A facile and high-yield approach to synthesize one-dimensional CeO₂ nanotubes with well-shaped hollow interior as a photocatalyst for degradation of toxic pollutants

Abstract

A facile, template-free and high-yield synthesis of single-crystalline cerium dioxide nanotubes (CeO₂-NT) has been reported via a “casually-modified” approach based on the hydrothermal treatment of Ce(OH)CO₃ precursors with alkali solution in an aqueous phase. This simple modification in synthesis procedures not only improves the yield of CeO₂-NT remarkably, but also gives rise to the formation of CeO₂-NT featuring excellent nanotubular open-ended structure with a well-shaped hollow interior. The collection techniques of BET, UV/visible diffuse reflectance spectra (DRS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis have been employed to characterize the morphology and optical properties of the as-prepared CeO₂-NT. Significantly, we demonstrate that CeO₂-NT exhibits a markedly enhanced photocatalytic activity and stability as compared with its counterpart of CeO₂ nanoparticles and commercial TiO₂ (P25) toward the degradation of aromatic benzene, a well-known toxic pollutant that commonly occurs in urban ambient air and is of significant concern regarding environmental health because of its toxic, mutagenic, or carcinogenic properties. This represents a first example to demonstrate the advantage of CeO₂ nanotubes as photocatalyst as compared to its counterpart of CeO₂ nanoparticles, clearly suggesting the morphology/shape-dependent photocatalytic behaviour of CeO₂ materials. Therefore, our current work not only offers a simple approach for fabrication of open-ended CeO₂-NT with well-shaped hollow interior, but also demonstrates the promising potential of the applications of CeO₂-NT, CeO₂-NT-based and other metal oxide nanotube-based materials in the area of photocatalysis, which will inevitably enrich the intriguing chemistry of morphology/shape-dependent heterogeneous photocatalysis and thermal catalysis.

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Introduction

One-dimensional organic and inorganic nanostructures including nanotubes, nanorods and nanowires have been attracting tremendous interest owing to their unique structural and electronic properties. To date, they have found broad applications in numerous areas, for example, light waveguides, solar energy conversion, catalysis, lithium battery, resonant Raman spectra, biomedical implants and gas sensing.^1–9 In particular, one-dimensional nanotubes have well-defined hollow interiors that allow for the introduction of other matter including metal oxides, metal halides and organic molecules into the cavities, thus leading to nanotube-based composite materials with intriguing properties different from the bulk.^10–13 For example, Bao and co-workers reported that a synergetic confinement of Rh metal particles inside the channel of carbon nanotubes results in a striking enhancement of catalytic activity for the conversion of CO and H₂ to ethanol.^14,15 Noteworthily, metal oxide nanotubes not only feature the special one-dimensional tubular nanostructure similar to carbon nanotubes, but also have the intrinsic chemical properties of metal oxides that are quite different from carbon nanotubes, which therefore attracts a great deal of scientific interest in controlled synthesis of CNT-like metal oxide nanotubes and exploration of their potential applications in many technological fields, for example, catalysis, electronic devices and gene delivery vehicles.^16–20

CeO₂ is an important rare earth material because it is widely used for preparing catalysts, fuel cells, solar cells, UV blockers, polishing materials and hydrogen storage materials.^21–29 It should be particularly noted that recent advances in the synthesis of CeO₂ nanostructured materials with different morphologies, including nanospheres, nanorods, nanowires, nanocubes, nanopolyhedra, and nanotubes, offer new opportunities of enabling CeO₂ and CeO₂-based materials with desired structural and functional properties.^30–36 For example, it has been shown that CeO₂ nanotubes are very active for CO oxidation, and at 250 °C, the conversion rates of CO over CeO₂ nanotubes are 3 times higher than that of the bulk counterpart, commercial ceria powder.³⁷ So far, different methods have been reported to synthesize CeO₂ nanotubes. Han et al. reported the production of CeO₂ nanotubesvia a two-step procedure: precipitation at 100 °C and aging at 0 °C for 45 days.³⁸ Tang et al. reported the synthesis of CeO₂ nanotubes by annealing of Ce(OH)₃ nanotubes prepared by an alkali thermal-treatment process under oxygen-free conditions.³⁹ Zhou and co-workers reported a facile synthesis of CeO₂ nanotubes with large cavities by etchingCe(OH)₃ nanotubes/nanorods with H₂O₂.⁴⁰ In addition to these template-free methods, the synthesis of CeO₂ nanotubes using carbon nanotubes and porous alumina membrane as hard templates has also been reported.^41,42 Most recently, Chen and co-workers reported the synthesis of three different types of CeO₂ nanotubes by a hydrothermal treatment of Ce(OH)CO₃ nanorods as precursor.^37,43 Although significant progress has been achieved in the synthesis of CeO₂ nanotubes based on a literature survey as mentioned above, there is still a lack of simple and effective methods for high-yield synthesis of open-ended CeO₂ nanotubes with a well-shaped hollow interior that constitutes an important starting platform for anchoring other matter,^10–15 such as noble metal particles and organic molecules, into the inside channel so as to investigate the synergetic confinement effect of CeO₂ nanotubes. Furthermore, to the best of our knowledge, there is also no report on utilizing CeO₂ nanotubes as photocatalysts for the degradation of environmental organic pollutants.

Herein, we report a facile template-free and high-yield synthesis of single-crystalline fluorite-structured cerium dioxide nanotubes (CeO₂-NT) by a “casually-modified” approach based on the hydrothermal treatment of Ce(OH)CO₃ precursors with alkali solution in an aqueous phase. This simple modification in synthesis procedure leads to the finding that the as-obtained CeO₂-NT features an excellent nanotubular open-ended structure with a well-shaped hollow interior. Furthermore, the yield is remarkably enhanced. Significantly, we demonstrate for the first time that CeO₂-NT exhibits an obviously enhanced photocatalytic activity as compared with its counterpart of CeO₂ nanoparticles and commercial TiO₂ (P25) toward the degradation of aromatic benzene, a well-known toxic pollutant that commonly occurs in urban ambient air and is of significant concern regarding environmental health because of its toxic, mutagenic, or carcinogenic properties.^44–47 Thus, our present work not only offers a simple approach to synthesize open-ended CeO₂ nanotubes with well-shaped hollow interior, but also demonstrates a promising potential of the applications of CeO₂-NT, CeO₂-NT-based and other metal oxide nanotube-based functional materials in the realm of photocatalysis, which consequently would enrich the chemistry of morphology/shape-dependent heterogeneous photocatalysis and thermal catalysis.

Experimental

Synthesis of CeO₂ nanotubes

All reagents are of analytic grade and used without further purification. The synthesis of CeO₂ nanotubes is via a “casually-modified” method based on the previous report regarding hydrothermal treatment of Ce(OH)CO₃ precursors with alkali solution.^37,43 The first step towards the synthesis of Ce(OH)CO₃ precursors was based on the work reported by Chen et al.³⁷ Typically, 1.7 g of Ce(NO₃)₃·6H₂O and 3.6 g of urea were added to 80 mL of water under vigorous magnetic stirring for 30 min. Then, the clear suspension was transferred into a 100 mL round bottom flask for treatment in an oil bath at 80 °C for 24 h with stirring. After reaction, the suspension was cooled down to room temperature. The as-obtained powder samples were centrifuged and washed with distilled water, and dried completely in an oven by which the Ce(OH)CO₃ precursors were obtained. The second step was the hydrothermal treatment of Ce(OH)CO₃ precursors with NaOH solution in order to synthesize the CeO₂ nanotubes. The key modification for this step was to improve the amount of Ce(OH)CO₃precursors and the concentration of NaOH aqueous solution. This modification leads to the yield of the as-prepared CeO₂ nanotubes being improved significantly; meanwhile, the as-synthesized CeO₂ nanotubes feature a well-shaped hollow interior with open-ended structure. In a typical experiment, 0.25 g of the Ce(OH)CO₃ precursors were redispersed into 40 mL of distilled water. Upon the addition of 4.8 g of NaOH, the mixture suspension was stirred for 30 min and then transferred into a 50 mL Teflon-lined stainless steel autoclave for a hydrothermal reaction at 120 °C for 24 h. After that, the products were cooled to room temperature, washed with distilled water and anhydrous ethanol, and then they were dried in air at 100 °C for 20 h. For comparison, the CeO₂ nanoparticles of size 15–30 nm were supplied by Alfa Aesar.

Characterization

The X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance X-ray diffractometer at 40 kV and 40 mA with Ni-filtered Cu-Kα radiation. The optical properties were analyzed by UV-vis diffuse reflectance spectroscopy (DRS) using a UV-vis spectrophotometer (Cary-500, Varian Co.), in which BaSO₄ was used as the internal standard. The nitrogen adsorption–desorption isotherm was recorded using a Micromeritics-ASAP2020 instrument. The morphology of the samples was determined by field emission scanning electron microscopy (SEM) on a FEI Nova NANOSEM 230 spectrophotometer. Transmission electron microscopy (TEM) images were obtained using a JEOL model JEM 2010 EX instrument at the accelerating voltage of 200 kV. Electron spin resonance (ESR) spectra were measured using a Bruker A300 EPR electron paramagnetic resonance spectrometer equipped with a quanta-Ray Nd:YAG laser system as their radiation light source (λ = 254 nm). The settings were the center field at 3480.00 G, microwave frequency at 9.83 GHz, and power at 6.35 mW.

Photocatalytic activity test

The gas-phase photocatalytic degradation of benzene was carried out in a tubular vessel microreactor operating in a continuous flow mode.^45–47 All of the catalysts were sieved to obtain the particles of 0.21–0.25 mm size. Four 4 W UV lamps with wavelength centered at 254 nm (Philips, TUV 4W/G4 T5) were used as the light source because the as-prepared CeO₂ nanotubes has the strongest light absorption around the region of 254 nm. A bubbler that contained benzene was immersed in an ice-water bath, and benzene (about 250 ppm) bubbled with oxygen from the bubbler was fed to 0.3 g catalyst at a total flow rate of 20 mL min⁻¹. The reaction temperature was controlled at 30 ± 1 °C by an air-cooling system. Simultaneous determination of benzene and CO₂ concentrations was performed with an online gas chromatograph (HP6890) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD). Conversion of benzene was defined as follows: % conversion = [(C_o − C)/C_o] × 100 where C_o is the initial concentration of benzene and C is the concentration of benzene after photocatalytic reaction.

Results and discussion

Fig. 1 shows the X-ray powder diffraction (XRD) patterns of the as-prepared cerium dioxide nanotubes (CeO₂-NT). It can be readily indexed to the pure cubic fluorite-structured CeO₂ (JCPDS34-394), which is the same as that of bulk CeO₂ nanoparticles. The Brunauer–Emmett–Teller (BET) adsorption–desorption isotherm of the as-synthesized CeO₂-NT is shown in Fig. 2. The specific BET surface area and pore volume of CeO₂-NT are 72 m² g⁻¹ and 0.14 cm³ g⁻¹, respectively. The morphology and microscopic structure of CeO₂-NT were disclosed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis, as displayed in Fig. 3. It is clear that the as-synthesized CeO₂-NT sample has an inner diameter of around 10–25 nm along with a wall thickness of ca. 5–10 nm. Quite interesting is the observation from the TEM analysis that CeO₂-NT consists of well-shaped and distinct hollow interior with open-ended structure. This structure character is evidently different from those reported in previous research works on the synthesis of CeO₂ nanotubes.^37–43 Thus, similar to the case of filling carbon nanotubes,^10–15 this tubular CeO₂-NT should be also suitable for introduction of noble metal particles, metal oxides, molten metal halides and organic molecules by nanocapillary wetting.

Fig. 1 XRD pattern of the as-prepared CeO₂ nanotubes (CeO₂-NT); inset is the XRD pattern of CeO₂ nanoparticles (CeO₂-NP).

Fig. 2 BET adsorption–desorption isotherm curve of the as-prepared CeO₂ nanotubes (CeO₂-NT).

Fig. 3 Typical SEM (a), TEM (b, c, d) and HRTEM images (e, f) of the as-prepared CeO₂ nanotubes (CeO₂-NT). Inset is the selected area electron diffraction (SAED) pattern.

The high-resolution transmission electron microscopy (HRTEM) analysis and selected area electron diffraction (SAED) pattern, as shown in Fig. 3f, reveal that the as-prepared CeO₂-NT has a good single-crystalline structure, which is the same as that of L-type CeO₂ nanotubes as reported by Chen et al.⁴³ Noteworthily, our “casually-modified” approach not only gives rise to the formation of open-ended CeO₂-NT with distinct well-shaped hollow interior, but also remarkably improves the synthesis yield of CeO₂-NT that is beneficial for further exploration of its applications in catalysis. With a 50 mL autoclave, we can obtain about 0.21 g of CeO₂-NT product per batch, which corresponds to a high yield of ca. 84% with reference to 0.25 g of Ce(OH)CO₃ precursor as raw material.

The UV-vis diffuse reflectance spectra (DRS) are displayed in Fig. 4. It is clear that the sample of CeO₂-NT possesses higher light absorption intensity than its counterpart of CeO₂ nanoparticles (CeO₂-NP) in both the UV and visible light region. Nevertheless, in the UV region, commercial P25 nanoparticles display higher light absorption capability than CeO₂-NT and CeO₂-NP. In addition, the light absorption edge of CeO₂-NT displays a red shift as compared to the CeO₂ nanoparticles. According to the Kubelka–Munk function transformation, the estimated band gap values of CeO₂-NT, CeO₂-NP and P25 are approximately 2.92 eV, 3.03 eV and 3.32 eV, respectively. These results suggest that the optical absorption property of CeO₂ exhibits a morphology/shape-dependent behaviour and can be finely tailored by changing its microscopic morphology. The higher light absorption intensity of CeO₂-NT than CeO₂-NP also indicates that it may have enhanced photocatalytic performance for a specific target reaction. This inference is true as has been confirmed by the photocatalytic degradation of benzene in the gas phase as discussed below.

Fig. 4 UV-vis diffuse reflectance spectra (DRS) of the as-prepared CeO₂ nanotubes (CeO₂-NT), CeO₂ nanoparticles (CeO₂-NP) and commercial P25 nanoparticles (a), and the plot of transformed Kubelka–Munk function versus the energy of light (b).

Fig. 5 displays the time-online photocatalytic results for the gas-phase degradation of benzene over the samples of CeO₂-NT, CeO₂-NP and P25. As can be seen, during the reaction of 22 h, the sample of CeO₂-NT exhibits the best photocatalytic performance toward the gas-phase degradation of benzene in view of the constant conversion ratio of benzene. Namely, no obvious deactivation trend is observed over the sample of CeO₂-NT. In addition, after the reaction of 10 h, the produced amount of CO₂ over CeO₂-NT is nearly stable at 29 ppm. In contrast, the samples of both CeO₂-NP and commercial P25-TiO₂ nanoparticles display a quite unstable photocatalytic activity toward the gas-phase degradation of benzene. With regard to the sample of CeO₂-NP, the conversion ratio of benzene at the initial stage is 2.2%; after reaction for 22 h, it is decreased to 1.4%. Furthermore, the produced amount of CO₂ is remarkably decreased to only 8 ppm as compared to 19 ppm at the initial stage. In analogy, commercial P25 nanoparticles also exhibit the significant deactivation behaviour for the gas-phase degradation of benzene. The enhanced photocatalytic performance of CeO₂-NT as compared to CeO₂-NP seems consistent with its higher light absorption intensity as shown in UV-vis DRS spectra in Fig. 4. However, such a single correlation between light absorption intensity and photocatalytic activity is one-sided because other factors can also have an important and synergetic effect on photocatalytic performance, such as morphology, particle size and lifetime of photogenerated electron–hole pairs.^45–47 Furthermore, it should be noted that P25 shows lower and more unstable activity than CeO₂-NT, although it has higher light absorption intensity than CeO₂-NT.

Fig. 5 Time-online data for gas-phase photocatalytic degradation of benzene over the samples of commercial P25, CeO₂ nanoparticles (CeO₂-NP) and the as-prepared CeO₂ nanotubes (CeO₂-NT).

To further understand why CeO₂-NT shows more stable and higher photocatalytic activity than CeO₂-NP and commercial P25, we have used electron spin resonance (ESR) spectra to observe the possible radical species formed over these samples under the irradiation of light. Information in this regard can help us learn the lifetime of electron–hole pairs and intensity of radical species, which are key photoactive species responsible for the degradation of benzene.^45–47 The ESR data are shown in Fig. 6. It can be seen that the total intensity of hydroxyl radical (˙OH) species and superoxide radical (O₂˙⁻) species as detected over the CeO₂-NT sample is higher than that over the CeO₂-NP and commercial P25 nanoparticles. In addition, it should be noted that, upon light irradiation, the O₂˙⁻ radical species are clearly observed over only the CeO₂-NT sample but they are not observed over the samples of P25 and CeO₂-NP. These suggest the prolonged lifetime of electron–hole pairs generated over the sample of CeO₂-NT upon light irradiation. As a result, as reflected in Fig. 6, a greater amount of radical species with strong oxidation power, including ˙OH species and superoxide radical O₂˙⁻ is able to be photogenerated over the CeO₂-NT than its counterpart CeO₂-NP and commercial P25 nanoparticles. This in turn explains reasonably why the higher photocatalytic activity of CeO₂ nanotubes than CeO₂ and P25 nanoparticles has been observed toward the gas-degradation of benzene, as schematically shown in Fig. 7.

Fig. 6 ESR spectra of radical adducts trapped by DMPO; (top) DMPO–˙OH radical species detected for the samples dispersion in water; (bottom) DMPO–O₂˙⁻ radical species detected for the sample dispersions in methanol.

Fig. 7 Proposed illustration showing the CeO₂ nanotube as photocatalyst with enhanced electron–hole pair lifetime for degradation of the volatile organic pollutant benzene in the gas phase.

In addition, the nanotubular morphology of CeO₂-NT also imparts intrinsic advantages as a photocatalyst as compared with its counterpart of CeO₂ nanoparticles, because it is well documented that: (1) the one-dimensional nanotube geometry facilitates the fast and long-distance electron transport; (2) the nanotubular morphology is beneficial for the enhancement of light absorption and scattering owing to the high length-to-diameter ratio.^47–55

Our research work presents a first example that the photocatalytic activity of CeO₂ can be tailored by changing its microscopic morphology. That is, one-dimensional nanotubular CeO₂ possesses higher and more stable photocatalytic performance that bulk CeO₂ nanoparticles toward the gas-phase degradation of the organic pollutant benzene in air. Such morphology/shape-dependent catalytic properties have been also observed previously in thermal heterogeneous catalysis. For example, the activity of CeO₂ nanotubes has been shown to be three times higher than that of their bulk counterpart for CO oxidation.³⁷ It has been demonstrated that the CeO₂ nanorods are more reactive for CO oxidation than irregular nanoparticles of CeO₂.⁵⁶ In addition, nanopolyhedral CeO₂ exhibit higher catalytic activity for CO oxidation than mesoporous CeO₂ owing to the higher particle dispersity of the former than that of the latter.⁵⁷ Therefore, it is reasonable to believe that, similar to morphology-dependent catalytic activity of CeO₂ in thermal heterogeneous catalysis, there would be a wide scope to further tune the photocatalytic activity of semiconductor CeO₂ through adjusting the morphology of CeO₂ material by a variety of morphology-controlled synthesis approaches.

Conclusions

In summary, we have reported a facile, template-free and high-yield synthesis of single-crystalline cerium dioxide nanotubes (CeO₂-NT) by a “casually-modified” approach based on the hydrothermal treatment of Ce(OH)CO₃ precursors with alkali solution in an aqueous phase. This simple modification in synthesis procedure not only improves the yield of CeO₂-NT remarkably, but also gives rise to the formation of CeO₂-NT featuring excellent nanotubular open-ended structure with a well-shaped hollow interior, which could offer solid inroads into further anchoring other matter into the inside channel of CeO₂ nanotubes. In addition, we have presented a first example that CeO₂-NT exhibit a markedly enhanced photocatalytic activity as compared with its counterpart CeO₂ nanoparticles and commercial TiO₂ (P25) toward the degradation of aromatic benzene, a well-known toxic pollutant that commonly occurs in urban ambient air. This clearly illustrates the morphology/shape-dependent photocatalytic behaviour of CeO₂ materials. Therefore, our work demonstrates the promising potential of the applications of CeO₂-NT, CeO₂-NT-based and other metal oxide nanotube-based materials in photocatalysis, which will therefore enrich the chemistry of morphology/shape-dependent functional properties related to CeO₂ materials as heterogeneous photocatalysts. Finally, it is hoped that the current work will stimulate further interest in this intriguing topic.

Acknowledgements

The support by the National Natural Science Foundation of China (20903022, 20903023), the Award Program for Minjiang Scholar Professorship, Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818), National Basic Research Program of China (973 Program: 2007CB613306), the Science and Technology Development of Foundation of Fuzhou University (2009-XQ-10), the Open Fund of Photocatalysis Institute of Fuzhou University (0380038004), and Program for Returned High-Level Overseas Chinese Scholars of Fujian province is gratefully acknowledged.

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