymer supported copper grid and allowed to dry at room temperature for TEM observations. Crystal identification was carried out using a Bruker D8 Advance X-ray diffractometer with a Cu Kα X-ray source operating at 40 kV and 100 mA. Fourier transform infrared (FTIR) spectra were recorded with a PerkinElmer spectrometer using KBr pellets, and the thickness of the pellet being about 1.3 mm. The nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010m instrument (Micromeritics Instrument Corp., Norcross, GA). The Brunauer–Emmett–Teller (BET) specific surface area was calculated using adsorption data. UV–visible absorption
Y. Chi et al. / Ceramics International 42 (2016) 5094–5099
Fig. 4. (A) TEM image of mesoporous ZnTiO3 rods (S1). (B) Nitrogen adsorption–desorption isotherm of S1.
spectra were recorded using a UV–visible spectrophotometer (UV-2550). The samples were placed in a 1 cm 1 cm 3 cm quartz cuvette, and the spectra were recorded at room temperature. 3. Results and discussion Mesoporous ZnTiO3 rods can be co-assembled by Zn (NO3)2 6H2O, Ti(OC4H9)4, and PVP through a one-step sol–gel method, followed by calcination at 600 1C for 3 h to eliminate any organic species. SEM image of calcined mesoporous ZnTiO3 (S1) synthesized at 50 1C shows 100% rod-like morphologies and smooth surface, while other morphologies can be hardly observed (Fig. 1A). The high yield 1D morphology may mainly be attributed to restriction of PVP, which can prevent the growth of ZnTiO3 rods in the radial direction (Fig. 1B). The length of these randomly oriented ZnTiO3 rods ranges from 3.1 to 8.9 μm, and the
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diameters ranges from 0.35 to 1.19 μm. The large dimension is beneficial to the separation and recovery of ZnTiO3 rods after photocatalytic degradation [18,19]. EDX spectrum was measured to determine the chemical composition. As shown in Fig. 1C, the existence of Zn, Ti, O, Au, Si is detected. Au and Si peak comes from the conductive coating and silicon wafer when operating SEM. It is worth pointing out that the atomic ratio of Zn to Ti is close to 1:1, indicating the well defined composition of ZnTiO3 rods. The XRD pattern of ZnTiO3 (S1) can be readily indexed to a cubic crystal phase of ZnTiO3 according to JCPDS Card no. 39-0190 (Fig. 1D). No peak corresponding to the TiO2, ZnO or zinc titanates of other stoichiometry is observed. The broadening of diffraction peaks indicates ZnTiO3 rod is composed of nano-sized particles with a mean size of about 20.5 nm calculated by Scherrer equation. In the control experiments, the same procedures were carried out for the preparation of samples S2 and S3 except for the reaction temperature. As shown in Fig. 2, S2 and S3 (synthesized at 40 1C and 75 1C respectively) keep the rodlike morphologies. However, the regularity of rod-like morphologies deteriorates. During a cooperative assembly process, PVP can alleviate the growth tendency of ZnTiO3 rods in radial direction, which is sensitive to synthesis temperature. When reaction temperature is 50 1C, the cooperative assembly among precursors and PVP is more effective to obtain rod-like morphology. Fig. 3A shows the FTIR spectrum of the synthesized mesoporous ZnTiO3 rods. The bands above 400 cm 1 originate from Ti–O stretching vibrations [23]. It is also seen that these main characteristic peaks of S1, S2 and S3 are nearly unchanged. This implies that the bond strengths of Zn–Ti–O are not affected by synthesis temperature. In addition, the XRD patterns of S2 and S3 exhibit the same feature as S1 (Fig. 3B), indicating that the crystal phase is not influenced by synthesis temperature. In the TEM image (Fig. 4A), mesopores with wormholeslike shapes were observed throughout ZnTiO3 rods (S1). The mesoporosity is derived from accumulation of nanoparticles, which is expected to contribute to the enhanced catalytic performances. The wormhole-like mesopores are also confirmed by N2 adsorption–desorption measurements. The nitrogen adsorption–desorption isotherm of ZnTiO3 rods (S1) exhibits a characteristic type IV isotherm (Fig. 4B), indicating the presence of mesoporous structure. The BET specific surface area and the average pore size are calculated to be 32.8 cm2/g and 3.3 nm, respectively. Similar to S1, ZnTiO3 rods S2 and S3 also exhibit mesoporous structure, and the BET specific surface area is 27.9 and 23.1 cm2/g, respectively. The regular morphologies of S1 lead to the higher surface area, which is paramount to achieve desired performance of photocatalyst. To evaluate the photoactivity of mesoporous ZnTiO3 rods (S1) for degradation of organic pollutants, we carried out the experiments of photocatalytic decomposition of RhB as a model reaction. For comparison, S2 and S3 were used as photocatalytic references. The degradation efficiency of the asprepared samples was defined as C/C0, where C and C0 stood for the remnants and initial concentration of RhB, respectively.
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Fig. 5. (A) Self-degradation of RhB with UV irradiation but in absence of photocatalysts and degradation profiles of RhB over photocatalysts in the dark, (B) degradation profiles of RhB over different photocatalysts, (C) kinetic linear simulation curves for the degradation of RhB with different photocatalysts, and (D) photocatalytic activity of S1 for the degradation of RhB during three cycles of use.
Before UV illumination, the time from 30 to 0 min in Fig. 5 showed the adsorption–desorption of RhB over photocatalysts in the dark. In the control experiments, no appreciable decomposition of RhB could be observed in absence of photocatalysts (Fig. 5A). Also, there was no appreciable degradation of RhB by photocatalysts in the absence of UV light irradiation, indicating an adsorption–desorption equilibrium of RhB on the surface of ZnTiO3 rods was established in the dark within 30 min. After UV illumination for 70 min, the degradation rates of RhB were 97%, 95%, and 92% for S1, S2, and S3, respectively (Fig. 5B). The photocatalytic degradation of RhB follows pseudofirst-order kinetics, and the apparent degradation rate constant k could be defined by ln(C/C0)¼ kt. Linear relationships between ln(C/C0) and reaction time were obtained for all the photocatalysts (Fig. 5C), and the corresponding value of k was calculated from the slope. The relative photo-catalytic activity of photocatalysts for RhB degradation follows the order: S1 4 S2 4S3. The higher surface area of S1 increases the surface reaction sites and might modulate the catalytic activity of the surface atoms. Furthermore, we have also monitored the cyclic stability of S1 by monitoring the catalytic activity during three cycles of use.
As shown in Fig. 5D, each experiment was carried out under identical conditions, and the photocatalytic activity of S1 exhibited no obvious decrease. The mesoporous ZnTiO3 rods could be easily recovered by sedimentation and maintain their photocatalytic activity after three cycles of use, which is significant for the potential use for elimination of organic pollutants at an industrial scale. The good activity of photocatalyst mainly originates from the mesoporosity for improved mass transfer and the 1D architecture for easy recovery. 4. Conclusions In summary, herein we describe an effective route to synthesize mesoporous ZnTiO3 rods. The mesoporous 1D structure could be constructed by co-assembly of precursors with the assistance of PVP. The mesoporous ZnTiO3 rods exhibit a good photocatalytic activity and can be easily recycled by sedimentation without significant decrease in photocatalytic activity. Combining the advantage of mesoporous 1D structure, good photocatalytic activity with good reusability, and low cost, the mesoporous ZnTiO3 rods will provide possibilities for environmental remediation. And this
Y. Chi et al. / Ceramics International 42 (2016) 5094–5099
synthesize route can be extended to synthesize other multimetallic oxides with mesoporous 1D structures. [11]
Acknowledgments This work was supported by the Natural Science Foundation of Jilin Province of China (20150520020JH). References
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 5094–5099 www.elsevier.com/locate/ceramint
Synthesis and characterization of mesoporous ZnTiO3 rods via a polyvinylpyrrolidone assisted sol–gel method Yue Chia,n, Qing Yuanb, Shushan Houa, Zhankui Zhaoa,n a
College of Material Science and Engineering, Key Laboratory of Advanced Structural Materials, Ministry of Education, Changchun University of Technology, Changchun 130012, China b Institute of Nano-Photonics, School of Physics and Materials Engineering, Dalian Nationalities University, Dalian 116600, China Received 28 October 2015; received in revised form 2 December 2015; accepted 3 December 2015 Available online 12 December 2015
Abstract Mesoporous ZnTiO3 rods were fabricated via a polyvinylpyrrolidone assisted sol–gel method. In this method, the control of nanostructure growth was achieved by the cooperative assembly among precursors and polyvinylpyrrolidone, through which well-designed one-dimensional morphology and mesoporosity could be obtained. The regularity of rod-like morphologies was sensitive to cooperative assembly temperature. Furthermore, the mesoporous ZnTiO3 rods were used for photodegradation of organic dyes and proved to be useful photocatalysts with excellent reusability thanks to the well-designed nanostructure and one-dimensional structure. Hence mesoporous ZnTiO3 rods with good photocatalytic activity and low cost could offer broad opportunities for environmental remediation. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Mesoporous ZnTiO3 rods; Sol–gel method; Photocatalysis; Recyclability.
1. Introduction Semiconductors have been fervently researched over the past few decades due to their potential applications such as hydrogen production through photocatalytic water splitting, dye-sensitized solar cells, and photocatalytic remediation of harmful organics from air and water [1,2]. Among numerous investigations, semiconductors such as TiO2, ZnO have received extensive attention owing to the wide application as photoelectrochemical devices, photocatalysts, gas sensors, and the high activity involved in fabricating delicate composites [3,4]. Recently, ZnO–TiO2 composites have show promise in area such as paint pigments, gas sensor and catalytic sorbents. It has been reported that three compounds are known to exist in ZnO–TiO2 system, including Zn2TiO4 (cubic, tetragonal), ZnTiO3 (cubic, hexagonal), and Zn2Ti3O8 (cubic) [4–9]. n
Corresponding authors. Tel./fax: þ 86 431 85716644. E-mail addresses: [email protected] (Y. Chi), [email protected] (Z. Zhao). http://dx.doi.org/10.1016/j.ceramint.2015.12.024 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Among them, ZnTiO3 with high reduction potential and low oxidation potential has attracted particular attention due to the outstanding potential in photocatalysis [10,11]. Similar to other photocatalysts, rapid recombination of photogenerated electrons and holes is unfavorable to photocatalytic efficiency of ZnTiO3. As we know, large surface-to-volume ratio of materials can significantly increase the surface reaction sites and might even modulate the catalytic activity. Therefore, a useful strategy to improve decomposition efficiency of ZnTiO3 may rely on the development of well-engineered morphologies, nanostructures, geometry, and integration. One-dimensional (1D) structures, such as nanorods, nanowires and nanobelts, have attracted tremendous attention within the last decade. Similar to nanoparticles and nanosheets or thin films, 1D materials exhibit large surface area and quantum confinement effects. Beside, geometrical characteristics offer 1D materials unique properties over the other two categories, such as 1D confined transport of electrons or photons and excellent mechanical properties [12]. Therefore, the interest towards exploiting diverse strategies for controllable construction 1D materials has
Y. Chi et al. / Ceramics International 42 (2016) 5094–5099
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Fig. 1. (A) SEM image of mesoporous ZnTiO3 rods (S1), (B) schematic illustration of formation process for mesoporous ZnTiO3 rods, (C) EDX spectrum of S1, and (D) XRD pattern of S1.
continuously increased. During the past decades, many strategies including vapor–liquid–solid processes, vapor–solid processes, electrochemical deposition and solution growth have been developed to fabricate 1D materials [13,14]. Especially, because of excellent compositional control and homogeneity at molecular level, the sol–gel method has been successfully applied in the synthesis of 1D materials with well-designed nanostructures [15– 17]. To get enhanced performances, a further step beyond preparation of 1D materials is the construction of mesoporosity into 1D architecture, due to the high surface area, large pore size and pore volume of mesoporous materials [18–20]. In addition, the candidates with 1D structure have been proved beneficial to separate nanostructural photocatalysts from solution [21,22]. Based on above analysis, 1D mesoporous ZnTiO3 may be a good candidate for enhanced photocatalytic activity. However, a crucial challenge to this kind of materials is the ability to control the nanostructures growth by a facile synthesis method that is critical to the control of dimensions, mesoporosity and function. In this paper, we describe the synthesis of mesoporous ZnTiO3 rods via a polyvinylpyrrolidone (PVP) assisted sol–gel process, aiming to obtain an efficient, recoverable, stable, and cost-effective photocatalyst. The photocatalytic activity was investigated by measuring the decomposition of dye RhB as a test substance. Well-designed 1D morphology and mesoporosity could lead to high light-harvesting efficiency, facilitating
the high photocatalytic activity. Moreover, ZnTiO3 rods could be easily recycled without a significant decrease of photocatalytic activity due to large length to diameter ratio of 1D structure and well-designed nanostructure. 2. Experimental section 2.1. Materials and reagents PVP (K30, analytical grade) was purchased from Sinopharm Chemical Reagent Co. Ltd. Other chemicals were purchased from Beijing Chemical Corp. All chemicals were used as received without any further purification. 2.2. Materials synthesis In a typical synthesis, 0.32 g Zn(NO3)2 6H2O and 0.37 g Ti (OC4H9)4 were dissolved in 20 mL glycol under magnetic stirring, respectively. The two solutions were mixed together, and then slowly added into a mixture of glycol (60 mL), PVP (1.0 g) and concentrated ammonia aqueous solution (28 wt%, 300 μL) by stirring at 50 1C for 20 h. The product was collected by centrifugation, washed with ethanol for several times, and dried at 60 1C for 10 h. Finally, to remove PVP and obtain meosoporous ZnTiO3 rods, the as-prepared powder was
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Fig. 2. (A, B) SEM images of S2 and S3.
calcined at 600 1C for 3 h with a heating rate of 2 1C min 1. For simplicity, meosoporous ZnTiO3 rods were denoted as S1. The same procedures were carried out for the preparation of samples S2 and S3, except that the reaction temperature was 40 1C and 75 1C for S2 and S3 respectively.
Fig. 3. (A) FTIR spectra of (a) S1, (b) S2, and (c) S3. (B) XRD patterns of (b) S2 and (c) S3.
The degradation rate (D%) can be expressed as follows: C0 C D% ¼ C0
2.3. Photocatalytic test 2.4. Characterization Photocatalytic activity testing on the degradation of RhB was carried out in a 200 mL beaker containing 100 mL of 15 mg/L RhB solution and 15 mg of solid catalyst under 300 rpm stirring at room temperature. The photoreactor was designed with an internal light source surrounded by a quartz jacket (50 W high pressure mercury lamp with main emission wavelength 313 nm). Before the photocatalytic reaction was initiated, the solution was stirred in dark for 30 min to obtain a good dispersion and reach an adsorption–desorption equilibrium between the organic molecules and the catalyst surface. Subsequently, all the photocatalytic performances were carried out at the same place with the same UV intensity to make the data reasonably comparable. At given intervals of illumination, the samples (3 mL) of the reaction solution were taken out and centrifuged for investigation. For the evaluation of photocatalytic activities, C is the concentration of the RhB molecule at a real time t, and C0 stands for the initial concentration of RhB.
Scanning electron microscope (SEM) and energy dispersive X-ray (EDX) were measured on JSM-5600. Transmission electron microscopy (TEM) was performed on JEM 3010. A few droplets of a suspension of the sample in ethanol were put on a microgrid carbon polymer supported copper grid and allowed to dry at room temperature for TEM observations. Crystal identification was carried out using a Bruker D8 Advance X-ray diffractometer with a Cu Kα X-ray source operating at 40 kV and 100 mA. Fourier transform infrared (FTIR) spectra were recorded with a PerkinElmer spectrometer using KBr pellets, and the thickness of the pellet being about 1.3 mm. The nitrogen adsorption and desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010m instrument (Micromeritics Instrument Corp., Norcross, GA). The Brunauer–Emmett–Teller (BET) specific surface area was calculated using adsorption data. UV–visible absorption
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Fig. 4. (A) TEM image of mesoporous ZnTiO3 rods (S1). (B) Nitrogen adsorption–desorption isotherm of S1.
spectra were recorded using a UV–visible spectrophotometer (UV-2550). The samples were placed in a 1 cm 1 cm 3 cm quartz cuvette, and the spectra were recorded at room temperature. 3. Results and discussion Mesoporous ZnTiO3 rods can be co-assembled by Zn (NO3)2 6H2O, Ti(OC4H9)4, and PVP through a one-step sol–gel method, followed by calcination at 600 1C for 3 h to eliminate any organic species. SEM image of calcined mesoporous ZnTiO3 (S1) synthesized at 50 1C shows 100% rod-like morphologies and smooth surface, while other morphologies can be hardly observed (Fig. 1A). The high yield 1D morphology may mainly be attributed to restriction of PVP, which can prevent the growth of ZnTiO3 rods in the radial direction (Fig. 1B). The length of these randomly oriented ZnTiO3 rods ranges from 3.1 to 8.9 μm, and the
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diameters ranges from 0.35 to 1.19 μm. The large dimension is beneficial to the separation and recovery of ZnTiO3 rods after photocatalytic degradation [18,19]. EDX spectrum was measured to determine the chemical composition. As shown in Fig. 1C, the existence of Zn, Ti, O, Au, Si is detected. Au and Si peak comes from the conductive coating and silicon wafer when operating SEM. It is worth pointing out that the atomic ratio of Zn to Ti is close to 1:1, indicating the well defined composition of ZnTiO3 rods. The XRD pattern of ZnTiO3 (S1) can be readily indexed to a cubic crystal phase of ZnTiO3 according to JCPDS Card no. 39-0190 (Fig. 1D). No peak corresponding to the TiO2, ZnO or zinc titanates of other stoichiometry is observed. The broadening of diffraction peaks indicates ZnTiO3 rod is composed of nano-sized particles with a mean size of about 20.5 nm calculated by Scherrer equation. In the control experiments, the same procedures were carried out for the preparation of samples S2 and S3 except for the reaction temperature. As shown in Fig. 2, S2 and S3 (synthesized at 40 1C and 75 1C respectively) keep the rodlike morphologies. However, the regularity of rod-like morphologies deteriorates. During a cooperative assembly process, PVP can alleviate the growth tendency of ZnTiO3 rods in radial direction, which is sensitive to synthesis temperature. When reaction temperature is 50 1C, the cooperative assembly among precursors and PVP is more effective to obtain rod-like morphology. Fig. 3A shows the FTIR spectrum of the synthesized mesoporous ZnTiO3 rods. The bands above 400 cm 1 originate from Ti–O stretching vibrations [23]. It is also seen that these main characteristic peaks of S1, S2 and S3 are nearly unchanged. This implies that the bond strengths of Zn–Ti–O are not affected by synthesis temperature. In addition, the XRD patterns of S2 and S3 exhibit the same feature as S1 (Fig. 3B), indicating that the crystal phase is not influenced by synthesis temperature. In the TEM image (Fig. 4A), mesopores with wormholeslike shapes were observed throughout ZnTiO3 rods (S1). The mesoporosity is derived from accumulation of nanoparticles, which is expected to contribute to the enhanced catalytic performances. The wormhole-like mesopores are also confirmed by N2 adsorption–desorption measurements. The nitrogen adsorption–desorption isotherm of ZnTiO3 rods (S1) exhibits a characteristic type IV isotherm (Fig. 4B), indicating the presence of mesoporous structure. The BET specific surface area and the average pore size are calculated to be 32.8 cm2/g and 3.3 nm, respectively. Similar to S1, ZnTiO3 rods S2 and S3 also exhibit mesoporous structure, and the BET specific surface area is 27.9 and 23.1 cm2/g, respectively. The regular morphologies of S1 lead to the higher surface area, which is paramount to achieve desired performance of photocatalyst. To evaluate the photoactivity of mesoporous ZnTiO3 rods (S1) for degradation of organic pollutants, we carried out the experiments of photocatalytic decomposition of RhB as a model reaction. For comparison, S2 and S3 were used as photocatalytic references. The degradation efficiency of the asprepared samples was defined as C/C0, where C and C0 stood for the remnants and initial concentration of RhB, respectively.
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Fig. 5. (A) Self-degradation of RhB with UV irradiation but in absence of photocatalysts and degradation profiles of RhB over photocatalysts in the dark, (B) degradation profiles of RhB over different photocatalysts, (C) kinetic linear simulation curves for the degradation of RhB with different photocatalysts, and (D) photocatalytic activity of S1 for the degradation of RhB during three cycles of use.
Before UV illumination, the time from 30 to 0 min in Fig. 5 showed the adsorption–desorption of RhB over photocatalysts in the dark. In the control experiments, no appreciable decomposition of RhB could be observed in absence of photocatalysts (Fig. 5A). Also, there was no appreciable degradation of RhB by photocatalysts in the absence of UV light irradiation, indicating an adsorption–desorption equilibrium of RhB on the surface of ZnTiO3 rods was established in the dark within 30 min. After UV illumination for 70 min, the degradation rates of RhB were 97%, 95%, and 92% for S1, S2, and S3, respectively (Fig. 5B). The photocatalytic degradation of RhB follows pseudofirst-order kinetics, and the apparent degradation rate constant k could be defined by ln(C/C0)¼ kt. Linear relationships between ln(C/C0) and reaction time were obtained for all the photocatalysts (Fig. 5C), and the corresponding value of k was calculated from the slope. The relative photo-catalytic activity of photocatalysts for RhB degradation follows the order: S1 4 S2 4S3. The higher surface area of S1 increases the surface reaction sites and might modulate the catalytic activity of the surface atoms. Furthermore, we have also monitored the cyclic stability of S1 by monitoring the catalytic activity during three cycles of use.
As shown in Fig. 5D, each experiment was carried out under identical conditions, and the photocatalytic activity of S1 exhibited no obvious decrease. The mesoporous ZnTiO3 rods could be easily recovered by sedimentation and maintain their photocatalytic activity after three cycles of use, which is significant for the potential use for elimination of organic pollutants at an industrial scale. The good activity of photocatalyst mainly originates from the mesoporosity for improved mass transfer and the 1D architecture for easy recovery. 4. Conclusions In summary, herein we describe an effective route to synthesize mesoporous ZnTiO3 rods. The mesoporous 1D structure could be constructed by co-assembly of precursors with the assistance of PVP. The mesoporous ZnTiO3 rods exhibit a good photocatalytic activity and can be easily recycled by sedimentation without significant decrease in photocatalytic activity. Combining the advantage of mesoporous 1D structure, good photocatalytic activity with good reusability, and low cost, the mesoporous ZnTiO3 rods will provide possibilities for environmental remediation. And this
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synthesize route can be extended to synthesize other multimetallic oxides with mesoporous 1D structures. [11]
Acknowledgments This work was supported by the Natural Science Foundation of Jilin Province of China (20150520020JH). References
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