Bioinspired Synthesis of Ce1-x O2:x%Cu2+ Nanobelts for CO Oxidation and Organic Dye Degradation.

Ce1-x O2:x%Cu2+ nanobelts were bioinspired, designed, and fabricated using commercial filter papers as scaffolds by adding Cu(NO3)2 in the original sol solution of CeO2 nanobelts, which display excellent catalyst properties for CO oxidation and photocatalytic activity for organic dyes. Compared with pure CeO2, CuO belts were synthesized using the same method and the corresponding Ce0.5O2:50%Cu2+ bulk materials were synthesized without filter paper as scaffolds; the synthesized Ce1-x O2:x%Cu2+ nanobelts, especially Ce0.5O2:50%Cu2+ nanobelts, can decrease the reaction temperature of CO to CO2 at 100 °C with the conversion rate of 100%, much lower than the formerly reported kinds of Ce1-x O2:x%Cu2+ catalysts. Meanwhile, the synthesized Ce1-x O2:x%Cu2+ nanobelts also display better photocatalytic activity for organic dyes. All of these results provide useful information for the potential applications of the synthesized Ce1-x O2:x%Cu2+ nanobelts in catalyst fields.

Introduction

Because of the special electronic, optical, and chemical characteristics arising from 4f electrons, CeO2 nanomaterials have been widely investigated in light of their kinds of applications.[1−3] They have been well recognized as an efficient oxidation catalyst and catalyst support for their high oxygen storage capacity and excellent redox properties arising from their fast switching between the redox couple Ce3+/Ce4+.[4] As a strategy to improve the catalytic performance of CeO2, incorporation of additional dopants (Zr, Cu, La, Mn, etc.) in the CeO2 support has been proven to be a good method to enhance the physicochemical and catalytic properties of composites. The thermal stability, redox properties, surface area, and other properties of the composites can be adjusted through changing surface element compositions.[5−9] Meanwhile, the doping of additional metal ions can form oxygen vacancies and other defective structures, which can further improve the oxygen-storage capacity and oxygen anion mobility of the final composites,[10−13] resulting in the striking enhancement of catalytic properties in many reactions, such as water–gas shift reactions, preferential CO oxidation, formaldehyde and methane oxidation, and other reactions.[14−16] Up to now, many kinds of dopants have been cultivated to change the surface element composition with the purpose of promoting the active oxygen content on the CeO2 surface, which ultimately reveals obviously the high catalytic activity.[5−9,14−16]

As one kind of cultivated ions, Cu2+ ions doped in CeO2 nanocomposites are of great interest in light of their high catalytic activity for CO oxidation because of the special structure complement of CuO and CeO2.[17−20] The catalytic performance of Ce1–O2:x%Cu2+ composites is much better than those of pure CuO and CeO2 in some aspects, which include promising application in CO oxidation,[17,18] PROX,[19] SO2 catalysis,[20] and so on.[21] Compared with the most effective oxidation catalysts to eliminate CO, noble metal (Pd, Pt, and Rh) catalysts,[22] Ce1–O2:x%Cu2+ composites show high activity, low cost, and relatively affluent properties, which makes them an ideal alternative for CO oxidation. Meanwhile, as a low-valence state material, Cu2+ ions play an important role in Ce1–O2:x%Cu2+ photocatalysts with their stable oxidation state, inducing the charge compensation and the promotion of oxygen vacancies (Vo•) with the Cu2+ impurity phase, leading to higher photocatalytic behavior.[23,24] Meanwhile, the substitution of oxygen vacancies with Ce and Cu states also plays a significant role in the stronger visible catalytic response in the Ce1–O2:x%Cu2+ systems.[25]

Besides the interactions between CuO and CeO2, which can strongly affect the catalytic activity of the final composites, the loading amounts and dispersion states of CuO in the CeO2 matrix can also strikingly influence the interaction between CuO and CeO2.[26] The catalytic performance of the final composites can be improved dramatically by improving the dispersion of CuO particles.[27,28] However, it is difficult to prepare binary oxides with good dispersion owing to the difference of the phase structures and lattices of CuO and CeO2. Meanwhile, the tendency of Cu(II) ions to separate from the CeO2 matrix in the form of nonactive tenorite (CuO) particles makes the loading amount of Cu(II) ions hardly increased in a common procedure to prepare Ce1–O2:x%Cu2+ composites with better properties. Therefore, the choice of the preparation method is very important; to date, various methods have been tried, such as coprecipitation,[26] conventional wet impregnation method and deposition–precipitation method,[27] homogeneous thermal decomposition of urea,[28] flame spray pyrolysis,[29] surfactant-template method, and so on.[17] However, there have been a few literature studies reporting the method using natural cellulosic substances as a scaffold to prepare Ce1–O2:x%Cu2+ composites. As one kind of bioinspired synthetic strategy, it is believed that this kind of synthesis method can introduce some excellent properties of biological organisms into artificial materials, providing diverse functional materials with complex morphologies. Among the kinds of natural substances employed as template substrates, natural cellulose substances possess sophisticated three-dimensional morphological hierarchies, which have been adopted as ideal biotemplates or scaffolds to yield various functional nanomaterials.[30,31]

In this paper, Ce1–O2:x%Cu2+ nanobelt composites were synthesized in a yield of about 68% using commercial filter papers as scaffolds by adding Cu(NO3)2 into the original sol solution of CeO2 nanobelts. The scanning electron microscopy (SEM) images indicate that there were no separated CuO particles appearing in the obtained nanobelts when the doping amounts of Cu2+ ions were not higher than 50%. The H2 temperature-programmed reduction (TPR) curves and the catalytic evaluation results indicate that the obtained Ce1–O2:x%Cu2+ complex catalysts have an excellent catalyst effect on CO oxidation. In particular, for the Ce0.5O2:50%Cu2+ sample, which can decrease the reaction temperature of CO to 100 °C with the conversion rate reaching 100% (T100), this temperature is much lower than the formerly reported kinds of Ce1–O2:x%Cu2+ composites. Meanwhile, the photocatalytic activity of the synthesized Ce1–O2:x%Cu2+ composites in the degradation of organic dyes was also evaluated, and the results indicate that under the same conditions, the photocatalytic activity of the synthesized Ce1–O2:x%Cu2+ composites was much better than that of other species. In short, all results indicate that the synthesized Ce1–O2:x%Cu2+ nanobelt composite catalysts may have potential applications in the catalyst field.

Experimental Procedures

Materials and Reagents

Ce(NO3)3·6H2O (99.9%) was purchased from Aladdin Industrial Inc. Cu(NO3)2·3H2O (99.9%) was obtained from Shanghai Jingbao Chemical Corp. Citric acid monohydrate (C6H8O7·H2O) (CA) was provided by Beijing Chemical Corp. All reagents were of analytical grade and used as received without further purification. The filter papers were obtained from Hangzhou Wohua Filter Paper Co., Ltd.; before being used as the scaffold, they were washed by suction filtration of ethanol, followed by drying with airflow.

Synthesis of CeO2 and CeO2–CuO Nanobelts

First, 10 mmol of Ce(NO3)3 and 15 mmol of CA were dissolved in certain volumes (20, 30, and 40 mL) of deionized water to obtain the solution with different concentrations (0.5, 0.33, and 0.25 M), and the obtained solution was maintained at 65 °C for 24 h to form the final sol solution. The preparation of Ce1–(NO3)3:x%Cu2+ sol solution was similar to that of Ce(NO3)3 sol solution except for the use of certain amounts (molar) of Cu(NO3)2 to substitute Ce(NO3)3.

One typical synthesis procedure of the Ce1–O2:x%Cu2+ nanobelts was as follows: first, a piece of pretreated filter paper was placed in a suction filtering unit. Then, Ce1–(NO3)3:x%Cu2+ sol solution with different concentrations was passed through the filter paper. The obtained filter paper was washed using ethanol to remove the surface sol solution. Finally, the filter paper was dried in flowing air, and thin Ce1–(NO3)3:x%Cu2+ gel layers covering the surface of the cellulose fibers were obtained. The resultant Ce1–(NO3)3:x%Cu2+/filter paper composites were calcined at 500 °C for 2 h in air to obtain Ce1–O2:x%Cu2+ composites.

Catalytic Performance on CO

The catalytic performance of Ce1–O2:x%Cu2+ composites on CO was evaluated in a fixed-bed glass reactor using 200 mg of composites and a total inlet flow of 100 mL/min, with 0.5% CO, 10% O2, and 89.5% Ar. The catalytic performance was evaluated at several temperatures in the range from room temperature to 260 °C. The catalyst was directly exposed to the reaction gas as the reactor temperature was stabilized at the reaction temperature without any pretreatment. The reaction temperature was monitored by a thermocouple placed in the middle of the catalyst bed. The CO concentration in the reactor effluent was analyzed using a Jindao GC-8A gas chromatograph.

Photocatalytic Performances on Rhodamine B

The photocatalytic activity of the synthesized Ce1–O2:x%Cu2+ composites on rhodamine B (RhB) was conducted in a 250 mL cylindrical glass vessel with a 300 W halogen tungsten lamp as the light source (λ ≥ 400 nm). In a typical photocatalytic experiment, 25 mg of Ce1–O2:x%Cu2+ composites were dispersed in 50 mL of RhB (20 mg/L) and H2O2 (5 mL) solution under ultrasonication for 5 min. Then, the mixture was placed in a dark environment with 1 h stirring to obtain adsorption–desorption equilibrium. Every 15 min, 5 mL of suspension was extracted and centrifuged to separate the photocatalyst materials. The UV–vis spectra of the supernatant were recorded using an Agilent Cary 100 to determine the concentration of RhB at each time point.

Other Characterizations

TG measurements were operated on a Pyris Diamond TG with a heating rate of 10 °C/min from 40 to 800 °C. XRD patterns were measured using a Rigaku-D X-ray powder diffractometer with Cu Kα radiation (λ = 1.54 Å). The SEM imaging of the products was examined using a field-emission SEM (HITACHI SU8010). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) imaging were obtained on a JEOL-2100 microscope with an accelerating voltage of 200 kV. XPS experiments were carried out using an ESCALAB 250 spectrometer. The spectra were recorded using monochromatic Al Kα radiation (hv = 1486.6 eV) as the excitation source. Photoelectrons were selected in energy with a hemispheric electron analyzer. All of the binding energy (BE) values were referenced to the C 1s peak of carbon at 284.6 eV. TPR experiments were performed with a thermal conductivity detector, 30 mg samples in a gas mixture of 98% (molar) nitrogen and 2% (molar) hydrogen with a gas flow rate of 100 mL/min, and a temperature range of 80–900 °C with a temperature ramp rate of 5 °C/min. Prior to TPR tests, the samples were treated at 450 °C for 1 h under airflow (ramp rate: 10 °C /min; flow 100 mL/min) to clean the surface.

Results and Discussion

Phase Structure and Morphology

To find a proper temperature to anneal the Ce(NO3)3/filter paper composites, TG measurements were first performed. As can be seen in Figure a, the result indicates that the weight loss mainly occurs in three stages. The first one below 130 °C corresponds to the release of water molecules adsorbed on the sample, and the weight loss is about 20%. The second one is in the range of 130–200 °C, and the weight loss is about 10%, which may be attributed to the progressive dehydration. The third one ranging from 200 to 500 °C with weight loss about 60% should be ascribed to the burning of filter paper, the decomposition of the organic reagent, Ce(NO3)3, and the formation of CeO2. Based on the above TG results, 500 °C was chosen as the annealing temperature because the sample has reached equilibrium before this temperature. Then, an annealing treatment at 500 °C for 2 h with a temperature increase rate of 1 °C/min was performed for the Ce(NO3)3/filter paper composites. The phase structure as well as the morphology of the obtained samples with different concentrations was characterized. Figure b displays the XRD patterns of the annealed samples prepared using Ce(NO3)3 sol solution with two different concentrations, which shows that the formation of pure CeO2 with all diffraction peaks is in good agreement with those of the cubic CeO2 in the JCPDS file PDF no. 34-0394, and no other phase structure can be observed. Meanwhile, it can also be observed that the concentration of Ce(NO3)3 in sol solution can influence the peak intensity of the obtained XRD patterns. When the Ce(NO3)3 concentration in the sol solution decreased to half, the peak intensity of the corresponding XRD pattern also decreased nearly half, and this phenomenon should be due to the decrease in the belt’s thickness, which further influences the crystal structure. Meanwhile, the crystallite size of the two samples was estimated using Scherrer’s equation D = 0.89λ/(β cos θ), in which D is the average grain size, the factor 0.89 is the characteristic of spherical objects, λ is the X-ray wavelength (0.15405 nm), and β and θ are the full width at half-maximum and diffraction angle of the observed peak, respectively. The strongest peak (111) at 2θ (28.555°) was used to calculate the average crystallite size of the calcined CeO2 samples, which decreased with decreasing the Ce(NO3)3 concentration (100 nm for 0.5 M and 71 nm for 0.25 M). This speculation can be further confirmed through the SEM images in Figure .

Figure 1

TG curve of the Ce(NO3)3/filter paper composites (a) and the XRD patterns of the samples prepared using Ce(NO3)3 sol solution with different concentrations and annealed at 500 °C for 2 h with a temperature increase rate of 1 °C/min (b).

Figure 3

SEM images of the annealed CeO2 samples prepared using different concentrations of Ce(NO3)3 sol solution: (a,b) 0.5, (c,d) 0.33, and (e,f) 0.25 M.

The SEM imaging of CeO2 obtained under different conditions as well as filter paper used in the synthesis was first characterized. The SEM imaging in Figure displays the possible influential factor of CA and filter paper on the morphology of the final sample, while the SEM imaging in Figure S1 depicts the variation process of the synthesized samples. Compared with the CeO2 prepared without CA and filter paper (BM) (Figure a), it can be found that there are a large number of holes in the CeO2 prepared with CA and without filter paper as scaffolds (BMCA) besides the increasing size (Figure b). This may be due to the existence of CA, which makes the solution more viscous and leaves a large number of holes at high temperature. However, only particles with different sizes can be obtained for the sample of BM and BMCA, and no belt structure can be observed. When filter paper was used as scaffolds, it can be found that the morphology of the obtained CeO2 is similar with that of the cellulose fiber of the filter paper (Figure c,d), indicating that the usage of filter paper can change the final morphology of CeO2 sample. The morphology changes of the filter paper shown in Figure S1 further confirm the using of filter paper as scaffolds can change the morphology of the final CeO2 sample.

Figure 2

SEM imaging of (a) BM, (b) BMCA, (c) filter paper, and (d) CeO2 nanobelts.

Then, the influence of Ce(NO3)3 concentration in the sol solution on the thickness and surface morphology of the final CeO2 microbelts was studied and is shown in Figure . When the Ce(NO3)3 concentration was controlled at 0.5 or 0.33 M, the annealed samples displayed narrow, uneven microbelts with lots of accumulation and holes on them (Figure a–d). When the Ce(NO3)3 concentration was decreased to 0.25 M, the product mainly consisted of broad micrometer belts with a relatively smooth surface. Comparing with the former two samples (0.5 and 0.3 M Ce(NO3)3), the thickness of the film and the number of holes on the film decreased greatly; meanwhile, the breadth of the microbelts increased significantly, as shown in Figure e,f. This phenomenon should be due to the obvious shrinking of the microbelts during the annealing treatment, and the original morphology of the microbelts could not be retained well because of the decomposition of large amounts of Ce(NO3)3, resulting in the obvious shrinkage of microbelts.

Because of the structure complement of CuO and CeO2 and striking improvement of the catalytic activity when Cu2+ ions were doped in CeO2 nanocomposites, different amounts of Cu2+ ions were added in the original 0.25 M Ce(NO3)3 sol solution to substitute Ce3+ ions, and Ce1–O2:x%Cu2+/filter paper composites were prepared. The TG curves of the synthesized Ce1–(NO3)3:x%Cu2+/filter paper composites are shown in Figure a. It can be found that the doping amounts of Cu2+ ions in the original Ce(NO3)3 sol solution did not change the balance temperature dramatically, and when the annealing temperature reached 500 °C, all of the samples have reached the equilibrium. Thus, 500 °C was further chosen as the annealing temperature in the following annealing treatments of Ce1–(NO3)3:x%Cu2+/filter paper composites. Then, the XRD patterns of the annealed samples with different doping amounts of Cu2+ ions were characterized. As shown in Figure b, it can be observed that when the doping amounts of Cu2+ ions are small (25%), the peaks of CuO are too weak to be observed. With an increase in the doping amounts of Cu2+ ions, the peaks ascribed to CuO become obvious gradually. When the doping amounts of Cu2+ ions are increased to 50%, the diffraction peaks of CuO can be observed in the XRD pattern besides the peaks of CeO2, which can be indexed to PDF card no. 43-1548. The intensity of the CuO diffraction peaks increases with an increase in the doping amounts of Cu2+ ions. When the doping amounts of Cu2+ ions are increased to 100%, pure CuO can be obtained with no other phase structures, indicating that the pure cubic CuO was formed. Furthermore, it can be found that the diffraction peaks become broader for Ce1–O2:x%Cu2+ composites compared with pure CeO2 or pure CuO films. This should be due to the change in the morphology and size of the synthesized Ce1–O2:x%Cu2+ composites, which can be confirmed by the following SEM, TEM, and HRTEM imaging of the samples (Figures and 6).

Figure 4

(a) TG curves of the synthesized Ce1–(NO3)3:x%Cu2+/filter paper composite with different doping amounts of Cu2+ ions. (b) XRD patterns of Ce1–O2:x%Cu2+ composites.

Figure 5

SEM images of the composites: (a,b) CeO2, (c,d) Ce0.75O2:25%Cu2+, (e,f) Ce0.5O2:50%Cu2+, (g,h) Ce0.25O2:75% Cu2+, and (i,j) CuO.

Figure 6

TEM images with different magnifications: (a) 5000×, (b) 20 000×, and (c) 80 000×. (d) HRTEM images of the Ce0.5O2:50%Cu2+ nanobelts.

The SEM imaging of the synthesized Ce1–O2:x%Cu2+ composites is first executed to verify the possible influence on the morphology by doping Cu2+ ions, and the obtained results are shown in Figure . Compared with the SEM images of pure CeO2 (Figure a,b), it can be found that the doping of little amounts of Cu2+ ions can increase the number of the surface rumples in a certain degree (Figure c–f). When the doping amount of Cu2+ ions increases to 50% (Figure e,f), many rumples can be observed in the images, which can increase the surface areas of the samples and further enhance the catalyst properties of the sample by supplying more active sites. However, the number of the surface rumples can hardly increase always with an increase in the doping amounts of Cu2+ ions. When the amounts of Cu2+ ions increase to 75% (Figure g,h), the integrity of the film is destroyed, and many holes can be observed in the microbelts. In the amplified imaging (Figure h), it can be found that surface rumples decrease dramatically and many detritus and particles coexist in the microbelts. For pure CuO (Figure i,j), it can be found that the microbelts are composed of CuO microparticles. These changes in the morphology should be due to an increase in Cu2+ ions, which separate from the CeO2 matrix and form tenorite (CuO) particles. The separation of CuO further influences the catalyst properties of the sample, and this speculation is verified in the following catalyst evaluation.

Meanwhile, the TEM and HRTEM images of Ce0.5O2:50%Cu2+ composites are further characterized to study the details. As shown in Figure , the TEM imaging (Figure a–c) further confirms the repeat of the morphology of the cellulose fibers, and many dark fiber-like parts can be observed (Figure a). Besides, from the enlarged part of the TEM image (Figure c), it can be found that many nanoparticles with size of about 3–5 nm exist in the film uniformly. HRTEM imaging indicates that two kinds of lattice fringes with different d spacings can be observed; for instance, the lattice fringes with a d spacing of 0.3517 nm belong to the lattice fringe of the [113] plane of CuO, while the lattice fringes with a d spacing of 0.306 nm belong to the lattice fringe of the [311] plane of CeO2. Combining the TEM image shown in Figure c, it can be concluded that the nanoparticles should be CuO, which disperse in CeO2 nanobelts uniformly.

Then, the possible influence of the doped Cu2+ ions on the valence state of the composites was also studied, and XPS of the Ce0.5O2:50%Cu2+ composites was performed. Figure depicts the characteristic spin–orbit split of Cu 2p, Ce 3d, O 1s, and C 1s signals for the synthesized composite. As shown, all of the Cu 2p profiles present two sets of peaks, corresponding to Cu 2p3/2 and Cu 2p1/2. A broad Cu 2p3/2 signal composing of two contributions centered at 932.7 (1) and 934.9 eV (2), accompanied by a shakeup satellite in the 939–946 eV region (3 and 4), can be observed. According to previous results,[32] the two peaks centered at 932.7 (1) and 934.9 eV (2) should be ascribed to Cu2+ and reduced copper species (Cu+ or Cu0). However, the chemical states of Cu+ and Cu0, whose peaks appear at 932.4 and 932.6 eV, are not distinguishable on the basis of their Cu 2p3/2 BE.[33] The Auger Cu LMM was further measured to distinguish the different species of Cu (Figure ; Cu LMM). It can be found that two peaks can be observed in the spectrum: a weak peak at 916 eV should correspond to Cu+, while the other one at ∼918 eV should be ascribed to Cu2+. It is reasonable to speculate that the presence of reduced states (Cu+ or Cu0) should depend on the preparation method, especially the existence of citric acid, which has reduction ability. Meanwhile, the formation of Cu+ might also be induced by substitution at the interface of the two oxide phases because of the analogous Ce4+ and Cu+ ionic radii.[34] The Ce 3d spectrum is known to be complicated because of the hybridization of the Ce 4f with ligand O 2p orbitals and fractional occupancy of the valence 4f orbitals. The spectra can be decomposed in nine contributions: 1 and 6 (Ce 3d94f2, O 2p4) and 3 and 8 (Ce 3d94f1, O 2p5); 5 and 9 (final state of Ce 3d94f0, O 2p6) assigned to Ce(IV); 4 (Ce 3d94f2, O 2p5) and 2 and 7(Ce 3d94f1, O 2p6) assigned to Ce(III). The peaks 1, 3, 5, 6, 7, and 9 are associated with the Ce4+ initial state, while the peaks 2, 4, and 7 are ascribed to the Ce3+ initial state of CeO2. It is believed that the reducibility of Ce4+ to Ce3+ forces the copper ions to adapt to a different oxidation state, maintaining the charge balance of the lattice. Therefore, it is reasonable to suggest that the redox cycles between Ce4+/Ce3+ and Cu2+/Cu+ are more facile to occur in this process and play an important role in the catalytic reaction.[35] The O 1s XPS data exhibit a main peak (1) at 529 eV and two smaller peaks (2 and 3) located at 531 and 533 eV, respectively. The main peak (1) can be ascribed to the lattice oxygen of CeO2 and CuO, and the shoulder peak (2) can be attributed to surface chemisorbed oxygen, which contains O–, O2–, O22–, etc. Meanwhile, the smaller peak (3) can be attributed to covalent oxygen, such as surface H2O and CO2.[36] The C 1s spectra have similar features to those of O 1s spectra, which contain a main peak (1) at 284.5 eV and two smaller peaks (2 and 3) at 286 and 288.8 eV, respectively. The main peak (1) can be ascribed to C–C, and the shoulder peak (2) can be attributed to C–O, while the smaller peak (3) can be attributed to O=C–C. The existence of C should be due to the addition of citric acid in the raw materials as well as the CO2 in the air. The energy-dispersive X-ray spectra of the as-synthesized Ce0.5O2:50%Cu2+ nanobelts and the corresponding Ce0.5O2:50%Cu2+ BMCA further confirm the presence of element C in the sample (Figure S2c,d).

Figure 7

XPS of the synthesized Ce0.5O2:50%Cu2+ nanobelts and Cu LMM.

For being a potential catalyst, it is well known that the specific surface area is an important parameter that can influence the number of active sites. The N2 adsorption–desorption isotherms of the synthesized Ce0.5O2:50%Cu2+ nanobelts as well as the corresponding bulk material are synthesized with CA (BMCA) and without CA (BM). As shown in Figure a, the Brunauer–Emmett–Teller (BET) surface area of the nanobelts is measured to be 58 m2/g (BJH desorption), which is much higher than 31 m2/g of BMCA and 23 m2/g of the BM, combining the SEM imaging of the three samples; the addition of CA and the usage of the filter paper as scaffolds facilitate the improvement of the BET surface area. Besides, the pore size distribution curve of the synthesized Ce0.5O2:50%Cu2+ nanobelts was also measured through mercury intrusion porosimetry (Figure b), and the results indicate that the obtained nanobelts have large macroporosity, mainly in the range of 50–400 nm. All of these results indicate that the special nanobelt structure of Ce1–O2:x%Cu2+ composites can supply much more active sites. Then, the evaluation of the catalytic properties of the obtained Ce1–O2:x%Cu2+ composites was executed, including their catalytic properties for CO oxidation and photocatalytic activity for RhB.

Figure 8

(a) N2 adsorption–desorption isotherms of the synthesized Ce0.5O2:50%Cu2+ nanobelts: Ce0.5O2:50%Cu2+ BMCA and Ce0.5O2:50%Cu2+ BM. (b) Pore size distribution curve of the synthesized Ce0.5O2:50%Cu2+ nanobelts.

Catalytic Performance on CO

The H2–TPR profiles of the nanobelts with different doping amounts of Cu2+ ions are compared in Figure . For the pure CeO2 sample, two weak peaks can be observed in the high-temperature area centered at 427 and 688 °C. The low-temperature peak accounts for the surface reduction of cerium atoms from the +4 to +3 oxidation state,[37,38] while the peak at a higher temperature corresponds to the reduction of ceria bulk. They are associated with the reduction of surface capping oxygen and bulk oxygen atoms, respectively. Besides, the position of the reduction peak is much lower than those reported at 450 and 900 °C.[39] For the synthesized CuO sample, a single peak can be observed at around 220 °C, which is also much lower than that reported at 380 °C.[27] Compared with the CeO2 and CuO profiles, the profiles of Ce1–O2:x%Cu2+ nanobelts show a strong reduction peak in the lower temperature region (Figure a,b). Two or three overlapped reduction peaks in the low-temperature reduction region indicate that different copper species are present in the samples. As reported,[40] the peaks located below ∼160 °C should be associated with the presence of the most easily reducible CuO species, which is highly dispersed on the surface of the nanobelts or interacting strongly with ceria, whereas the peaks located in the range of 160–200 °C should be due to larger CuO particles. Finally, the peak centered around ∼220 °C should be assigned to the reduction of bulk CuO. It can be found that when the doping amounts of Cu2+ are lower than 50%, the peaks are mainly located lower than 160 °C, indicating the excellent dispersion of CuO on the nanobelts and good interaction with ceria. When the doping amounts of Cu2+ reach 75%, the position of the peaks red-shift to high temperatures, around 200 °C, indicating the increase in the size of CuO particles. However, this temperature is still much lower than that of literature, which reported the position of the reduction peak was in the ranged of 150–280 °C,[19] combining with the reduction peak positions of pure CeO2 and CuO, which are also much lower than that reported previously,[27,39] it can be concluded that the special nanobelt structure can make the reduction peak shift to low temperatures. These results display that the presence of Cu2+ ions can affect both the surface redox properties of CeO2 and the bulk ones, as demonstrated by Martinez-Arias et al.[36,39] The redox properties of Cu2+ and Ce4+ are mutually affected by the presence of each other, and which further influence their sequential catalyst properties.

Figure 9

(a) TPR curves of Ce1–O2:x%Cu2+ composites as well as the peak fit of the TPR curves of CeO2:25%Cu2+, CeO2:50%Cu2+, and CeO2:75%Cu2+ (b).

Then, the catalytic activities of Ce1–O2:x%Cu2+ nanobelts on CO as a function of reaction temperature were determined, and the obtained results are shown in Figure . It can be noted that the CO conversion rate increases with an increase in the reaction temperature for all samples. For the full CO conversion rate 100% (T100), the temperatures needed for different samples are also different. It is only at 100 °C can the CeO2:50%Cu2+ sample make the CO conversion rate reach 100%. This temperature is much lower than those of other samples. For the selected five Ce1–O2:x%Cu2+ nanobelts and two corresponding bulk materials (BMCA and BM), the needed temperatures to make the conversion rate of CO reach 100% are as follows: Ce0.5O2:50%Cu2+ (100 °C) < Ce0.75O2:25%Cu2+ (109 °C) < Ce0.25O2:75%Cu2+ (121 °C) < CuO (221 °C) < Ce0.5O2:50%Cu2+ BMCA (223 °C) < CeO2 (261 °C) < Ce0.5O2:50%Cu2+ BM (261 °C, 98%). These results indicate that the complex nanobelts of CeO2 and CuO can improve their catalytic activities on CO conversion than those of pure CeO2 or CuO nanobelts as well as the corresponding bulk materials with different morphologies. Meanwhile, the synthesized Ce1–O2:x%Cu2+ nanobelts also show better catalytic activity and lower temperature than the previously reported catalysts used for CO conversion, such asCeO2:Au catalyst, (165 °C),[39] CuO-doped CeO2 nanoparticles (140 °C),[19] copper–ceria nanosheets (122 °C),[41] Cu–ceria nanorods (125 °C),[42] and so on, with the CO conversion rate reaching 100%. The better catalytic property in our samples should be due to the better synergistic interaction between CuO and CeO2 in this nanobelt structure, which is responsible for the higher activity.

Figure 10

Catalytic activities of Ce1–O2:x%Cu2+ nanobelts, Ce0.5O2:50%Cu2+ BMCA, and Ce0.5O2:50%Cu2+ BM on CO as a function of reaction temperature.

Photochemical Performances on Rhodamine B

CuO nanomaterials are also well known for their photocatalytic activity, which can be used in the photodegradation of organic dyes under the irradiation of visible light at an ambient temperature.[43] In this paper, the photocatalytic activity of Ce1–O2:x%Cu2+ complex nanobelts was also evaluated. RhB was chosen as a degradation sample under the irradiation of visible light at an ambient temperature in the presence of hydroxide water (H2O2). The characteristic absorption peak of RhB at 553 nm is monitored to follow the catalytic degradation process, and the optical absorption spectra of RhB measured at different intervals are shown in Figure . For pure RhB aqueous solution, only a slight degradation of RhB (10.88%) can be detected after 90 min irradiation (Figure a). Figure b shows the optical absorption spectra of RhB measured at different intervals only with the H2O2; only 16.06% RhB can be degraded after 90 min irradiation (Figure b). The addition of CeO2 to the RhB and H2O2 solution can enhance the degradation level of RhB, and about 66.72% RhB can be degraded after 90 min irradiation (Figure c). The degradation degree of RhB by adding pure CuO to the solution of RhB and H2O2 is higher than that of CeO2, and about 93.2% RhB can be degraded after 90 min irradiation (Figure d). The degradation performance of the CeO2:50%Cu2+ complex nanobelts is better than that of pure CuO, and about 98.43% RhB can be degraded after 90 min irradiation (Figure e). Meanwhile, the photodegradation property of CeO2:50%Cu2+ BMCA was also evaluated, and only about 49.06% RhB can be degraded after 90 min irradiation, which is much lower than that of the CeO2:50%Cu2+ complex nanobelts. Thus, the degradation degree of RhB at 90 min in the presence of the above four Ce1–O2:x%Cu2+ samples is as follows: Ce0.5O2:50%Cu2+ nanobelts (98.43%) > CuO film (93.2%) > CeO2 film (66.72%) > Ce0.5O2:50%Cu2+ BMCA (49.06%). These results indicate that the complex nanobelts of the CeO2 and CuO can improve their photochemical performances under the same condition (Figure c–e). Meanwhile, the morphology of the samples also plays an important role in the catalytic performance (Figure e,f), and the nanobelt structure of the sample has better photocatalytic activity. This should be due to the higher BET surface area of nanobelts structure of the sample, which can supply more catalytic sites.

Figure 11

Absorption spectra of RhB under the irradiation of visible light with different catalyst samples. (a) Without any catalyst and H2O2, (b) H2O2, (c) CeO2 and H2O2, (d) CuO and H2O2, (e) Ce0.5O2:50%Cu2+ nanobelts and H2O2, and (f) Ce0.5O2:50%Cu2+ BMCA and H2O2. (g) Plot of the extent of photodegradation of RhB, which corresponds to (a–f).

To verify these results, the optical energy band gaps of the synthesized CeO2, Ce0.5O2:50%Cu2+, and CuO nanobelts were further studied, and their UV–vis spectra were characterized. As shown in Figure a,b, it can be found that the UV–visible spectra of the as-prepared CeO2 products occur at 460 nm, while the UV–visible spectra of the as-prepared CuO products occur at 910 nm, which is near to that of Ce0.5O2:50%Cu2+. The band gaps of CeO2, CuO, and Ce0.5O2:50%Cu2+ nanobelts are estimated to be 2.75, 1.36, and 1.32 eV, respectively. The doping of CuO on CeO2 nanobelts can decrease its band gap, which can further enhance the photochemical performances.

Figure 12

(a) UV–vis spectra and (b) plot of photon energy of the as-prepared CeO2, Ce0.5O2:50%Cu2+, and CuO products.

The fundamental photodegradation mechanism involves the acceleration in the decomposition of H2O2 over the catalyst to generate free-radical species, such as •OH, •OOH, or •O2–, which are deemed to be liable for the degradation of the dyes. The related chemical reactions including the electrons (e–) in the VB can be excited to the CB and at the same time can generate the same number of holes (h+) in the VB. The formed e– and h+ pairs can be captured by H2O2 molecules, leading to the formation of •OH, •HOO, or •O2–; oxidant species react with dye and finally realize complete mineralization with the formation of CO2, H2O, or other inorganic ions.[19]

To investigate the possible mechanism for the enhancement of the catalytic performance of the Ce1–O2:x%Cu2+ complex film, the crystal structure of CeO2 and CuO are studied, and their simulated crystal structures are displayed in Figure . It is known that CeO2 is of a cubic fluorite structure (space group Fm3m) with a lattice constant of a = 0.541 nm (Figure a,b). Ce4+ ions arranged with a face-centered cubic phase and O2– atoms occupy all tetrahedra in the cell. Each Ce atom is surrounded by eight O atoms, and every O atom can form ligand bonds with four Ce atoms. This structure is also called as an open structure as there are many cubic spaces in it, and ions can disperse quickly in this crystal structure. For monoclinic CuO (Figure c,d), the crystal can be described as alternating planes composed of O2– and Cu2+ ions, which are stacked alternately along the [002] axis. The oppositely charged ions form positively charged (002)-Cu and negatively charged (002)-O polar on the two sides. For the Ce1–O2:x%Cu2+ complex nanobelts, the O atoms in CeO2 cell can be easily lost, and the presence of oxygen vacancies in the remaining CeO2– cell can induce the charge variation of a small amount of the copper in the lattice as a consequence of charge compensation upon the incorporation of Cu2+. Alternatively, the remaining CeO2– can also be easily oxidized into CeO2 and further reduce the charge variation of the surrounding ions.[44,45] Thus, the active sites at the interface between CuO and CeO2 cyclic changed. For the improvement of the catalytic performance of the synthesized Ce0.5O2:50%Cu2+ nanobelts than the other species, especially the bulk materials, there are mainly several reasons. First, the increasing of the amount of interfacial active sites of Ce0.5O2:50%Cu2+ film by increasing the surface area and the doping amounts of Cu2+. Second, the changing of the physicochemical properties of the Ce0.5O2:50%Cu2+ on the exposed face, which can also influence the catalytic performance of the final sample.

Figure 13

Simulated crystal structures of CeO2 and CuO. (a) Crystal structure of CeO2. (b) Packing view along the c axis of CeO2. (c) Crystal structure of CuO. (d) Packing view along the c axis of CuO.

Conclusions

In this paper, a new method was attempted for the synthesis of CeO2 films using commercial filter papers as the scaffold, and Cu2+ ions were doped in the original Ce(NO3)3 sol solution to prepare Ce1–O2:x%Cu2+ complex nanobelt catalysts. The SEM images indicate that there are no separated CuO particles appearing in the obtained films when the doping amounts of Cu2+ ions are no higher than 50% (mol). The H2–TPR curves and the catalytic evaluation results display that the obtained Ce1–O2:x%Cu2+ catalysts have excellent catalyst property for CO oxidation. Especially for the Ce0.5O2:50%Cu2+ sample, which can decrease the reaction temperature of CO to 100 °C with the conversion rate reaching to 100%, this temperature was much lower than the formerly reported Ce1–O2:x%Cu2+ catalysts or other metal ion-doped CeO2 matrix. Meanwhile, the synthesized Ce0.5O2:50%Cu2+ has also enhanced photochemical performance compared with pure CeO2 or CuO belts obtained under the same condition, and the morphology of the sample can also influence the catalytic properties. These results indicate that the synthesized Ce1–O2:x%Cu2+ films may find potential applications in the catalyst field. Meanwhile, the demonstrated method also can be extendable to the synthesis of other inorganic oxide compounds.