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n-type CeO2 heterojunctions with enhanced photocatalytic activity
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity Zheng-Mei Yang, Su-Cheng Hou, Gui-Fang Huang n, Hui-Gao Duan n, Wei-Qing Huang Department of Applied Physics, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 April 2014 Accepted 27 June 2014
Novel p-type NiO/n-type CeO2 heterojunctions, with cubic NiO particles embedded on the CeO2 nanofibers, were successfully prepared by the electrospinning technique. The photocatalytic activity for MB degradation under UV light irradiation of the NiO/CeO2 heterojunction is much higher than that of pure NiO or CeO2. The rate constant of MB degradation by NiO/CeO2 is about 4 times and 2 times than those of pure NiO and CeO2 under UV light irradiation, respectively. The excellent photocatalytic activity of the NiO/CeO2 heterostructures is closely related to the fast transfer and efficient separation of electron–hole pairs between NiO and CeO2 due to the formation of the heterojunction and their matching band positions. & 2014 Published by Elsevier B.V.
Keywords: Heterostructures Semiconductors Nanocomposites Nanofibers Electrospinning Photocatalytic activity
1. Introduction Semiconductor photocatalysis is regarded as a promising avenue toward solving the worldwide energy shortage and environment pollution with abundant solar light [1,2]. Of the well-known photocatalysts, cerium dioxide (CeO2), naturally n-type semiconductors with a wide bandgap (Eg ¼3.2 eV), has been deemed as one of the most promising photocatalysts for the degradation of organic pollutants and water splitting for hydrogen generation owing to its high incident photon-to-electron conversion efficiencies, low cost, remarkable chemical stabilities along with safety advantage [3,4]. Nevertheless, the photocatalytic activity of CeO2 system is largely limited by the quick recombination of the photoinduced electrons and holes [5]. To overcome this bottleneck, an effective strategy is the coupling of two semiconductors, CeO2 and another semiconductor with the matching band positions, to form heterojunctions to increase the separation efficiency of photogenerated electron–hole pairs of CeO2 photocatalysts [6,7]. So far, various CeO2-based heterojunctions, such as Ag3PO4–CeO2 [5], ZnO–CeO2 [8] and BiVO4–CeO2 [9], have been investigated to suppress the recombination of photogenerated electron–hole pairs by the mutual transfer of photogenerated electrons or holes in the heterojunctions [10–12]. It is known that nickel oxide (NiO), a p-type semiconductor (Eg ¼ 3.5 eV) with high hole mobility and low lattice mismatch with CeO2, is conductive to
n
Corresponding authors. E-mail addresses: [email protected] (G.-F. Huang), [email protected] (H.-G. Duan).
the fabrication of p–n heterojunction with CeO2 [13,14]. Theoretically, the formed p–n heterojunction could provide a potential driving force and facilitate the transfer of electron–hole pairs the owing to the building of the internal electric field, with its field direction from the n-type CeO2 to the p-type NiO [14–17]. Thus, it is significant to synthesize p-type NiO/n-type CeO2 heterojunction, which might facilitate the easier transfer and efficient separation of photogenerated electron–hole pairs and improve the photocatalytic efficiency. Herein, novel p-type NiO/n-type CeO2 heterostructures were successfully prepared by the electrospinning technique. It is interesting that the cubic NiO nanoparticles are uniformly embedded in the CeO2 nanofibers and form a heterostructure. Moreover, the synthesized NiO/CeO2 heterojunction photocatalyst displays much higher activity than that of single NiO or CeO2 under the irradiation of UV light. To the best of our knowledge, this is the first report on the electrospinning preparation of NiO/ CeO2 heterojunction photocatalyst.
2. Experimental section The p-type NiO/n-type CeO2 heterojunctions were synthesized using an electrospinning and calcination method. In a typical procedure, 0.0005 mol Ni(CH3COO)2 and 0.0005 mol CeCl3 were mixed with 2.2 g ethanol and 2.2 g N,N-dimethylformamide (DMF) under vigorous stirring for 30 min. Subsequently, 0.38 g of poly (vinyl pyrrolidone) (PVP, Mw ¼300,000) was added to the above solution. After stirring for 12 h, the precursor solution was transferred to a syringe for electrospinning. The positive terminal of a
http://dx.doi.org/10.1016/j.matlet.2014.06.169 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Yang Z-M, et al. Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.169i
Z.-M. Yang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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variable highvoltage power supply was connected to the needle tip of the syringe while the other terminal was connected to the collector plate. In a typical electrospinning process, the precursor solution was electrospun at 20 kV voltage, 20 cm working distance, and 0.4 mL/h flow rate at room temperature. The as-spun samples were calcined at 500 1C for 3 h in air to obtain NiO/CeO2 heterojunctions with original Ni/Ce molar ratios of 1. For comparision, pure CeO2 and NiO were also prepared using similar experimental conditions. The crystal structure of the samples was characterized by a power X-ray diffraction (XRD, Siemens D-5000 diffractometer with Cu Kα irradiation) and high-resolution transmission electron microscopy (HRTEM, JEOL JSM-2010F). The morphological details of the prepared samples were investigated by field emission scanning electron microscopy (FESEM, S-4800) and transmission electron microscopy (TEM, JEOL JEM 2010F). The Brunauer– Emmett–Teller (BET) specific surface area of the samples was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The photocatalytic performance for the degradation of methylene blue (MB, 10 mg/L, 80 mL) with 30 mg photocatalysts (CeO2, NiO or NiO/CeO2) under UV light was explored. A 300 W UV lamp was chosen as the UV light source. Prior to irradiation, solutions suspended with photocatalysts were sonicated in the dark for 10 min to ensure the adsorption–desorption equilibrium of MB on the surface of the photocatalysts. During irradiation, the samples were withdrawn at regular time intervals and centrifuged to remove the catalysts. The photodegradation efficiency was monitored by measuring the absorbance of the solution samples at its characteristic absorption wavelength of 663 nm (MB) with a UV–Vis spectrophotometer at room temperature.
CeO2
3. Results and discussion The typical SEM images of pure CeO2, NiO and the NiO/CeO2 heterojunction are shown in Fig. 1. It can be seen that the as-spun CeO2 nanofibers (Fig. 1a), with diameters of 110 nm, possess smooth and uniform surfaces, whereas the NiO nanofibers (Fig. 1b), with diameter ranging from 50 to 100 nm, are rough and consist of smaller nanoparticles. The SEM image in Fig. 1c and the TEM micrograph shown in the inset of Fig. 1c reveal that the NiO/CeO2 heterojunction is composed of two distinct phases. The cubic-shaped particles are NiO. It is noticeable that the NiO nanoparticles are uniformly embedded in the CeO2 nanofibers and form a heterostructure. To further confirm the crystallographic structure of the NiO/CeO2 heterojunction, HRTEM measurement was carried out. The HRTEM images of NiO/CeO2 heterojunction in Fig. 1d show two distinct sets of lattice fringes. The uniform lattice fringes have spacings corresponding to the (1 1 1) plane of cubic fluorite-type CeO2 and the (1 1 1) plane of cubic NiO, respectively. The XRD patterns of all the samples are shown in Fig. 2. It is observed that all of the diffraction peaks of pure CeO2 correspond to the cubic fluorite-type CeO2 structure (JCPDS no. 81-0792), while those of NiO can be indexed to the body-centered cubic structure of NiO (JCPDS no. 78-0643). As expected, the XRD pattern of the NiO/CeO2 composite shows the mixed patterns of CeO2 and NiO, and excludes the possibility of any third phase formation, indicating that the NiO/CeO2 heterojunction has been successfully prepared. The photocatalytic activity of these samples was evaluated by photodegradation of MB. The degradation efficiency of MB over pure CeO2, NiO and the NiO/CeO2 heterojunction photocatalyst, or without the photocatalyst, under UV light irradiation is presented
NiO d=0.24nm NiO CeO2 d=0.31nm
Fig. 1. SEM images of (a) CeO2, (b) NiO, (c) NiO/CeO2 heterojunction, and (d) HRTEM image of the NiO/CeO2 heterojunction.
Please cite this article as: Yang Z-M, et al. Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.169i
Z.-M. Yang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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in Fig. 3. The results show that the decrease in the concentration of MB without photocatalyst is negligible under UV light irradiation, testifying the stabilization of MB. The degradation efficiency of MB obviously increases with the addition of photocatalyst. It can be seen that 96.0% of the MB is photocatalytically degraded after 40 min irradiation for the NiO/CeO2 sample. However, for the pure CeO2 and NiO samples, the MB is degraded by only 69.2% and 47.3%, respectively. The average rates of the MB decomposition over NiO, CeO2 and NiO/CeO2 are estimated to be about 0.017 min 1, 0.033 min 1 and 0.068 min 1, respectively. It is obvious that the NiO/CeO2 heterojunction exhibits the highest photocatalytic degradation efficiency, followed by pure CeO2 and NiO. In addition, the specific surface area of pure CeO2 is evaluated to be about 18.9 m2/g by BET measurement, which is a little larger than that of the NiO/CeO2 heterojunction (about 18.1 m2/g). Thus the possible reason for the enhanced photocatalytic activity of NiO/CeO2 could be attributed to the effective transfer and separation of the electron–hole pairs due to the formation of heterojunction between CeO2 and NiO. The band gap structure of p-type NiO/n-type CeO2 is elucidated in Fig. 4 to clarify its enhanced photocatalytic activity. When the p–n heterojunction formed, the holes in NiO with high hole
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2 Theta (degree) Fig. 2. XRD patterns of the prepared NiO, CeO2 and CeO2/NiO heterojunction.
Fig. 4. Schematic diagram showing the energy band structure and electron–hole pair separation in the p-type NiO/n-type CeO2 heterojunction.
mobility will transfer to CeO2, while the electrons will transfer from CeO2 to NiO owing to the concentration gradient of carriers. Meanwhile, an inner electric field, with its field direction from the n-type CeO2 to the p-type NiO, is built in the interface of p–n heterojunctions with the transfer of electrons and holes. The net carrier diffusion between the NiO and CeO2 will stop until the system attains equilibration. During the photocatalytic degradation process, CeO2 and NiO can be simultaneously photoexcited and then generate the same amount of electrons and hole in the CB and VB, respectively. The photogenerated electrons in the NiO particles quickly transfer to the CB of CeO2 and, conversely, the photogenerated holes in the CeO2 nanofibers quickly transfer to the VB of NiO due to their matching band potential, resulting in the efficient separation of the photogenerated charge carriers. Subsequently, the holes in the VB of NiO can oxidize OH to form OH radicals. Meanwhile, the accumulated electrons in the CB of CeO2 can be transferred to O2 molecules adsorbed on the surface of the heterojunction and react with H þ to produce active OH radicals. These formed OH radicals will further oxidize the MB molecules. As discussed above, the coupling of CeO2 and NiO to form heterojunction is helpful to improve the transfer of photoexcited electron–hole pairs, inhibit the recombination of photoinduced carriers and facilitate to produce more OH radicals, resulting in the enhanced photocatalytic activity of NiO/CeO2 heterojunction.
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Time (min) Fig. 3. MB concentration changes over photocatalyst-free solution, NiO, CeO2 and the NiO/CeO2 heterojunction.
In conclusion, p-type NiO/n-type CeO2 heterostructures were prepared by the electrospinning. The unique p–n heterojunction structure enhances the interaction between NiO and CeO2. The prepared NiO/CeO2 heterojunction exhibits excellent photocatalytic activity for organic contaminant degradation under UV light irradiation. The rate constant of MB degradation over the NiO/ CeO2 heterojunction is much faster than those over pure NiO and CeO2 by a factor of 4 and 2 under UV light irradiation, respectively. The enhanced photocatalytic activity can be attributed to the easier transfer and separation of photogenerated electron–hole pairs between NiO and CeO2 due to the formation of the heterojunction. Thus, it is expected that the p-type NiO/n-type CeO2 heterojunction with excellent photocatalytic activity may greatly promote their industrial application in the elimination of organic pollutants from wastewater.
Please cite this article as: Yang Z-M, et al. Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.169i
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 11274107, 61204109), Program for New Century Excellent Talents in University (Grant no. NCET13-0185), Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant no. 201318), Interdisciplinary Program of Hunan University, and Science and Technology Plan Projects of Hunan Province (2013SK3148). References
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Shen ZY, Chen G, et al. J Mater Chem 2012;22:19646. Yang ZM, Huang GF, et al. J Mater Chem A 2014;2:1750–6. Zhang J, Li L, et al. J Mater Chem 2012;22:10480–7. Hu S, Zhou F, et al. Catal Commun 2011;12:794–7. Li C, Chen R, et al. Mater Lett 2011;65:1327–30. Wetchakun N, Chaiwichain S, et al. ACS Appl Mater Interfaces 2012;4:3718–23. Pouretedal HR, Tofangsazi Z, et al. J Alloys Compd 2012;513:359–64. Li CR, Chen R, et al. Mater Lett 2011;65:1327–30. Cao TP, Li YJ, et al. Mater Res Bull 2010;45:1406–12. Pan C, Ding R, et al. Physica E 2013;54:138–43. Zhang ZY, Shao CL, et al. ACS Appl Mater Interfaces 2010;2:2915–23. Luo CZ, Li DL, et al. RSC Adv 2014;4:3090–5. Tian FS, Liu YL. Scr Mater 2013;69:417–9. Ibupoto ZH, Abbasi MA, et al. J Nanomater 2014:928658.
[1] Mohamed SH, El-Hagary M, et al. Eur Phys J Appl Phys 2012;57:20301. [2] Yang ZM, Tian Y, et al. Mater Lett 2014;116:209–11. [3] Hoang LH, Hai PV, et al. Mater Lett 2011;65:3047–50.
Please cite this article as: Yang Z-M, et al. Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.169i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity Zheng-Mei Yang, Su-Cheng Hou, Gui-Fang Huang n, Hui-Gao Duan n, Wei-Qing Huang Department of Applied Physics, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, Hunan University, Changsha 410082, China
art ic l e i nf o
a b s t r a c t
Article history: Received 8 April 2014 Accepted 27 June 2014
Novel p-type NiO/n-type CeO2 heterojunctions, with cubic NiO particles embedded on the CeO2 nanofibers, were successfully prepared by the electrospinning technique. The photocatalytic activity for MB degradation under UV light irradiation of the NiO/CeO2 heterojunction is much higher than that of pure NiO or CeO2. The rate constant of MB degradation by NiO/CeO2 is about 4 times and 2 times than those of pure NiO and CeO2 under UV light irradiation, respectively. The excellent photocatalytic activity of the NiO/CeO2 heterostructures is closely related to the fast transfer and efficient separation of electron–hole pairs between NiO and CeO2 due to the formation of the heterojunction and their matching band positions. & 2014 Published by Elsevier B.V.
Keywords: Heterostructures Semiconductors Nanocomposites Nanofibers Electrospinning Photocatalytic activity
1. Introduction Semiconductor photocatalysis is regarded as a promising avenue toward solving the worldwide energy shortage and environment pollution with abundant solar light [1,2]. Of the well-known photocatalysts, cerium dioxide (CeO2), naturally n-type semiconductors with a wide bandgap (Eg ¼3.2 eV), has been deemed as one of the most promising photocatalysts for the degradation of organic pollutants and water splitting for hydrogen generation owing to its high incident photon-to-electron conversion efficiencies, low cost, remarkable chemical stabilities along with safety advantage [3,4]. Nevertheless, the photocatalytic activity of CeO2 system is largely limited by the quick recombination of the photoinduced electrons and holes [5]. To overcome this bottleneck, an effective strategy is the coupling of two semiconductors, CeO2 and another semiconductor with the matching band positions, to form heterojunctions to increase the separation efficiency of photogenerated electron–hole pairs of CeO2 photocatalysts [6,7]. So far, various CeO2-based heterojunctions, such as Ag3PO4–CeO2 [5], ZnO–CeO2 [8] and BiVO4–CeO2 [9], have been investigated to suppress the recombination of photogenerated electron–hole pairs by the mutual transfer of photogenerated electrons or holes in the heterojunctions [10–12]. It is known that nickel oxide (NiO), a p-type semiconductor (Eg ¼ 3.5 eV) with high hole mobility and low lattice mismatch with CeO2, is conductive to
n
Corresponding authors. E-mail addresses: [email protected] (G.-F. Huang), [email protected] (H.-G. Duan).
the fabrication of p–n heterojunction with CeO2 [13,14]. Theoretically, the formed p–n heterojunction could provide a potential driving force and facilitate the transfer of electron–hole pairs the owing to the building of the internal electric field, with its field direction from the n-type CeO2 to the p-type NiO [14–17]. Thus, it is significant to synthesize p-type NiO/n-type CeO2 heterojunction, which might facilitate the easier transfer and efficient separation of photogenerated electron–hole pairs and improve the photocatalytic efficiency. Herein, novel p-type NiO/n-type CeO2 heterostructures were successfully prepared by the electrospinning technique. It is interesting that the cubic NiO nanoparticles are uniformly embedded in the CeO2 nanofibers and form a heterostructure. Moreover, the synthesized NiO/CeO2 heterojunction photocatalyst displays much higher activity than that of single NiO or CeO2 under the irradiation of UV light. To the best of our knowledge, this is the first report on the electrospinning preparation of NiO/ CeO2 heterojunction photocatalyst.
2. Experimental section The p-type NiO/n-type CeO2 heterojunctions were synthesized using an electrospinning and calcination method. In a typical procedure, 0.0005 mol Ni(CH3COO)2 and 0.0005 mol CeCl3 were mixed with 2.2 g ethanol and 2.2 g N,N-dimethylformamide (DMF) under vigorous stirring for 30 min. Subsequently, 0.38 g of poly (vinyl pyrrolidone) (PVP, Mw ¼300,000) was added to the above solution. After stirring for 12 h, the precursor solution was transferred to a syringe for electrospinning. The positive terminal of a
http://dx.doi.org/10.1016/j.matlet.2014.06.169 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Yang Z-M, et al. Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.169i
Z.-M. Yang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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variable highvoltage power supply was connected to the needle tip of the syringe while the other terminal was connected to the collector plate. In a typical electrospinning process, the precursor solution was electrospun at 20 kV voltage, 20 cm working distance, and 0.4 mL/h flow rate at room temperature. The as-spun samples were calcined at 500 1C for 3 h in air to obtain NiO/CeO2 heterojunctions with original Ni/Ce molar ratios of 1. For comparision, pure CeO2 and NiO were also prepared using similar experimental conditions. The crystal structure of the samples was characterized by a power X-ray diffraction (XRD, Siemens D-5000 diffractometer with Cu Kα irradiation) and high-resolution transmission electron microscopy (HRTEM, JEOL JSM-2010F). The morphological details of the prepared samples were investigated by field emission scanning electron microscopy (FESEM, S-4800) and transmission electron microscopy (TEM, JEOL JEM 2010F). The Brunauer– Emmett–Teller (BET) specific surface area of the samples was analyzed by nitrogen adsorption in a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The photocatalytic performance for the degradation of methylene blue (MB, 10 mg/L, 80 mL) with 30 mg photocatalysts (CeO2, NiO or NiO/CeO2) under UV light was explored. A 300 W UV lamp was chosen as the UV light source. Prior to irradiation, solutions suspended with photocatalysts were sonicated in the dark for 10 min to ensure the adsorption–desorption equilibrium of MB on the surface of the photocatalysts. During irradiation, the samples were withdrawn at regular time intervals and centrifuged to remove the catalysts. The photodegradation efficiency was monitored by measuring the absorbance of the solution samples at its characteristic absorption wavelength of 663 nm (MB) with a UV–Vis spectrophotometer at room temperature.
CeO2
3. Results and discussion The typical SEM images of pure CeO2, NiO and the NiO/CeO2 heterojunction are shown in Fig. 1. It can be seen that the as-spun CeO2 nanofibers (Fig. 1a), with diameters of 110 nm, possess smooth and uniform surfaces, whereas the NiO nanofibers (Fig. 1b), with diameter ranging from 50 to 100 nm, are rough and consist of smaller nanoparticles. The SEM image in Fig. 1c and the TEM micrograph shown in the inset of Fig. 1c reveal that the NiO/CeO2 heterojunction is composed of two distinct phases. The cubic-shaped particles are NiO. It is noticeable that the NiO nanoparticles are uniformly embedded in the CeO2 nanofibers and form a heterostructure. To further confirm the crystallographic structure of the NiO/CeO2 heterojunction, HRTEM measurement was carried out. The HRTEM images of NiO/CeO2 heterojunction in Fig. 1d show two distinct sets of lattice fringes. The uniform lattice fringes have spacings corresponding to the (1 1 1) plane of cubic fluorite-type CeO2 and the (1 1 1) plane of cubic NiO, respectively. The XRD patterns of all the samples are shown in Fig. 2. It is observed that all of the diffraction peaks of pure CeO2 correspond to the cubic fluorite-type CeO2 structure (JCPDS no. 81-0792), while those of NiO can be indexed to the body-centered cubic structure of NiO (JCPDS no. 78-0643). As expected, the XRD pattern of the NiO/CeO2 composite shows the mixed patterns of CeO2 and NiO, and excludes the possibility of any third phase formation, indicating that the NiO/CeO2 heterojunction has been successfully prepared. The photocatalytic activity of these samples was evaluated by photodegradation of MB. The degradation efficiency of MB over pure CeO2, NiO and the NiO/CeO2 heterojunction photocatalyst, or without the photocatalyst, under UV light irradiation is presented
NiO d=0.24nm NiO CeO2 d=0.31nm
Fig. 1. SEM images of (a) CeO2, (b) NiO, (c) NiO/CeO2 heterojunction, and (d) HRTEM image of the NiO/CeO2 heterojunction.
Please cite this article as: Yang Z-M, et al. Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.169i
Z.-M. Yang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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in Fig. 3. The results show that the decrease in the concentration of MB without photocatalyst is negligible under UV light irradiation, testifying the stabilization of MB. The degradation efficiency of MB obviously increases with the addition of photocatalyst. It can be seen that 96.0% of the MB is photocatalytically degraded after 40 min irradiation for the NiO/CeO2 sample. However, for the pure CeO2 and NiO samples, the MB is degraded by only 69.2% and 47.3%, respectively. The average rates of the MB decomposition over NiO, CeO2 and NiO/CeO2 are estimated to be about 0.017 min 1, 0.033 min 1 and 0.068 min 1, respectively. It is obvious that the NiO/CeO2 heterojunction exhibits the highest photocatalytic degradation efficiency, followed by pure CeO2 and NiO. In addition, the specific surface area of pure CeO2 is evaluated to be about 18.9 m2/g by BET measurement, which is a little larger than that of the NiO/CeO2 heterojunction (about 18.1 m2/g). Thus the possible reason for the enhanced photocatalytic activity of NiO/CeO2 could be attributed to the effective transfer and separation of the electron–hole pairs due to the formation of heterojunction between CeO2 and NiO. The band gap structure of p-type NiO/n-type CeO2 is elucidated in Fig. 4 to clarify its enhanced photocatalytic activity. When the p–n heterojunction formed, the holes in NiO with high hole
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Fig. 4. Schematic diagram showing the energy band structure and electron–hole pair separation in the p-type NiO/n-type CeO2 heterojunction.
mobility will transfer to CeO2, while the electrons will transfer from CeO2 to NiO owing to the concentration gradient of carriers. Meanwhile, an inner electric field, with its field direction from the n-type CeO2 to the p-type NiO, is built in the interface of p–n heterojunctions with the transfer of electrons and holes. The net carrier diffusion between the NiO and CeO2 will stop until the system attains equilibration. During the photocatalytic degradation process, CeO2 and NiO can be simultaneously photoexcited and then generate the same amount of electrons and hole in the CB and VB, respectively. The photogenerated electrons in the NiO particles quickly transfer to the CB of CeO2 and, conversely, the photogenerated holes in the CeO2 nanofibers quickly transfer to the VB of NiO due to their matching band potential, resulting in the efficient separation of the photogenerated charge carriers. Subsequently, the holes in the VB of NiO can oxidize OH to form OH radicals. Meanwhile, the accumulated electrons in the CB of CeO2 can be transferred to O2 molecules adsorbed on the surface of the heterojunction and react with H þ to produce active OH radicals. These formed OH radicals will further oxidize the MB molecules. As discussed above, the coupling of CeO2 and NiO to form heterojunction is helpful to improve the transfer of photoexcited electron–hole pairs, inhibit the recombination of photoinduced carriers and facilitate to produce more OH radicals, resulting in the enhanced photocatalytic activity of NiO/CeO2 heterojunction.
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Time (min) Fig. 3. MB concentration changes over photocatalyst-free solution, NiO, CeO2 and the NiO/CeO2 heterojunction.
In conclusion, p-type NiO/n-type CeO2 heterostructures were prepared by the electrospinning. The unique p–n heterojunction structure enhances the interaction between NiO and CeO2. The prepared NiO/CeO2 heterojunction exhibits excellent photocatalytic activity for organic contaminant degradation under UV light irradiation. The rate constant of MB degradation over the NiO/ CeO2 heterojunction is much faster than those over pure NiO and CeO2 by a factor of 4 and 2 under UV light irradiation, respectively. The enhanced photocatalytic activity can be attributed to the easier transfer and separation of photogenerated electron–hole pairs between NiO and CeO2 due to the formation of the heterojunction. Thus, it is expected that the p-type NiO/n-type CeO2 heterojunction with excellent photocatalytic activity may greatly promote their industrial application in the elimination of organic pollutants from wastewater.
Please cite this article as: Yang Z-M, et al. Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.169i
Z.-M. Yang et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant nos. 11274107, 61204109), Program for New Century Excellent Talents in University (Grant no. NCET13-0185), Foundation for the Author of National Excellent Doctoral Dissertation of China (Grant no. 201318), Interdisciplinary Program of Hunan University, and Science and Technology Plan Projects of Hunan Province (2013SK3148). References
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Please cite this article as: Yang Z-M, et al. Electrospinning preparation of p-type NiO/n-type CeO2 heterojunctions with enhanced photocatalytic activity. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.06.169i