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Nanoscale Research Letters

, 13:89 | Cite as

A Facile Method for Loading CeO2 Nanoparticles on Anodic TiO2 Nanotube Arrays

  • Yulong Liao
  • Botao Yuan
  • Dainan Zhang
  • Xiaoyi Wang
  • Yuanxun Li
  • Qiye Wen
  • Huaiwu Zhang
  • Zhiyong Zhong
Open Access
Nano Idea
  • 516 Downloads

Abstract

In this paper, a facile method was proposed to load CeO2 nanoparticles (NPs) on anodic TiO2 nanotube (NT) arrays, which leads to a formation of CeO2/TiO2 heterojunctions. Highly ordered anatase phase TiO2 NT arrays were fabricated by using anodic oxidation method, then these individual TiO2 NTs were used as tiny “nano-containers” to load a small amount of Ce(NO3)3 solutions. The loaded anodic TiO2 NTs were baked and heated to a high temperature of 450 °C, under which the Ce(NO3)3 would be thermally decomposed inside those nano-containers. After the thermal decomposition of Ce(NO3)3, cubic crystal CeO2 NPs were obtained and successfully loaded into the anodic TiO2 NT arrays. The prepared CeO2/TiO2 heterojunction structures were characterized by a variety of analytical technologies, including XRD, SEM, and Raman spectra. This study provides a facile approach to prepare CeO2/TiO2 films, which could be very useful for environmental and energy-related areas.

Keywords

Anodic TiO2 nanotubes CeO2 nanoparticles CeO2/TiO2 heterojunctions Green chemistry 

Abbreviations

EDS

Energy-dispersive spectrometry

NT

Nanotube

SEM

Scanning electron microscopy

XRD

X-ray diffraction

Background

As is well known, titanium dioxide (TiO2) materials have been widely used for a great number of applications such as solar cells, water treatment materials, catalysts and so on [1, 2, 3, 4, 5, 6]. The reason for TiO2 and TiO2-derived materials have so many applications is they have outstanding photocatalytic, electrical, mechanical, and thermal properties [7, 8, 9]. In nature, TiO2 has three most commonly encountered crystalline polymorphs, including anatase, rutile, and brookite. Among the three TiO2 polymorphs, anatase is the most photoactive polymorph used for degradation of organic pollutants or electrodes for energy applications [10, 11]. Anatase TiO2 have a band gap of ~ 3.2 eV, and it has shown good catalytic activity, corrosion resistance, and light resistance. Alone with its stable performance, low cost, non-toxic harmless, TiO2 in anatase phase was recognized as the best photocatalyst.

Recently, TiO2 nanotube (NT) arrays have attracted great attention due to its unique tubular structure-induced advantages [12, 13, 14, 15, 16, 17, 18]. However, their performances were still limited by inherent material faults, such as relatively wide gaps (~ 3.2 eV) [19, 20, 21, 22]. In order to achieve better application, narrow band semiconductors with proper energy level were proposed to further modify TiO2 NT arrays [23, 24]. The band gap of cubic CeO2 is about 2.92 eV and has good chemical stability. TiO2 modified by CeO2 were found very useful in the field of photocatalysis, gas sensors, and so on [25, 26, 27]. In the field of photocatalysis, the rapid recombination of photogenerated electron-hole pairs of reduces the photocatalytic performance of TiO2. However, the modification of CeO2 changes the recombination rate of the electron-hole pairs inside a CeO2/TiO2 composite material. As shown in the Fig. 1a, once CeO2/TiO2 heterojunctions are formed, more superoxide and hydroxyl radicals could be produced, leading to improved photocatalytic performance. In the field of gas sensors, CeO2 is a promising material for oxygen gas sensing at high temperature. TiO2 modified by CeO2 could effectively improve the adaptability of gas sensor, because the CeO2/TiO2 heterostructures enable the sensing of oxygen gas at low operating temperatures (< 500 °C) [28]. In order to prepare CeO2/TiO2 heterostructures, many approaches have been proposed including sol-gel method and hydrothermal method [29, 30, 31]. The former works were found very interesting and their products had shown good performances. However, the traditional methods are always used to prepare CeO2/TiO2 heterostructures in powder form and often with complicated procedures. For preparing CeO2/TiO2 heterostructures based on TiO2 NTs as shown in Fig. 1b, developing facile method to load CeO2 nanoparticles (NPs) on the TiO2 NT arrays is highly desired. To this end, we proposed a novel method for the preparation of CeO2/TiO2 heterojunctions in this study.
Fig. 1

a Energy levels of TiO2 NTs and CeO2 NPs with electron-hole pair transfer and separation. b Illustration diagram of CeO2 NP and TiO2 NT heterojunction

Highly ordered anatase phase TiO2 NT arrays were fabricated by anodic oxidation method, then the individual TiO2 NTs were prepared as tiny “nano-containers” to load Ce(NO3)3 solutions. The loaded anodic TiO2 NTs were heated to a high temperature, under which the Ce(NO3)3 were thermal decomposed. After the thermal decomposition of Ce(NO3)3, cubic crystal CeO2 NPs were obtained and successfully loaded into the anodic TiO2 NT arrays. CeO2/TiO2 heterojunctions prepared by this method was recognized as simple operation, low cost, non-toxic harmless.

Experimental Section

Synthesis of TiO2 Nanotube Arrays

Firstly, we used anodic oxidation method to prepare TiO2 nanotube arrays [32, 33, 34]. Briefly, titanium pieces were cut into small pieces (5 cm × 1.5 cm) and flattened. After being washed in detergent water, the titanium pieces were washed in an ultrasonic cleaner for 1 h with deionized water and alcohol, respectively. The dried titanium sheets with a counter electrode were immersed in the prepared electrolyte (500 ml glycol, 10 ml H2O and 1.66 g NH4F) under room temperature. A constant voltage of 60 V was applied to the two electrodes for 2 h. Then, TiO2 NT films were annealed at 450 °C for 3 h, and the rate of anatase TiO2 NTs were obtained.

Synthesis of CeO2/TiO2 Heterojunction

The individual TiO2 NTs inside the anodic films were taken as thousands small nano-containers to load the raw materials of CeO2, which will be full with the Ce contained solutions. As shown in Fig. 2, the TiO2 NTs were immersed in the Ce(NO3)3 solution (concentration were 0.05, 0.1, 0.2,0.5, and 1 mol/L respectively) for 3 s. In order to ensure the open tube mouth of the TiO2 NTs, it is worthy of attention that superfluous solution on the surface of the TiO2 NT films should be absorbed by using a qualitative filter paper immediately. The films were tilted as much as possible, making the solution flow to the edge of the films, and the filter paper was used to dry out the superfluous solution to ensure uniformity of solution. Then, the loaded films were dried at 70 °C for 1 h, during which the Ce(NO3)3 solute will be deposited inside the TiO2 NT nano-containers. And the dried films were further annealed at 450 °C for 2 h, during which the deposited Ce(NO3)3 will be thermally decomposed into CeO2 NPs at a high temperature. Finally, CeO2 NPs were obtained and attached to each single TiO2 NT of the arrays.
Fig. 2

Synthesis flow of CeO2/TiO2 heterojunction: (a) preparation of empty TiO2 NTs, (b) loading the TiO2 NTs with Ce(NO3)3 solution, and (c) formation of CeO2/TiO2 heterojunction structures

Characterization

Crystalline structure of the CeO2/TiO2 heterojunction was analyzed by X-ray diffraction (XRD; D/max 2400 X Series X-ray diffractometer). XRD was applied to characterize the samples at a step of 0.03° in the range of 10° to 80°. The microstructure of the heterojunctions and the morphology of the nanotubes were characterized by scanning electron microscopy (SEM; JSM-7000F, JEOL Inc. Japan). The elemental distribution of the microscopic region of the materials was qualitatively and quantitatively analyzed by energy-dispersive spectrometry (EDS). The crystal structure of the CeO2/TiO2 heterojunction was also analyzed by Raman spectra (inVia, Renishaw, UK). Resonant Raman scattering spectra were recorded at room temperature to obtain a more clear display of components.

Results and Discussion

Crystalline Properties of the Prepared CeO2/TiO2 Heterojunction Films

XRD patterns of the prepared CeO2/TiO2 heterojunction films are shown in Fig. 3. The diffraction peak could be identified as the anatase phase of TiO2 and cubic phase of CeO2. The diffraction peaks located at 25.28°, 36.80°, 37.80°, 48.05°, 53.89°, 55.06°, 62.68°, 70.30°, 75.03°, and 76.02° were attributed to the anatase lattice plane (101), (103), (004), (200), (105), (211), (204), (220), (215), and (301), respectively. Moreover, the minor diffraction peaks at 40.1° and 53.0° were attributed to (101) and (102) of Ti (see Fig. 3a). This indicates the anodic TiO2 NT films have an anatase crystalline structure in this study. In the crystallization process, anatase grains usually have a smaller size and a larger specific surface area. Therefore, anatase TiO2 surface has strong adsorption capacity of H2O, O2, and OH and its photocatalytic activity is greatly high [35, 36]. The adsorption capacity of the anatase TiO2 NT films is enormously influenced in the photocatalytic reaction, and the strong adsorption capacity is beneficial to its activity. Meanwhile, the diffraction peak located at 28.55° and 33.08° was indexed to crystal face (111) and (200) of CeO2, respectively [37, 38]. Figure 3b shows the XRD patterns of the CeO2/TiO2 heterojunction films with different initial Ce(NO3)3 concentration. When the concentration of Ce(NO3)3 was too low, only diffraction peaks of the anatase TiO2 could be observed. With the concentration of Ce(NO3)3 gradually increasing, the cubic phase of cerium oxide appeared and the diffraction peaks of cubic CeO2 became stronger. According to the tested XRD data, the standard PDF showed CeO2 has a face-centered cubic (FCC) crystal structure. The calculated lattice parameters were a = b = c = 0.5411 nm and α = β = γ = 90°, which matched with the standard PDF. It could be summarized that TiO2 was modified by CeO2 perfectly in lattice matching so that their heterojunctions are tighter and better to produce a special electron transfer process which is able to facilitate the separation of the electron/hole pairs.
Fig. 3

a XRD pattern of the anatase phase of TiO2 and cubic CeO2. b XRD pattern of the anatase phase of TiO2 and cubic CeO2 with different concentrations of Ce(NO3)3

Microscopic Morphologies of the CeO2/TiO2 Heterojunction Films

Figure 4 shows SEM images of the anatase TiO2 nanotube arrays before and after being modified by CeO2. Top profile of the TiO2 NT arrays without loading CeO2 is shown as Fig. 4a, and the self-organized NT arrays were found quite dense and had an open-mouth top morphology, which provides a passage way for the Ce(NO3)3 solution filling into the NTs in this study. The average tube diameter is evaluated about 110 nm. Figure 4b shows the microstructure of anodic TiO2 NTs modified by CeO2 NPs. It can be seen that there are lots of long strips on the tube-pore mouths by comparing to the pure TiO2 NTs. Meanwhile, the tube wall thickness could be found getting increased by taking a close look. These observations indicate that the morphologies of the anodic TiO2 NT arrays have an obvious change after the loading and annealing process. Also, from the SEM images, most CeO2 NPs were deposited on the top of the TiO2 NTs, because when the superfluous Ce(NO3)3 solution was treated, the superfluous solution on the top of tubes was not completely disposed, and after thermally decomposed, the CeO2 NPs were deposited on the top of tubes. Morphologies of the CeO2/TiO2 heterojunction films with Ce(NO3)3 solution concentration varying from 0.05 mol to 0.5 mol are shown in Fig. 5. It could be clearly seen that with the Ce(NO3)3 solution concentration increasing, the nanoparticles in the TiO2 NTs gradually became more abundant and more elongated particles appeared on the TiO2 NTs. These results reveal that the CeO2 nanoparticles are successfully attached to tube wall of the anodic TiO2 NT arrays, forming a CeO2/TiO2 heterojunction structure. The large specific surface area of the TiO2 NTs provides a good substrate for CeO2 NPs to load onto the anodic TiO2 NT films.
Fig. 4

Typical SEM images of a pure TiO2 nanotube arrays without modification and b the CeO2/TiO2 heterojunction, indicating the highly ordered structure with open tube mouth morphology, and after modification, CeO2 was successfully loaded into the TiO2 nanotube arrays

Fig. 5

SEM images of the CeO2/TiO2 heterojunctions with different Ce(NO3)3 solution concentration: a sample immersed in 0.05 mol/L Ce(NO3)3; b sample immersed in 0.1 mol/L Ce(NO3)3; c sample immersed in 0.2 mol/L Ce(NO3)3; and d sample immersed in 0.5 mol/L Ce(NO3)3

Components Analysis of the CeO2/TiO2 Heterojunction Films

In order to coordinate with the SEM test results, energy-dispersive X-ray spectroscopy (EDS) was used to analyze the elemental composition of the CeO2/TiO2 heterojunction films. EDS comparison diagram between TiO2 NTs and CeO2/TiO2 heterojunction is shown in Fig. 6. As shown in the Fig. 6a, only Ti and O could be detected. The atomic percentage of Ti and O elements is 27.37 and 65.36%, respectively. The sample of CeO2/TiO2 heterojunction film which is prepared in the 0.1 mol/L Ce(NO3)3 solution is shown in Fig. 6b. Ce, O, and Ti could be detected. The atomic percentage of Ce, Ti, and O elements is 11.91, 12.04, and 59.98%, respectively. It can be concluded from the EDS results that CeO2 NPs were successfully deposited on the TiO2 NTs.
Fig. 6

EDS results of a pure TiO2 NTs and b CeO2/TiO2 heterojunction, showing the existence of element Ti, Ce, and O after loading Ce(NO3)3. The results confirm the successful loading of CeO2 on the TiO2 NTAs

In order to further investigate the obtained films, Raman spectroscopy was used to analyze the properties of the CeO2-loaded TiO2 film. Figure 7 shows two typical Raman spectra of the pure anodic TiO2 film and the CeO2/TiO2 heterojunction film which is prepared in the 1 mol/L Ce(NO3)3 solution. Peaks located at around 400, 530, and 645 cm−1 could be clearly observed, which could be attributed to anatase TiO2 phase. Along with these characteristic peaks of anatase TiO2, there is a new peak at about 460 cm−1 that could be observed for the CeO2/TiO2 films. According to the Raman-active mode, this peak could be ascribed to the cubic phase of CeO2 [39]. The Raman spectra results also confirm that the CeO2/TiO2 heterojunction was successfully prepared.
Fig. 7

Raman spectra of pure TiO2 NTs and CeO2/TiO2 heterojunction, indicating CeO2 NPs were successfully loaded into the TiO2 NTAs

Mechanism of the CeO2/TiO2 Heterojunction Formation

According to the reported studies, the most common used method for preparing CeO2/TiO2 heterojunction is the sol-gel method or the secondary redox method [40]. In order to obtain the CeO2/TiO2 heterojunction in a very simple procedure with low cost, in this paper, the preparation of CeO2/TiO2 heterojunction is achieved by filling TiO2 NT nano-container with Ce(NO3)3 solution and then thermal decomposition of Ce(NO3)3. The high temperature breaks the chemical bonds of Ce(NO3)3 molecules, and the decomposed Ce, O, and N atoms then reform into CeO2 NPs and NO/O2. This process is schematically shown as Fig. 8. Firstly, the Ce(NO3)3 aqueous solution with different concentrations were filled into the TiO2 NT nano-container. Then, the film were baked at 70 °C for 1 h, during which Ce(NO3)3 will be deposited from water in the form of Ce(NO3)3·6H2O and finally change into Ce(NO3)3 loaded inside those TiO2 NT nano-container. Then, the Ce(NO3)3-loaded TiO2 NT films were annealed at a high temperature of 450 °C for 2 h. Under high temperature conditions, the chemical bonds in the Ce(NO3)3 molecule will be broken and recombine, resulting in the generation of CeO2 NPs inside the TiO2 NTs. Two involved chemical reaction are expressed as following eq. (1) and (2):
$$ \mathrm{Ce}{\left({\mathrm{NO}}_3\right)}_3\bullet 6{\mathrm{H}}_2\mathrm{O}\to \mathrm{Ce}{\left({\mathrm{NO}}_3\right)}_3 $$
(1)
$$ \mathrm{Ce}{\left({\mathrm{NO}}_3\right)}_3\to {\mathrm{CeO}}_2\kern0.5em +\mathrm{NO}\uparrow \kern0.5em +{\mathrm{O}}_2\uparrow $$
(2)
Fig. 8

Schematic synthesis diagram of the CeO2/TiO2 heterojunctions and involved chemical equations

In short, we have shown a facile method using TiO2 NT nano-container to load Ce(NO3)3 to prepare CeO2/TiO2 heterojunction films. Ce(NO3)3 thermal decomposition inside each individual anodic TiO2 NTs allows for a good formation and distribution of the CeO2 NPs. CeO2/TiO2 heterojunction films have lots of potential applications. In the field of photocatalysis, it can be used to degrade water pollution, because CeO2 can inhibit the rapid electron-hole recombination of TiO2 and the heterojunction films can adsorb organic pollutants efficiently. In the field of the photocatalytic hydrogen production and the improvement of TiO2 oxygen sensor, CeO2 NPs/TiO2 NTA films can also be used well.

Conclusions

Self-organized TiO2 NT arrays were prepared through an electrochemistry process, and they were taken as nano-containers to load CeO2 raw materials. After thermal treatment, well-distributed CeO2 NPs were successfully obtained and loaded onto TiO2 NT arrays, forming CeO2/TiO2 heterojunction films. The formation of cubic CeO2 and anatase TiO2 were confirmed by XRD. Microscopic morphologies of different CeO2/TiO2 heterojunction are characterized by SEM, which shows the CeO2 NPs were tightly deposited both around the tube and inside the inner wall of the TiO2 NT arrays. The successful preparation of CeO2/TiO2 heterojunction films were also confirmed by EDS and Raman spectra. In summary, this study provides a simple method to prepare CeO2/TiO2 heterojunction films with good morphology, heterogeneous stability, and low cost, which would be promising for environmental and energy-related applications.

Notes

Funding

This work was financially supported by the National R&D Program of China under No. 2017YFA0207400; National Key Research and Development Plan under No. 2016YFA0300801; National Natural Science Foundation of China under Nos. 51502033, 61571079, 61131005, and 51572042; National Basic Research Program of China under Grant No. 2012CB933104; 111 Project No. B13042; International Cooperation Projects under Grant No. 2015DFR50870; and the Science and Technology project of Sichuan Province No. 2017JY0002.

Availability of Data and Materials

They are all in the main text and figures.

Authors’ Contributions

YL conceived and supervised the research. BY conducted the experiments and wrote the manuscript. DZ, XW, YL, QW, HZ, and ZZ made the theoretical analysis. All the authors discussed the results. All authors read and approved the final manuscript.

Competing Interests

The authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Authors and Affiliations

  1. 1.State Key Laboratory of Electronic Thin Film and Integrated DevicesUniversity of Electronic Science and Technology of ChinaChengduChina
  2. 2.Center for Applied ChemistryUniversity of Electronic Science and Technology of ChinaChengduChina

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