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Bi2O3 heterojunction and their solar light photocatalytic performance
Materials Research Bulletin 64 (2015) 82–87
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Room-temperature solid state synthesis of ZnO/Bi2O3 heterojunction and their solar light photocatalytic performance Xin Wang a, * , Pengrong Ren b , Huiqing Fan c a Shaanxi Province Thin Film Technology and Optical Test Open Key Laboratory, School of Photoelectrical Engineering, Xi’an Technological University, Xi’an 710032, China b School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China c State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 July 2014 Received in revised form 17 November 2014 Accepted 16 December 2014 Available online 18 December 2014
ZnO/Bi2O3 heterojunction was prepared by solid state reaction method at room temperature. The asprepared nanostructures were characterized as the assembled nanosheets of ZnO on which Bi2O3 nanoparticles were well dispersed. Further studies revealed that the morphology and photocatalytic property could be regulated by tuning the amount of Bi2O3. The introduction of Bi2O3 can lead to the enhancement of solar light absorption, and the 5% ZnO/Bi2O3 samples exhibited the best photocatalytic activity under the solar light irradiation. The excellent photocatalytic performances could be ascribed to the synergistic effects of the hierarchical nanostructures and the effective separation of photogenerated carriers. Moreover, the hydroxyl radicals (OH) are found to be the main active species generated in the oxidation reaction of RhB over ZnO/Bi2O3 photocatalyst. Therefore, our work demonstrated a facile way to prepare hierarchical nanostructures with excellent photocatalytic performance using solid state method at room temperature. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Nanostructures B. Chemical synthesis D. Catalytic properties D. Microstructure
1. Introduction Oxide semiconductor photocatalysis is an environmentally friendly approach toward solving the energy shortage and realizing environmental remediation issues [1–3]. Considering the advantages of low cost and easy synthesis, ZnO has become a potential photocatalyst for industry. However, the universal use of ZnO is still limited due to its wide band gap and quick recombination of charge carriers [4]. And semiconductor heterojunction can be used to conquer these problems and improve the photoactivity efficiency by maximum utilization of spectral portion of the visible light [5,6]. On the one hand, semiconductors with narrow gap are employed to sensitize the wide gap ones. On the other hand, different band gaps can make photogenerated carriers flow from one semiconductor to another, resulting long time and effective separation. Therefore, the selection of heterostructure materials plays an important role in improving the photocatalytic property of the host materials. Here, we choose Bi2O3, which is an important p-type semiconductor photocatalyst with a good conducting ability and a narrow band gap of 2.8 eV [7]. It is
expected that ZnO/Bi2O3 heterojunction could improve the photocatalytic properties of pure ZnO. To date, various methods have been employed to regulate morphology of ZnO with the aim of tuning their performance, such as electrospinning [8], hydrothermal [9], solvothermal [10], sol–gel [11], solution combustion [12], etc. However, these synthetic methods are solution based and usually involve rigorous experimental conditions, and the as-synthesized products usually show poor photocatalytic activity [13]. Therefore, it is of great necessary to develop simple and reliable synthetic methods for the synthesis of heterojunction with designed chemical components and controlled morphologies, which can affect the properties of nanomaterials strongly. On the basis of the above discussion, ZnO/Bi2O3 heterojunction were prepared through a simple solid state reaction. The introduction quantity of Bi2O3 in the heterojunction system was found to play a great role in the morphology and photocatalytic activity-control. 2. Experimental 2.1. Preparation of ZnO/Bi2O3 heterojunction.
* Corresponding author. Tel.: +86 29 86173335; fax: +86 29 86173335. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.materresbull.2014.12.037 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
All reagents were analytically pure and used without further purification. In a typical procedure, certain Zn(CH3COO)22H2O
X. Wang et al. / Materials Research Bulletin 64 (2015) 82–87
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Abingdon, Oxfordshire, UK) and high resolution transmission electron microscopy (HRTEM; Tecnai F30G2, FEI, Hillsboro, OR, USA). UV–vis diffuse reflectance spectra (DRS) of the samples were recorded with an UV–vis spectrophotometer (UV3150; Shimadzu Corporation, Kyoto, Japan) using BaSO4 as reference. The ultraviolet-visible (UV–vis) absorption spectra were recorded by spectrophotometer (UV3150; Shimadzu Corporation, Kyoto, Japan). 2.3. Photocatalytic activity measurements
Fig. 1. XRD patterns of as-prepared powders.
(0.219 g, 1.0 mmol) and different amount of Bi2O3 were blended together in an agate mortar and ground thoroughly for 20 min at room temperature. Afterwards, NaOH (0.32 g, 8.0 mmol) was added to the mixture and ground for further 60 min. Final products were obtained by washing the mixtures several times with distilled water and absolute ethanol, followed by drying at 80 C for 12 h. Briefly, the samples with 0%, 2%, 5% and 10% (molar ratios) Bi2O3 were labeled as 0%, 2%, 5% and 10% ZnO/Bi2O3.
For photocatalytic activity evaluation, the suspension contained sample (40.00 mg) and aqueous RhB (40.0 mL, 10 mg/L) was stirred in dark for 30 min to establish adsorption/deposition equilibrium. The suspension was then exposed to simulated solar light irradiation provided by a 500 W high pressure Xe lamp (500 W high-pressure xenon lamps with useful spectral range of 290–800 nm) under continuous stirring. Analytical samples were taken from the reaction suspension at given time intervals; decreases in the concentrations of dyes were analyzed by UV spectrophotometer (UV-2450, Shimadzu, Kyoto, Japan). 3. Results and discussion
2.2. Characterization 3.1. Phase structure The phase structure and purity of the as synthesized nanocrystals were examined by X-ray diffraction (XRD; X’pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu-Ka radiation (l = 1.5406 A ) at 40 kV and 30 mA over the 2u range of 20–80 . The morphology and the elemental analysis of the obtained samples were investigated using field emission scanning electron microscopy (FE-SEM; JSM-6701F, JEOL, Tokyo, Japan) along with energy dispersive spectroscopy (EDS; FeatureMax Oxford Instruments,
Fig. 1 shows XRD patterns of the as-prepared powders. It can be seen that all the diffraction peaks of ZnO are well agreement with the standard spectra (JCPDS No. 36-1451). The heterojunction powders show both the ZnO and Bi2O3 spectra and no other peak can be found. With the increase of Bi2O3, the ZnO peak intensity is weakened by the high peak of well crystallized Bi2O3. Since the ZnO/Bi2O3 hierarchical nanostructures are prepared at room
Fig. 2. SEM images of (a) ZnO, (b) 2% ZnO/Bi2O3, (c) 5% ZnO/Bi2O3, (d) 10% ZnO/Bi2O3 nanopowders and EDX spectra of (e) ZnO and (f) 5% ZnO/Bi2O3.
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Fig. 3. TEM images of (a) ZnO and (b) 5% ZnO/Bi2O3 nanopowders. (c) and (d) are the enlarged images of the labelled regions of (a) and (b). HRTEM images of (e) ZnO and (f) 5% ZnO/Bi2O3 nanopowders. (g) and (h) are the enlarged images of the labelled regions of (e) and (f).
temperature, which is hardly for Bi2O3 getting into the ZnO lattice, no shift of the ZnO peak is observed. 3.2. Morphology and composition Fig. 2 shows SEM images of ZnO/Bi2O3 hierarchical nanostructures with different molar contents of Bi2O3. It indicates that the heterojunction are formed by many nanorods or nanosheets, and the Bi2O3 plays an important role in controlling the morphology. As depicted in Fig. 2(a), pure ZnO are consisted of large irregular aggregates of nanorods with inconsistent sizes. EDX spectra reveal that the sample contains only Zn and O, as shown in Fig. 2(e). The appearance of Al and Au is due to the Al foil and Au sputtering in the preparation of SEM samples. As shown in Fig. 2(b–c), upon introduction of Bi2O3, the aggregates become more regular, and tend to transform from nanorods to nanosheets. Especially, when the molar content of Bi2O3 reaches 5%, the nanosheets are organized to form the most regular 3D nanoflowers and Bi2O3 nanoparticles disperse well on the nanosheets, with single flowers having diameters in the range 2–3 mm, as confirmed in Fig. 2(c). With the molar content of Bi2O3 further increasing, an overdose of Bi2O3 aggregate to form some irregular bulks and destroy the overall morphology, as Fig. 2(d) displayed. The EDX analysis indicates Zn/O/Bi atomic ratio is approximately 8.05:14.86:1 where Bi2O3 nanoparticles disperse well on the nanosheets (5% ZnO/Bi2O3) and 4.07:9.38:1 where an overdose of Bi2O3 aggregate to form some irregular bulks (10% ZnO/Bi2O3). The morphologies of pure ZnO and 5% ZnO/Bi2O3 heterojunction are further investigated by TEM. As can be seen from Fig. 3(a), pure ZnO consist of many nanorods, and these nanoshrods selfassembled into 3D nanostructures. Fig. 3(b) shows the high magnification image of the selected area in Fig. 3(a), indicating the nanoshrods are 0.8–1 mm in length and 50–70 nm in width. As shown in Fig. 3(c), 5% ZnO/Bi2O3 shows similar structures to pure
ZnO. The high magnification image in Fig. 3(d), clearly demonstrates that there are some Bi2O3 nanoparticles dispersed well on the nanosheets. The spacings between adjacent lattice fringes are 0.26 (Fig. 3(g)) and 0.29 nm (Fig. 3(h)), which are close to the dspacings of the (0 0 1) of hexagonal wurtzite ZnO and (2 2 2) planes of g -Bi2O3, respectively. As we know, ZnO is usually formed according to the following equation: Zn(CH3COO)2 + 2NaOH ! Zn(OH)2 + 2CH3COONa + 2H2O
(1)
Zn(OH)2 ! ZnO + H2O
(2)
This reaction is self-initiated and self-sustained with H2O vapor releasing after grinding of the mixture of the precursors. When NaOH was added to the mixture and ground, Zn(OH)2 was obtained in the first step of the solid-state reaction, which has also been reported by Cao et al. [14]. The grinding process is one of the origins for producing ZnO. Considering that the decomposition temperature of Zn(OH)2 is relatively low and a lot of heat releases during grinding the mixture of the raw materials, Zn(OH)2 decomposes into ZnO and H2O due to the heat release by further grinding. To some degree, the absorption of moisture by NaOH and the heat release in the ground process of NaOH dissolution also have effect on the reaction. Based on above analysis, we proposed a plausible mechanism for the 3D ZnO/Bi2O3 hierarchical nanostructures, as schematically illustrated in Fig. 4. Initially, when adding NaOH to the agate mortar for grinding, the nucleation of Zn(OH)2 started and the Zn(OH)2 further transformed and aggregated into the rod-like intermediate through dehydration due to the heat release by grinding. Upon introduction of Bi2O3, the nanorods become more regular, and tend to form nanosheets. Then, ZnO nanosheets decorated by Bi2O3 was developed by the decomposition of the sheet-like intermediate Zn
Fig. 4. Schematic illustrating the formation mechanism of the ZnO/Bi2O3 hierarchical nanostructures assembled by nanorods.
X. Wang et al. / Materials Research Bulletin 64 (2015) 82–87
Fig. 5. UV–vis diffuse reflectance spectra of the as-prepared samples.
(OH)2 and gradually evolved into 3D nanostructures through an oriented attachment. Finally, the ZnO/Bi2O3 3D hierarchical nanostructures were obtained through further growth. 3.3. Optical properties The DRS of ZnO/Bi2O3 3D heterojunction are shown in Fig. 5. As expected, ZnO shows the characteristic spectrum of ZnO with its fundamental absorption sharp edge rising at 390 nm (3.20 eV) [15]. And Bi2O3 shows an absorption edge at the wavelength of 450 nm [16]. However, two noticeable absorption edges, respectively corresponding to the characteristic spectra of ZnO and Bi2O3, are observed for the ZnO/Bi2O3 3D heterojunction, indicating the introduction of Bi2O3 can extend the absorption wavelength to the visible light range. 3.4. Photocatalytic activity The simulated solar light photocatalytic for as-prepared powders was carried out by reduction RhB solution and the track
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of ultraviolet-visible (UV–vis) spectra was shown in Fig. 6(a–b). Evidently, the maximum absorbance of RhB diminishes gradually with time elapse. The absorption peak nearly disappears in 100 min under solar light irradiation in the presence of 5% ZnO/ Bi2O3 photocatalyst. Fig. 6(c) presents the RhB degradation curves of ln(Ct/C0) versus time during photodegradation with all the samples. As can be seen, with the adding of Bi2O3, the photocatalytic performance of ZnO/Bi2O3 heterojunction first increase and then decrease; the 5% ZnO/Bi2O3 sample has the best performance while the 10% sample has the lowest. Furthermore, the photocatalytic activity of the ZnO/Bi2O3 3D heterojunction are compared with bare ZnO by photo-degradation of a colorless pollutant phenol, thus the photo-sensitization effect can be excluded. The photo-degradation efficiencies of phenol as a function of irradiation time in the presence of ZnO/Bi2O3 compared to bare ZnO is plotted in Fig. 7(a). It can be seen that the ZnO/Bi2O3 heterojunction exhibit better photocatalytic performance than bare ZnO, which can degrade 100% and 94.8% of phenol in 3 h, respectively. Therefore, the above results demonstrate that the construction of ZnO/Bi2O3 heterostructure is an effective way to enhance the photocatalytic activity of ZnO. The stability of the photocatalyst is an important factor for its practical application. To demonstrate the potential applicability of ZnO/Bi2O3 heterojunction photocatalyst, circulating runs in the photocatalytic degradation of phenol were carried out under simulated solar light. As shown in Fig. 7(b), the ZnO/Bi2O3 heterojunction photocatalyst was stable under repeated use with constant photodecomposition rate of phenol in 12 h. After four recycles for the photodegradation of phenol, the photocatalyst did not exhibit any significant loss of activity, which indicates that the ZnO/Bi2O3 heterojunction photocatalyst has high stability and does not photocorrode during the photocatalytic oxidation of the model pollutant molecules. The radicals, electron and holes trapping experiments were designed to elucidate the photocatalytic degradation process of
Fig. 6. UV–vis spectra of RhB solution reduction for (a) 5% ZnO/Bi2O3 and (b) pure ZnO. (c) Kinetic curves of the degradation of RhB solution.
Fig. 7. (a) Photocatalytic degradation of phenol under solar light by the as-prepared samples; (b) cycling runs in the photocatalytic degradation of phenol by the 5% ZnO/Bi2O3.
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Fig. 8. Absorbance changes of RhB solution after different visible light irradiation times in the presence of (a) 5% ZnO/Bi2O3 + 1 mM EDTA and (b) 5% ZnO/Bi2O3 tBuOH. (c) Corresponding kinetic curves of the degradation of RhB solution.
ZnO/Bi2O3 heterojunction (Fig. 8(a–b)). As can be seen, under solar light irradiation, the photodegradation of RhB is slightly suppressed by the addition of hole scavenger, EDTA, while it is obviously inhibited when an electron and hydroxyl radical (OH) scavenger, tBuOH, is added [17]. This indicates that OH radicals are the main active species that can oxidize the adsorbed organic pollutants, which is also reported in other literatures [18]. As we known, OH radicals are the main active species that can oxidize the adsorbed organic pollutants, and they can be formed either through the hole oxidation of OH/H2O or the electron reduction of O2 (Reactions 3–5). According to our results, the hole scavenger EDTA could just exert limited influence on the photodegradation of RhB, meaning that OH in our system is mainly formed through action of electron reduction, which further prove the deduction of carbon sensitization and surface charge transfer. e– + O2 ! O2–
(3)
(4)
O2– + H2O ! HOO + OH–
2HOO ! O2 + H2O2
(5)
H2O2 + e– ! OH + OH–
(6)
3.5. Possible photocatalysis mechanism The excellent photocatalytic performance could be ascribed to two main reasons: (1) the effective separation of photogenerated
carriers, (2) the hierarchical nanostructures. Following are the detailed interpretation of the main reasons. (1) The enhanced photocatalytic activity of ZnO/Bi2O3 hetero-
junction can be attributed to the formation of p-n-type heterojunction between n-type ZnO and p-type Bi2O3 semiconductors, which promoted separation of photogenerated electrons (e) and holes (h+), as schemed in Fig. 9. The conduction band (CB) and valence band (VB) potentials of the p-type Bi2O3 semiconductor were 0.33 and 3.13 eV [19,20], respectively. The CB and VB for n-type ZnO were 0.31 and 2.89 eV [21], respectively. Before contact of p-type Bi2O3 semiconductor and n-type ZnO semiconductor, the CB potential of n-type ZnO was more negative than that of p-type Bi2O3, and the VB potential of p-type Bi2O3 was more positive than that of n-type ZnO. After light irradiation, photoexcited electrons on ZnO underwent vertical transfer to Bi2O3, whereas holes on Bi2O3 migrated to ZnO, which would produce an inner electric field [22]. In turn, the migrations of photogenerated electrons and holes would be promoted by the formed inner electric field [23], which facilitated the charge separation and higher photocatalytic ability. (2) The hierarchical nanostructures are propitious to the adsorption of the dye molecules, and therefore providing more active site for the photocatalytic reaction. However, when overdose amount of Bi2O3 is employed, Bi2O3 would aggregate to form some irregular bulks and destroy the 3D hierarchical nanostructures, leading to less active site for the photocatalytic reactions, and thus resulting poor photocatalytic performance.
4. Conclusions
Fig. 9. Schematic diagram showing the energy band structure and electron-hole pair separation in the ZnO/Bi2O3 heterostructure.
In summary, ZnO/Bi2O3 3D heterojunction were synthesized by a facile solid-state reaction route at room temperature without any surfactant or template. Their morphological features were characterized as self-assembled nanosheets on which Bi2O3 nanoparticles were well dispersed. It was found that the amount of Bi2O3 would significantly affect the overall morphology and the photocatalytic performance of the ZnO/Bi2O3 heterojunction under solar light. Results show that the 5% ZnO/Bi2O3 heterojunction have the most regular morphology and the highest photocatalytic activity, which is ascribed to the synergistic effects of the hierarchical morphology and the effective separation of photogenerated carriers, demonstrating that the ZnO/Bi2O3 heterojunction are a promising candidate as photocatalyst. Moreover, our work also hints that the present approach is a general and facile method with good
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potential for scale-up industrial applications that can be extended to fabricate other semiconductor heterojunction. Acknowledgement This work was supported by the Principal Fund (XAGDXJJ1402) and Dean Foundation(14GDYJY01) from Xi’an Technological University. References [1] [2] [3] [4]
B. Cheng, Y. Le, J.G. Yu, J. Hazard. Mater. 177 (2010) 971–977. L.M. Huang, H.Q. Fan, Sens. Actuators B: Chem. 171 (2012) 1257–1263. J.W. Fang, H.Q. Fan, G.Z. Dong, Mater. Lett. 120 (2014) 147–150. T.P. Cao, Y.J. Li, C.H. Wang, L.M. Wei, C.L. Shao, Y.C. Liu, Mater. Res. Bull. 45 (2010) 1406–1412. [5] X. Wei, T.F. Xie, L.L. Peng, W. Fu, J.S. Chen, Q. Gao, G.Y. Hong, D.J. Wang, J. Phys. Chem. C 115 (2011) 8637–8642. [6] Y. Hou, X.Y. Li, Q.D. Zhao, X. Quan, G.H. Chen, Appl. Phys. Lett. 95 (2009) 093108. [7] T.P. Xie, C.L. Liu, L.J. Xu, J. Yang, W. Zhou, J. Phys. Chem. C 117 (2013) 24601–24610.
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Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Room-temperature solid state synthesis of ZnO/Bi2O3 heterojunction and their solar light photocatalytic performance Xin Wang a, * , Pengrong Ren b , Huiqing Fan c a Shaanxi Province Thin Film Technology and Optical Test Open Key Laboratory, School of Photoelectrical Engineering, Xi’an Technological University, Xi’an 710032, China b School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China c State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 July 2014 Received in revised form 17 November 2014 Accepted 16 December 2014 Available online 18 December 2014
ZnO/Bi2O3 heterojunction was prepared by solid state reaction method at room temperature. The asprepared nanostructures were characterized as the assembled nanosheets of ZnO on which Bi2O3 nanoparticles were well dispersed. Further studies revealed that the morphology and photocatalytic property could be regulated by tuning the amount of Bi2O3. The introduction of Bi2O3 can lead to the enhancement of solar light absorption, and the 5% ZnO/Bi2O3 samples exhibited the best photocatalytic activity under the solar light irradiation. The excellent photocatalytic performances could be ascribed to the synergistic effects of the hierarchical nanostructures and the effective separation of photogenerated carriers. Moreover, the hydroxyl radicals (OH) are found to be the main active species generated in the oxidation reaction of RhB over ZnO/Bi2O3 photocatalyst. Therefore, our work demonstrated a facile way to prepare hierarchical nanostructures with excellent photocatalytic performance using solid state method at room temperature. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: A. Composites A. Nanostructures B. Chemical synthesis D. Catalytic properties D. Microstructure
1. Introduction Oxide semiconductor photocatalysis is an environmentally friendly approach toward solving the energy shortage and realizing environmental remediation issues [1–3]. Considering the advantages of low cost and easy synthesis, ZnO has become a potential photocatalyst for industry. However, the universal use of ZnO is still limited due to its wide band gap and quick recombination of charge carriers [4]. And semiconductor heterojunction can be used to conquer these problems and improve the photoactivity efficiency by maximum utilization of spectral portion of the visible light [5,6]. On the one hand, semiconductors with narrow gap are employed to sensitize the wide gap ones. On the other hand, different band gaps can make photogenerated carriers flow from one semiconductor to another, resulting long time and effective separation. Therefore, the selection of heterostructure materials plays an important role in improving the photocatalytic property of the host materials. Here, we choose Bi2O3, which is an important p-type semiconductor photocatalyst with a good conducting ability and a narrow band gap of 2.8 eV [7]. It is
expected that ZnO/Bi2O3 heterojunction could improve the photocatalytic properties of pure ZnO. To date, various methods have been employed to regulate morphology of ZnO with the aim of tuning their performance, such as electrospinning [8], hydrothermal [9], solvothermal [10], sol–gel [11], solution combustion [12], etc. However, these synthetic methods are solution based and usually involve rigorous experimental conditions, and the as-synthesized products usually show poor photocatalytic activity [13]. Therefore, it is of great necessary to develop simple and reliable synthetic methods for the synthesis of heterojunction with designed chemical components and controlled morphologies, which can affect the properties of nanomaterials strongly. On the basis of the above discussion, ZnO/Bi2O3 heterojunction were prepared through a simple solid state reaction. The introduction quantity of Bi2O3 in the heterojunction system was found to play a great role in the morphology and photocatalytic activity-control. 2. Experimental 2.1. Preparation of ZnO/Bi2O3 heterojunction.
* Corresponding author. Tel.: +86 29 86173335; fax: +86 29 86173335. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.materresbull.2014.12.037 0025-5408/ ã 2014 Elsevier Ltd. All rights reserved.
All reagents were analytically pure and used without further purification. In a typical procedure, certain Zn(CH3COO)22H2O
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83
Abingdon, Oxfordshire, UK) and high resolution transmission electron microscopy (HRTEM; Tecnai F30G2, FEI, Hillsboro, OR, USA). UV–vis diffuse reflectance spectra (DRS) of the samples were recorded with an UV–vis spectrophotometer (UV3150; Shimadzu Corporation, Kyoto, Japan) using BaSO4 as reference. The ultraviolet-visible (UV–vis) absorption spectra were recorded by spectrophotometer (UV3150; Shimadzu Corporation, Kyoto, Japan). 2.3. Photocatalytic activity measurements
Fig. 1. XRD patterns of as-prepared powders.
(0.219 g, 1.0 mmol) and different amount of Bi2O3 were blended together in an agate mortar and ground thoroughly for 20 min at room temperature. Afterwards, NaOH (0.32 g, 8.0 mmol) was added to the mixture and ground for further 60 min. Final products were obtained by washing the mixtures several times with distilled water and absolute ethanol, followed by drying at 80 C for 12 h. Briefly, the samples with 0%, 2%, 5% and 10% (molar ratios) Bi2O3 were labeled as 0%, 2%, 5% and 10% ZnO/Bi2O3.
For photocatalytic activity evaluation, the suspension contained sample (40.00 mg) and aqueous RhB (40.0 mL, 10 mg/L) was stirred in dark for 30 min to establish adsorption/deposition equilibrium. The suspension was then exposed to simulated solar light irradiation provided by a 500 W high pressure Xe lamp (500 W high-pressure xenon lamps with useful spectral range of 290–800 nm) under continuous stirring. Analytical samples were taken from the reaction suspension at given time intervals; decreases in the concentrations of dyes were analyzed by UV spectrophotometer (UV-2450, Shimadzu, Kyoto, Japan). 3. Results and discussion
2.2. Characterization 3.1. Phase structure The phase structure and purity of the as synthesized nanocrystals were examined by X-ray diffraction (XRD; X’pert PRO MPD, Philips, Eindhoven, The Netherlands) with Cu-Ka radiation (l = 1.5406 A ) at 40 kV and 30 mA over the 2u range of 20–80 . The morphology and the elemental analysis of the obtained samples were investigated using field emission scanning electron microscopy (FE-SEM; JSM-6701F, JEOL, Tokyo, Japan) along with energy dispersive spectroscopy (EDS; FeatureMax Oxford Instruments,
Fig. 1 shows XRD patterns of the as-prepared powders. It can be seen that all the diffraction peaks of ZnO are well agreement with the standard spectra (JCPDS No. 36-1451). The heterojunction powders show both the ZnO and Bi2O3 spectra and no other peak can be found. With the increase of Bi2O3, the ZnO peak intensity is weakened by the high peak of well crystallized Bi2O3. Since the ZnO/Bi2O3 hierarchical nanostructures are prepared at room
Fig. 2. SEM images of (a) ZnO, (b) 2% ZnO/Bi2O3, (c) 5% ZnO/Bi2O3, (d) 10% ZnO/Bi2O3 nanopowders and EDX spectra of (e) ZnO and (f) 5% ZnO/Bi2O3.
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Fig. 3. TEM images of (a) ZnO and (b) 5% ZnO/Bi2O3 nanopowders. (c) and (d) are the enlarged images of the labelled regions of (a) and (b). HRTEM images of (e) ZnO and (f) 5% ZnO/Bi2O3 nanopowders. (g) and (h) are the enlarged images of the labelled regions of (e) and (f).
temperature, which is hardly for Bi2O3 getting into the ZnO lattice, no shift of the ZnO peak is observed. 3.2. Morphology and composition Fig. 2 shows SEM images of ZnO/Bi2O3 hierarchical nanostructures with different molar contents of Bi2O3. It indicates that the heterojunction are formed by many nanorods or nanosheets, and the Bi2O3 plays an important role in controlling the morphology. As depicted in Fig. 2(a), pure ZnO are consisted of large irregular aggregates of nanorods with inconsistent sizes. EDX spectra reveal that the sample contains only Zn and O, as shown in Fig. 2(e). The appearance of Al and Au is due to the Al foil and Au sputtering in the preparation of SEM samples. As shown in Fig. 2(b–c), upon introduction of Bi2O3, the aggregates become more regular, and tend to transform from nanorods to nanosheets. Especially, when the molar content of Bi2O3 reaches 5%, the nanosheets are organized to form the most regular 3D nanoflowers and Bi2O3 nanoparticles disperse well on the nanosheets, with single flowers having diameters in the range 2–3 mm, as confirmed in Fig. 2(c). With the molar content of Bi2O3 further increasing, an overdose of Bi2O3 aggregate to form some irregular bulks and destroy the overall morphology, as Fig. 2(d) displayed. The EDX analysis indicates Zn/O/Bi atomic ratio is approximately 8.05:14.86:1 where Bi2O3 nanoparticles disperse well on the nanosheets (5% ZnO/Bi2O3) and 4.07:9.38:1 where an overdose of Bi2O3 aggregate to form some irregular bulks (10% ZnO/Bi2O3). The morphologies of pure ZnO and 5% ZnO/Bi2O3 heterojunction are further investigated by TEM. As can be seen from Fig. 3(a), pure ZnO consist of many nanorods, and these nanoshrods selfassembled into 3D nanostructures. Fig. 3(b) shows the high magnification image of the selected area in Fig. 3(a), indicating the nanoshrods are 0.8–1 mm in length and 50–70 nm in width. As shown in Fig. 3(c), 5% ZnO/Bi2O3 shows similar structures to pure
ZnO. The high magnification image in Fig. 3(d), clearly demonstrates that there are some Bi2O3 nanoparticles dispersed well on the nanosheets. The spacings between adjacent lattice fringes are 0.26 (Fig. 3(g)) and 0.29 nm (Fig. 3(h)), which are close to the dspacings of the (0 0 1) of hexagonal wurtzite ZnO and (2 2 2) planes of g -Bi2O3, respectively. As we know, ZnO is usually formed according to the following equation: Zn(CH3COO)2 + 2NaOH ! Zn(OH)2 + 2CH3COONa + 2H2O
(1)
Zn(OH)2 ! ZnO + H2O
(2)
This reaction is self-initiated and self-sustained with H2O vapor releasing after grinding of the mixture of the precursors. When NaOH was added to the mixture and ground, Zn(OH)2 was obtained in the first step of the solid-state reaction, which has also been reported by Cao et al. [14]. The grinding process is one of the origins for producing ZnO. Considering that the decomposition temperature of Zn(OH)2 is relatively low and a lot of heat releases during grinding the mixture of the raw materials, Zn(OH)2 decomposes into ZnO and H2O due to the heat release by further grinding. To some degree, the absorption of moisture by NaOH and the heat release in the ground process of NaOH dissolution also have effect on the reaction. Based on above analysis, we proposed a plausible mechanism for the 3D ZnO/Bi2O3 hierarchical nanostructures, as schematically illustrated in Fig. 4. Initially, when adding NaOH to the agate mortar for grinding, the nucleation of Zn(OH)2 started and the Zn(OH)2 further transformed and aggregated into the rod-like intermediate through dehydration due to the heat release by grinding. Upon introduction of Bi2O3, the nanorods become more regular, and tend to form nanosheets. Then, ZnO nanosheets decorated by Bi2O3 was developed by the decomposition of the sheet-like intermediate Zn
Fig. 4. Schematic illustrating the formation mechanism of the ZnO/Bi2O3 hierarchical nanostructures assembled by nanorods.
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Fig. 5. UV–vis diffuse reflectance spectra of the as-prepared samples.
(OH)2 and gradually evolved into 3D nanostructures through an oriented attachment. Finally, the ZnO/Bi2O3 3D hierarchical nanostructures were obtained through further growth. 3.3. Optical properties The DRS of ZnO/Bi2O3 3D heterojunction are shown in Fig. 5. As expected, ZnO shows the characteristic spectrum of ZnO with its fundamental absorption sharp edge rising at 390 nm (3.20 eV) [15]. And Bi2O3 shows an absorption edge at the wavelength of 450 nm [16]. However, two noticeable absorption edges, respectively corresponding to the characteristic spectra of ZnO and Bi2O3, are observed for the ZnO/Bi2O3 3D heterojunction, indicating the introduction of Bi2O3 can extend the absorption wavelength to the visible light range. 3.4. Photocatalytic activity The simulated solar light photocatalytic for as-prepared powders was carried out by reduction RhB solution and the track
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of ultraviolet-visible (UV–vis) spectra was shown in Fig. 6(a–b). Evidently, the maximum absorbance of RhB diminishes gradually with time elapse. The absorption peak nearly disappears in 100 min under solar light irradiation in the presence of 5% ZnO/ Bi2O3 photocatalyst. Fig. 6(c) presents the RhB degradation curves of ln(Ct/C0) versus time during photodegradation with all the samples. As can be seen, with the adding of Bi2O3, the photocatalytic performance of ZnO/Bi2O3 heterojunction first increase and then decrease; the 5% ZnO/Bi2O3 sample has the best performance while the 10% sample has the lowest. Furthermore, the photocatalytic activity of the ZnO/Bi2O3 3D heterojunction are compared with bare ZnO by photo-degradation of a colorless pollutant phenol, thus the photo-sensitization effect can be excluded. The photo-degradation efficiencies of phenol as a function of irradiation time in the presence of ZnO/Bi2O3 compared to bare ZnO is plotted in Fig. 7(a). It can be seen that the ZnO/Bi2O3 heterojunction exhibit better photocatalytic performance than bare ZnO, which can degrade 100% and 94.8% of phenol in 3 h, respectively. Therefore, the above results demonstrate that the construction of ZnO/Bi2O3 heterostructure is an effective way to enhance the photocatalytic activity of ZnO. The stability of the photocatalyst is an important factor for its practical application. To demonstrate the potential applicability of ZnO/Bi2O3 heterojunction photocatalyst, circulating runs in the photocatalytic degradation of phenol were carried out under simulated solar light. As shown in Fig. 7(b), the ZnO/Bi2O3 heterojunction photocatalyst was stable under repeated use with constant photodecomposition rate of phenol in 12 h. After four recycles for the photodegradation of phenol, the photocatalyst did not exhibit any significant loss of activity, which indicates that the ZnO/Bi2O3 heterojunction photocatalyst has high stability and does not photocorrode during the photocatalytic oxidation of the model pollutant molecules. The radicals, electron and holes trapping experiments were designed to elucidate the photocatalytic degradation process of
Fig. 6. UV–vis spectra of RhB solution reduction for (a) 5% ZnO/Bi2O3 and (b) pure ZnO. (c) Kinetic curves of the degradation of RhB solution.
Fig. 7. (a) Photocatalytic degradation of phenol under solar light by the as-prepared samples; (b) cycling runs in the photocatalytic degradation of phenol by the 5% ZnO/Bi2O3.
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Fig. 8. Absorbance changes of RhB solution after different visible light irradiation times in the presence of (a) 5% ZnO/Bi2O3 + 1 mM EDTA and (b) 5% ZnO/Bi2O3 tBuOH. (c) Corresponding kinetic curves of the degradation of RhB solution.
ZnO/Bi2O3 heterojunction (Fig. 8(a–b)). As can be seen, under solar light irradiation, the photodegradation of RhB is slightly suppressed by the addition of hole scavenger, EDTA, while it is obviously inhibited when an electron and hydroxyl radical (OH) scavenger, tBuOH, is added [17]. This indicates that OH radicals are the main active species that can oxidize the adsorbed organic pollutants, which is also reported in other literatures [18]. As we known, OH radicals are the main active species that can oxidize the adsorbed organic pollutants, and they can be formed either through the hole oxidation of OH/H2O or the electron reduction of O2 (Reactions 3–5). According to our results, the hole scavenger EDTA could just exert limited influence on the photodegradation of RhB, meaning that OH in our system is mainly formed through action of electron reduction, which further prove the deduction of carbon sensitization and surface charge transfer. e– + O2 ! O2–
(3)
(4)
O2– + H2O ! HOO + OH–
2HOO ! O2 + H2O2
(5)
H2O2 + e– ! OH + OH–
(6)
3.5. Possible photocatalysis mechanism The excellent photocatalytic performance could be ascribed to two main reasons: (1) the effective separation of photogenerated
carriers, (2) the hierarchical nanostructures. Following are the detailed interpretation of the main reasons. (1) The enhanced photocatalytic activity of ZnO/Bi2O3 hetero-
junction can be attributed to the formation of p-n-type heterojunction between n-type ZnO and p-type Bi2O3 semiconductors, which promoted separation of photogenerated electrons (e) and holes (h+), as schemed in Fig. 9. The conduction band (CB) and valence band (VB) potentials of the p-type Bi2O3 semiconductor were 0.33 and 3.13 eV [19,20], respectively. The CB and VB for n-type ZnO were 0.31 and 2.89 eV [21], respectively. Before contact of p-type Bi2O3 semiconductor and n-type ZnO semiconductor, the CB potential of n-type ZnO was more negative than that of p-type Bi2O3, and the VB potential of p-type Bi2O3 was more positive than that of n-type ZnO. After light irradiation, photoexcited electrons on ZnO underwent vertical transfer to Bi2O3, whereas holes on Bi2O3 migrated to ZnO, which would produce an inner electric field [22]. In turn, the migrations of photogenerated electrons and holes would be promoted by the formed inner electric field [23], which facilitated the charge separation and higher photocatalytic ability. (2) The hierarchical nanostructures are propitious to the adsorption of the dye molecules, and therefore providing more active site for the photocatalytic reaction. However, when overdose amount of Bi2O3 is employed, Bi2O3 would aggregate to form some irregular bulks and destroy the 3D hierarchical nanostructures, leading to less active site for the photocatalytic reactions, and thus resulting poor photocatalytic performance.
4. Conclusions
Fig. 9. Schematic diagram showing the energy band structure and electron-hole pair separation in the ZnO/Bi2O3 heterostructure.
In summary, ZnO/Bi2O3 3D heterojunction were synthesized by a facile solid-state reaction route at room temperature without any surfactant or template. Their morphological features were characterized as self-assembled nanosheets on which Bi2O3 nanoparticles were well dispersed. It was found that the amount of Bi2O3 would significantly affect the overall morphology and the photocatalytic performance of the ZnO/Bi2O3 heterojunction under solar light. Results show that the 5% ZnO/Bi2O3 heterojunction have the most regular morphology and the highest photocatalytic activity, which is ascribed to the synergistic effects of the hierarchical morphology and the effective separation of photogenerated carriers, demonstrating that the ZnO/Bi2O3 heterojunction are a promising candidate as photocatalyst. Moreover, our work also hints that the present approach is a general and facile method with good
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potential for scale-up industrial applications that can be extended to fabricate other semiconductor heterojunction. Acknowledgement This work was supported by the Principal Fund (XAGDXJJ1402) and Dean Foundation(14GDYJY01) from Xi’an Technological University. References [1] [2] [3] [4]
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