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Co3O4 heterojunction as visible-light stabilized photocatalysts
Materials Science in Semiconductor Processing 79 (2018) 24–31
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Bimetal-organic frameworks derived carbon doped ZnO/Co3O4 heterojunction as visible-light stabilized photocatalysts Nannan Liua,b,c, Zhonghua Lia,b,
T
⁎
a
Key Laboratory of Microsystems and Microstructures Manufacturing, Ministry of Education, Harbin Institute of Technology, Harbin 150001, China Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, China c School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal-organic framework Carbon doping Heterojunction Large specific area Visible-light
The construction of transition-metal oxygen heterojunction photocatalysts has been paid close attention for application of versatile functionalities. Many methods have been developed and applied to improve photocatalytic performance, such as doping and morphology control. In this study, carbon (C)-doped binary oxide heteronanostructures with hollow dodecahedra morphology are fabricated by means of topotactic transformation of metal-organic frameworks. The converted hollow C doping ZnO/Co3O4 nanocomposite with large specific area has advantage in charge transfer and charge-separation efficiency, and then is evaluated by degrading methylene blue (MB) under visible light irradiation (λ > 400 nm). The experiment demonstrates the optimal catalytic performance is about 8.4-fold k values of pure ZnO. Importantly, the in-situ synthesized composites show noteworthy stability and recyclability, even after five runs. A possible photocatalytic mechanism also is mentioned, the results illustrate that synergistic effect of coupled semiconductor system with C doping dramatically suppresses the recombination of photogenerated electron-hole pairs, and the hydroxyl radicals (•OH) and holes play a critical role. This work exhibits an effective method for the design of other nonmetal doping sound heterojunctions with well-controlled morphology that significantly enhance photocatalytic behavior.
1. Introduction Solar energy is the most abundant and available clean energy source today [1–3]. In this manner, it is essential to utilize solar energy for green environmental pollution management [4–6]. Inspiration by the pioneering work of Fujishima and Honda in 1972, semiconductor-based photocatalysts and photocatalytic processes have attracted much attention recently [7]. Compared with TiO2 investigated mostly, Co3O4 is an intriguing semiconductor revealing excellent optical, electronic, and other physical and chemical property [8–11]. Besides, the narrow band gap of Co3O4 could reduce the excitation energy of electron-hole pairs separating, while the high electron mobility of Co3O4 is benefit for photocatalytic activity improvement [12]. Unfortunately, it is generally accepted that the fast recombination of photogenerated electron-hole pairs is still a significant challenge. It is well established that the heterojunction is an ideal solution for efficacious electron collection and separation of semiconductor [13]. For the principle, the p-type and n-type semiconductor could create an interface, the formed p-n heterojunction with a space-charge region at the interfaces demonstrates an electronic field caused by the migration
of electrons and holes, which emerges an outstanding ability to separate the electron-hole pairs [14–18]. Numerous efforts for blending with other compounds such as Bi2O3, Bi2WO6, g-C3N4, etc. have been gotten to optimize Co3O4-based photocatalysis property [19–22]. However, the incongruous position of energy band and low efficiency remain an enormous challenge. It's worth noting that the n-type ZnO is attractive and utilized in catalytic field and energy storage, and the appropriate band gap would regulate the recombination of photogenerated electron-hole pairs after compositing with Co3O4. Nonetheless, there have been a number of high profile cases compromising to indicate that the normal synthetic methods of metal oxides invariably generate a low specific area [24], limited exposure of active sites by the poor surface area would restrict the catalytic efficiency [25]. Furthermore, the direct ZnO/Co3O4 heterojunction without the linkage is unbeneficial for the rapid charge transfer via the boundary. Hence, it's urgent to take measures to further advance photocatalytic performance. Recent developments of metal organic frameworks (MOFs) have attracted much attention because of its abundant porous and large specific area, which make it have potential utilization in energy storages, light harvesting, catalysis and drug delivery [26–28].
Abbreviations: MOFs, metal-organic frameworks; ZnCo12, ZnO/Co3O4 1:2; ZnCo11, ZnO/Co3O4 1:1; ZnCo21, ZnO/Co3O4 2:1; MB, methylene blue ⁎ Corresponding author at: Key Laboratory of Microsystems and Microstructures Manufacturing, Ministry of Education, Harbin Institute of Technology, Harbin 150001, China. E-mail address: [email protected] (Z. Li). https://doi.org/10.1016/j.mssp.2018.01.004 Received 5 November 2017; Received in revised form 18 December 2017; Accepted 2 January 2018 1369-8001/ © 2018 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 79 (2018) 24–31
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Jobin Yvon iHR320 imaging spectrometer.
Moreover, the carbon doping metal oxides ramifications derived from MOFs also continue to be a great capacity. In addition to this, the morphology of ramifications could be tailored and epitaxial prepared by the unique structure of MOF precursors, which provides a fresh route for nonmetal doping of metal oxides synthesis [29–31]. In this study, we design and synthesize hollow C doping ZnO/Co3O4 (ZnO/Co3O4/C) dodecahedron heterojunctions with MOF precursors. Although the ZnO/Co3O4 hybrid composites were mentioned in previous reports, to the best of our knowledge, the hollow ternary compound involved amorphous carbon is generated firstly with MOFs precursor as photocatalysts. Importantly, the as-obtained sample simultaneously possesses initial MOF morphology (porous structure with connected nanoparticles) along with abundant C doping, which promote the charge transfer and separation [32]. Therefore, the sufficient physical integration of nanoparticle and C doping is a promising approach to obtain superior performance photocatalysts. To sum up, The as-prepared ZnO/Co3O4/C composites reveal several extensional advantages: 1) large specific area of MOF based materials is benefit for exposure of active sites; 2) C doping can serve as a potentially vital pathway to increase the charge mobility of photocatalysts most efficiently; 3) the in situ synthesized composite could furnish more interoperable interface than ex situ process; 4) the composite was prepared with a simple method, which avoids the complex dislodge template process of hollow structure; 5) the hollow structures further magnify the contact of catalyst and dye. Moreover, The ZnO/Co3O4/C composite displays an enhancement of inherent absorbance than the pure ZnO for visible light.
2.4. Catalytic activity measurements The activities of the catalysts were evaluated in a 200 mL reaction vessel containing MB as a model organic contaminant with the initial concentration of 10 mg/L−1. A 300 W Xe lamp provided a visible light source with a 400 nm cutoff filter at 15 cm distance. The process temperature was maintained via using circulation condensed water. In each typical experiment, 100 mL of MB aqueous solution containing 50 mg of catalytic amount was first kept in the dark under magnetical stirring for 1.5 h to establish the adsorption-desorption equilibrium. Then, at each 20 min interval of irradiation time, 3–5 mL of suspensions was taken and centrifuged (13,000 rpm, 1 min). Soon afterwards, the liquid supernatants was collected to monitor the change process of MB concentration by using the UV–vis spectrophotometer at 664 nm [31]. 3. Results and discussion 3.1. Characterization of the as-prepared samples The typical synthesis process of the materials was exhibited in Scheme 1. The MOFs were prepared by dropping 2-methylimidazole into metallic solution and aged for 24 h，and the metallic oxides were synthesized with one-step carbonization process of the obtained MOFs. The molar radio of ZnO to Co3O4 in the preparation of different heterojunctions was approximately 1:2 for ZnCo12, 1:1 for ZnCo11, and 2:1 for ZnCo21. And the bare ZnO/C and Co3O4/C were also prepared with singular metallic solution as contrast. The understanding of crystal phases for ZnO/Co3O4/C composites supported by the XRD analysis was revealed in Fig. 1a. The three samples showed the existing of both crystal ZnO (JCPDS No. 76-0704) and Co3O4 (JCPDS No. 74-2120), which meant that the crystalline had no change by adjusting the metal ratios. However, the peak intensity of each oxide in the three simples was completely different. The peaks of ZnO turned weakness with the increment of Co3O4, this phenomenon indicated the oxides had the same crystal state but different concentration in different composites. Thermogravimetric analysis (TG) was carried out for the sake of clearness the process of synthesizing ZnCo12 by MOFs precursor (Fig. 1b). First weight loss before 200 °C corresponded to the removal of water molecules, and pyrolysis occurred at 310 °C, that generated carbon and metal oxide, and then quality rapidly reduced at 520 °C due to the oxidization of carbon in the material. Until 600 °C, weightlessness was to stop and the carbon in the material remove. Experiment showed that ZnO, Co3O4 and carbon species existed at the same time with calcination temperature of 400 °C. The morphologies of the simples were characterized by SEM and TEM images. As revealed in Fig. 2a, the MOFs had a normative regular dodecahedron structure with a diameter of approximately 400 nm, while after calcination, the simples of oxide turned into a sunken hollow dodecahedron structure. For the ZnO/Co3O4/C composites
2. Materials and methods 2.1. Preparation of MOFs In a typical synthesis process, 2.97 g Zn(NO3)2·6H2O (10 mmol) and 1.45 g Co(NO3)2·6H2O (5 mmol) were dissolved in 150 mL methyl alcohol and stirred for 30 min to form a solution A. 4.97 g 2-methylimidazole was added into 50 mL methyl alcohol to form a clear solution B. Then solution B was poured into solution A slowly with continuous stirring for 15 min. The resulted solution was aged for 24 h at room temperature. And the MOF after centrifugation was dried at 60 °C for 12 h. The different MOFs were prepared with different molar ratios of Zn(NO3)2·6H2O and Co(NO3)2·6H2O. 2.2. Preparation of metallic oxide The ZnO/Co3O4/C was synthesized by one-step calcination process. The as-prepared MOFs were transferred into a crucible and heated to 400 °C for 100 min. The ZnO/C and Co3O4/C were obtained with different MOF precursors. 2.3. Characterization The crystalline phase and phase composition analysis of all the prepared samples were carried out by the X-ray diffractometer (XRD) using Cu Kα (λ = 1.5406 Å) radiation source. The morphologies of all the samples were performed by Transmission electron microscopy (TEM) on a JEOL JEM-2100 at the acceleration voltage of 200 kV and Scanning electron microscopy (SEM), moreover, High resolution transmission electron microscopy (HRTEM) and elemental mapping were also used to characterize the samples. The surface chemistries of the as-prepared samples were measured by X-ray photoelectron spectroscopy (XPS) in ESCALAB250Xi system with Mg Kα X-ray sources. The C1s peak at 284.6 eV was utilized to calibrate the whole of XPS spectra. The specific surface area was detected by BET N2 adsorptiondesorption isotherms. UV–vis absorption spectra were acquired with the use of a UV-2550, Shimadzu, Japan spectrophotometer. The photoluminescence (PL) spectra analyses were conducted with Horiba
Scheme 1. Synthesis scheme of ZnO/C, Co3O4/C and ZnO/Co3O4/C.
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Fig. 1. XRD patterns of ZnCo12, ZnCo11, and ZnCo21.
observation from the XRD pattern. The exposure of active sites is proportional to the surface area of catalysts. The BET measurements revealed that the MOF had large specific area of 1273 m2/g (Fig. 3a). While after oxidation in the air, the specific area of ZnCo21, ZnCo11 and ZnCo12 was 43, 91 and 93 m2/g (Fig. 3b), and the pore volume of ZnCo21, ZnCo11 and ZnCo12 was 0.08, 0.20 and 0.24 cm3/g, respectively. The different specific area and pore volume identified with the SEM images, the hollow ZnCo12 displayed highest specific area and the solid ZnCo21 only showed half of the area to ZnCo12. In spite of this, the obtained catalysts revealed higher specific area than the most oxides, which were beneficial to enhance the photocatalytic activity. The surface chemical composition and bonding environment were
exhibited in Fig. 2d–f, the morphology of surfaces changed into more sinking with the decrease of Co3O4 content. The ZnCo12 had an ecumenical roughly surface, and the surface of ZnCo11 turned into a reticular structure, while the ZnCo21 displayed a considerable shrunken dodecahedron construction. The phenomenon explained the process of topotactic transformation not only depended on the morphology of precursors, but also was influenced by the species and content of reagents. The elemental mapping of ZnCo12 in Fig. 2g illustrated the Zn, Co, O and C elements were uniformly distributed in the composite. The hollow structure of ZnCo12 was further confirmed by the TEM results in Fig. 2h, and the HRTEM image in Fig. 2i exhibited lattice fringes of ZnCo12, which revealed the heterojunction of the (111) plane of Co3O4 and (101) plane of ZnO in ZnCo12. The results were consistent with the
Fig. 2. SEM images of (a) MOF, (b) ZnO/C, (c) Co3O4/C, (d) ZnCo12, (e) ZnCo11 and (f) ZnCo21. (g) Elemental mapping of Zn, Co and O in ZnCo12. (h) TEM and (i) HRTEM image of ZnCo12.
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Fig. 3. N2 adsorption–desorption isotherms of as-prepared (a) ZnCo MOF, (b) ZnCo11, ZnCo12 and ZnCo21 composites.
Fig. 4. XPS spectrum of ZnCo21, ZnCo11 and ZnCo12: (a) Zn 2p, (b) Co 2p and (c) O 1s.
heterojunctions were of importance factor for the improvement of photocatalytic performance. A pseudo first-order kinetic model was used to calculate the rate constant (k) of degradation of dye concentration. As shown in Fig. 5b, the k value of the ZnCo12 optimal activity equaled to 0.00741 min−1, 8.4 times of pure ZnO/C particle (0.00088 min−1), 5.6 times of Co3O4/C particle (0.00132 min−1), confirming the crucial role of p-n heterojunctions for highly enhanced catalysis activity. The photocatalytic degradation process was monitored at the peak of 664 nm (the characteristic absorption of MB) by the temporal UV–vis spectral changes of MB solutions as a function of irradiation time, in the presence of the ZnCo12 nanocomposite (Fig. 5c). With time passing, the major absorbance syllabify decreased. After 180 min illumination, corresponding absorption peaks faded away, illustrating that the change process of MB concentration was chromophores split, but being merely achromatized by adsorption [23,38]. Whether photocatalysts can be applied for depollution of environment, the stability is one of the most pivotal factors. Herein, the circulation experiments of the degradation of MB were carried out five runs to estimate the stability of as-synthesized samples. After the completion of each cycling test, the photocatalyst was centrifuged and collected, then washed with water and absolute alcohol for three times, the recycled sample was dried at 70 °C for 12 h for the next cycle of photocatalysis. It can be observed from Fig. 5d, the photocatalyst maintained reliable activity with imperceptible deactivation after successive cycles, indicating the value of practical application [39]. The XRD pattern of the recycled ZnCo12 sample exhibited in Fig. 6a disclosed there was no discernible change in the phase and structure after five cycles, and the morphology of recycled ZnCo12 identified with the fresh catalyst (Fig. 6b), which was in favor of the stability of the ZnO/ Co3O4/C dodecahedron nanostructure photocatalyst in the same way. To explore the optimal amount of photocatalyst for the degradation
investigated by XPS in Fig. 4. Fig. 4a illustrated the determined oxidation states of Zn from Zn 2p XPS spectrum, and the peak energies of 1019.8 and 1042.9 eV were attributed to the binding energies of the Zn 2p3/2 and the Zn 2p1/2 of ZnO. The XPS spectrum of Co 2p were exhibited in Fig. 4b, the peaks of 778.3 and 793.8 eV identified Co3+ 2p3/ 3+ 2p1/2, and the fitting peaks of 779.3, 787.7 and 803.2 eV 2 and Co were due to the existence of Co2+, and the difference between the peaks of Co3+ 2p1/2 and Co3+ 2p3/2 was 15.5 eV, which demonstrated the presence of the Co3O4 phase [11]. The XPS spectrum of O 1s consisting of three peaks (Fig. 4c) around 528.2, 530.0 and 531.2 eV were related to the oxygen in the Co3O4, ZnO crystal lattice and the surface chemisorbed oxygen, respectively. And the peak intensity was in agreement with the radio of ZnO and Co3O4. Moreover, there were no significant difference for the peak energies among the three samples, revealing a similar bonding environment but different content [33]. Fig. 4d shown the XPS spectrum of C 1s, the two peaks of 283.7 and 287.6 eV represented the presence of C-C and C-O bonds, respectively.
3.2. Photocatalytic activity The photocatalytic performance of the as-prepared ZnO/C, Co3O4/C and ZnO/Co3O4/C compounds was estimated via recording photocatalytic degradation process of MB under visible light irradiation, which had been seen as a representative dye [34]. Fig. 5a demonstrated ZnO/C and Co3O4/C had slight photocatalytic activities due to wide band gap resulting in the impossibility on absorbing visible light as well as high recombination of photogeneration electrons and holes, respectively [35–37]. However, the ZnO/Co3O4/C nanostructures showed remarkable enhancement of the degradation of MB. Among them, the dye degradation of the ZnCo12 photocatalysis reached up to 95% in 180 min, while the ZnCo11 and the ZnCo21 only 80% and 66% in the same procedure,which indicated that cobalt oxides in formational p-n 27
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Fig. 5. (a) The photocatalytic activities of as-prepared samples for MB degradation under visible-light. (b) The pseudo-first-order reaction kinetics apparent rate constants for MB degradation. (c) UV–visible absorption spectra of ZnCo12 photocatalyst under visible light irradiation. (d)Degradation efficiency of ZnCo12 with increasing number of catalytic cycles.
of organic pollutants, we prepared the samples with varying the dosage of 30 mg, 40 mg, 50 mg and 60 mg to study the effect of degrading MB under visible light irradiation (Fig. 7). The results indicated that photocatalytic performance enhanced with percent of catalyst in solution increased, which could be assigned to the fact that an increase in the number of samples improved the number of photons absorbed and active sites. However, with the dosage increased in excess of 50 mg, the photoactivity suffered fading over the course of 180 min time. It can be attributed to the fact the addition of a substantial amount of dark catalyst caused shading effect and had an impact on the penetration of light, which resulted in a loss of light harvesting and a decline of the photocatalytic performance. Hence, the optimum amount of catalyst is 50 mg for the MB degradation.
Fig. 7. (a) XRD pattern and (b) SEM image of ZnCo12 after photocatalytic runs.
Fig. 6. Photocatalytic activities of different amounts as-prepared ZnCo12 for MB degradation under visible-light.
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Fig. 9. Trapping experiments using different active species trappers for photodegradation of MB over ZnCo12 under visible light illumination.
Fig. 8. PL emission spectra of pure ZnO/C, pure Co3O4/C, ZnCo11, ZnCo21 and ZnCo12.
ZnCO12 catalyst reveals a higher degradation rate compared with reported Co3O4 based heterojunction [40], the shorter irradiation time and higher catalytic performance confirm the improvement of photocatalytic activity by the unique structure. It is generally acknowledged that, in a certain excitation energy, the electrons and holes generate and then shift to the surface of the semiconductor participating in redox reactions which lead to generate the reactive species such as the hydroxyl radical (•OH) and superoxide radical (•O2−) during the photocatalytic degradation process. So the species trapping tests were executed to insight the photocatalytic mechanism. As shown in Fig. 9, compared with the bare ZnCo12 remove dye under visible irradiation, the adding of the isopropyl alcohol (IPA, the hydroxyl radicals scavenger) and ammonium oxalate (AO, the holes scavenger) inhibited the degradation rate of MB (70% and 80%), deducing that the •OH radicals and the holes were the imperative active species. In addition, the adding of the benzoquinone (BQ, the superoxide radicals scavenger) caused trivial decline of the degradation efficiency, indicating the •O2− radicals had some help to some extent. Furthermore, when the AgNO3 (the electrons scavenger) was added into reaction system, the rate of the degradation was accelerated slightly, the probable reason was that the elimination of electrons decreased the recombination rate of the electrons and holes. In line accordance with above obtained experiment information and literature report, a possible photocatalytic mechanism for the MB photodegradation over the ZnO/Co3O4/C structures is put forward (Scheme 2). The higher photocatalytic activity of the hybrid composition can be ascribed to the synergistic effect of ZnO, Co3O4 and carbon. Co3O4 is a p-type semiconductor, while ZnO is an n-type semiconductor [41]. The p-type Co3O4 and n-type ZnO heterojunctions are formed in situ after the calcination of the ZnCo MOFs. At the interface of the heterostructures, the electrons transfer turn up from Co3O4 to ZnO until equilibrium owing to the difference of Fermi levels, then the p-type
3.3. Photocatalytic mechanism Further experiment was investigated for the enhancing photocatalytic mechanism. Photoluminescence emission spectra (PL) served as assessment system for the recombination and migration capacity of photoelectrons and holes (Fig. 8). As we can expect, the PL intensity of single semiconductor sample at 475 nm was higher than the ZnO/ Co3O4/C nanoparticle, which was relevant to their direct band gap structure leading to low separating efficiency of charge carriers. In the presence of p-n type ZnO/Co3O4 heterostructures, especially C doping hollow dodecahedron, the lower intensity of PL emission manifested that the design of ZnO/Co3O4/C system can impede the recombination rate of photoinduced electrons-holes, accelerate charge transfer, as well as give rise to photocatalytic activity [40]. As illustrated in Table 1, the
Table 1 Photocatalytic performance of ZnO/Co3O4/C compared with Co3O4 based heterojunction. Materials
Experimental conditions
Photodegradation efficiency
Reference
Co3O4 sheets
Catalyst = 0.5 g/L MB = 20 mg/L Irradiation time: 180 min Catalyst = 3 g/L Phenol = 18 mg/L Irradiation time: 300 min Catalyst = 1 g/L MO = 10 mg/L Irradiation time: 180 min Catalyst = 1 g/L Orange II = 10 mg/ L Irradiation time: 300 min Catalyst = 0.2 g/L MB = 1.2 ×10−5 M Irradiation time: 300 min Catalyst = 1 g/L RhB = 10 μM Irradiation time: 400 min Catalyst = 1 g/L 2,4-DCP = 20 mg/L Irradiation time: 150 min Catalyst = 0.5 g/L MB = 10 mg/L Irradiation time: 180 min
80%
Ref. [12]
95%
Ref. [10]
100%
Ref. [19]
84.7%
Ref. [20]
85%
Ref. [43]
89%
Ref. [47]
91%
Ref. [44]
95%
This work
Co3O4/BiVO4
Co3O4-g-C3N4
Bi2O3/Co3O4
ZnO/Co3O4
Co3O4-ZnO
ZnO/Co3O4graphene
ZnO/Co3O4/C
Scheme 2. The photocatalytic degradation mechanism scheme of ZnO/Co3O4/C heterojunction.
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Co3O4 regions bring negative charges and the n-type ZnO possess positive charges. Whereafter, the joint interface shapes an inner electrostatic field. When visible light beat down on as-synthesized samples, Co3O4 are excited to yield photoinduced electrons from valence band (VB) to conduction band (CB), on the impact of the decreased potential energy, the CB electrons of Co3O4 can spontaneously transfer to the CB of ZnO, thus improve the reduction efficiency of the recombination of the photogenerated electron-hole pairs [42]. The physical connection between carbon and heterojunctions would be regarded as a continuous path for rapid excited electrons transfer from CB to carbon matrix and further captured by adsorbed oxygen molecular (O2) to generate the superoxide radical anions (•O2-), and proceed to the next step to form the hydroxyl radicals (•OH). In the meantime, the excited holes left behind in the VB are trapped by H2O of catalysis surface and so forth to produce the •OH species, participating in the photodegradation process of organic dye [43–47]. Therefore, it comes to a conclusion that the formation of in-situ ZnO/Co3O4/C system can improve the separation and transfer of photoinduced charge carriers, display high-efficiency photocatalytic capacity.
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4. Conclusions In summary, this work reports a facile topotactic transformation approach to construct hollow ZnO/Co3O4/C photocatalysts with desired morphology via direct calcination of MOFs in air atmosphere. The as-synthesized hollow dodecahedra nanocomposite possesses high surface area, which benefits for exposure of active sites. Furthermore, the dramatical enhancement of photocatalytic performance exhibited by the degradation of MB as a model of dye pollutions can be explained in terms of effective charge separation and charge transfer mechanism arising from binary oxide heterogeneous structure and the introduction of the carbon. ZnCo12 demonstrated the optimal catalytic performance, which is about 8.4-fold k values of pure ZnO/C. In addition, the in-situ synthesized composite showed conspicuous stability and recyclability after five runs. It should be pointed out that this study could shed some light for the construction of high-efficiency photocatalytic material, which can be potentially utilized for environmental purification. Acknowledgements The authors feels very much indebted the partial financial support from the National Natural Science Foundation of China (Nos. 51272052 and 50902040). References [1] Y. Horiuchi, T. Toyao, M. Takeuchi, M. Matsuoka, M. Anpo, Recent advances in visible-light-responsive photocatalysts for hydrogen production and solar energy conversion-from semiconducting TiO2 to MOF/PCP photocatalysts, Phys. Chem. Chem. Phys. 15 (2013) 13243–13253. [2] J. Nowotny, M.A. Alim, T. Bak, M.A. Idris, M. Ionescu, K. Prince, et al., Defect chemistry and defect engineering of TiO2-based semiconductors for solar energy conversion, Chem. Soc. Rev. 44 (2015) 8424–8442. [3] S. Liu, Z.-R. Tang, Y. Sun, J.C. Colmenares, Y.-J. Xu, One-dimension-based spatially ordered architectures for solar energy conversion, Chem. Soc. Rev. 44 (2015) 5053–5075. [4] T. Edison, R. Atchudan, M.G. Sethuraman, Y.R. Lee, Reductive-degradation of carcinogenic azo dyes using Anacardium occidentale testa derived silver nanoparticles, J. Photochem. Photobiol. B 216 (2016) 604–610. [5] W.-J. Ong, L.-L. Tan, Y.H. Ng, S.-T. Yong, S.-P. Chai, Graphitic carbon nitride (gC3N4)-based photocatalysts for artificial photosynthesis and environmental remediation: are we a step closer to achieving sustainability? Chem. Rev. 116 (2016) 7159–7329. [6] C.J. Li, S.P. Wang, T. Wang, Y.J. Wei, P. Zhang, J.L. Gong, Monoclinic porous BiVO4 networks decorated by discrete g-C3N4 nano-Islands with tunable coverage for highly efficient photocatalysis, Small 10 (2014) 2783–2790. [7] Honda Fujishima, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [8] A. Gasparotto, D. Barreca, D. Bekermann, A. Devi, R.A. Fischer, P. Fornasiero, et al., F-Doped Co3O4 photocatalysts for sustainable H2 generation from water/ethanol, J. Am. Chem. Soc. 133 (2011) 19362–19365.
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g-C3N4 quantum dots (CN QDs): an efficient visible-light-driven Z-scheme hybridized photocatalyst, Appl. Catal. B-Environ. 202 (2017) 611–619. Y. Tak, K. Yong, A novel heterostructure of Co3O4/ZnO nanowire array fabricated by photochemical coating method, J. Phys. Chem. C 112 (2008) 74–79. T. Wang, L. Shi, J. Tang, V. Malgras, S. Asahina, G. Liu, et al., A Co3O4-embedded porous ZnO rhombic dodecahedron prepared using zeolitic imidazolate frameworks as precursors for CO2 photoreduction, Nanoscale 8 (2016) 6712–6720. X. Li, Y. Gao, J. Liu, X. Yu, Z. Li, Facile synthesis of Ti3+ doped Ag/AgI TiO2 nanoparticles with efficient visible-light photocatalytic activity, Int. J. Hydrog. Energy 42 (2017) 13031–13038. C. Dong, X. Xiao, G. Chen, H. Guan, Y. Wang, Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/shell nanorods, Mater. Chem. Phys. 155 (2015) 1–8. K. Li, B. Chai, T. Peng, J. Mao, L. Zan, Preparation of AgIn5S8/TiO2 heterojunction nanocomposite and its enhanced photocatalytic H2 production property under visible light, ACS Catal. 3 (2013) 170–177.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Bimetal-organic frameworks derived carbon doped ZnO/Co3O4 heterojunction as visible-light stabilized photocatalysts Nannan Liua,b,c, Zhonghua Lia,b,
T
⁎
a
Key Laboratory of Microsystems and Microstructures Manufacturing, Ministry of Education, Harbin Institute of Technology, Harbin 150001, China Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150001, China c School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Metal-organic framework Carbon doping Heterojunction Large specific area Visible-light
The construction of transition-metal oxygen heterojunction photocatalysts has been paid close attention for application of versatile functionalities. Many methods have been developed and applied to improve photocatalytic performance, such as doping and morphology control. In this study, carbon (C)-doped binary oxide heteronanostructures with hollow dodecahedra morphology are fabricated by means of topotactic transformation of metal-organic frameworks. The converted hollow C doping ZnO/Co3O4 nanocomposite with large specific area has advantage in charge transfer and charge-separation efficiency, and then is evaluated by degrading methylene blue (MB) under visible light irradiation (λ > 400 nm). The experiment demonstrates the optimal catalytic performance is about 8.4-fold k values of pure ZnO. Importantly, the in-situ synthesized composites show noteworthy stability and recyclability, even after five runs. A possible photocatalytic mechanism also is mentioned, the results illustrate that synergistic effect of coupled semiconductor system with C doping dramatically suppresses the recombination of photogenerated electron-hole pairs, and the hydroxyl radicals (•OH) and holes play a critical role. This work exhibits an effective method for the design of other nonmetal doping sound heterojunctions with well-controlled morphology that significantly enhance photocatalytic behavior.
1. Introduction Solar energy is the most abundant and available clean energy source today [1–3]. In this manner, it is essential to utilize solar energy for green environmental pollution management [4–6]. Inspiration by the pioneering work of Fujishima and Honda in 1972, semiconductor-based photocatalysts and photocatalytic processes have attracted much attention recently [7]. Compared with TiO2 investigated mostly, Co3O4 is an intriguing semiconductor revealing excellent optical, electronic, and other physical and chemical property [8–11]. Besides, the narrow band gap of Co3O4 could reduce the excitation energy of electron-hole pairs separating, while the high electron mobility of Co3O4 is benefit for photocatalytic activity improvement [12]. Unfortunately, it is generally accepted that the fast recombination of photogenerated electron-hole pairs is still a significant challenge. It is well established that the heterojunction is an ideal solution for efficacious electron collection and separation of semiconductor [13]. For the principle, the p-type and n-type semiconductor could create an interface, the formed p-n heterojunction with a space-charge region at the interfaces demonstrates an electronic field caused by the migration
of electrons and holes, which emerges an outstanding ability to separate the electron-hole pairs [14–18]. Numerous efforts for blending with other compounds such as Bi2O3, Bi2WO6, g-C3N4, etc. have been gotten to optimize Co3O4-based photocatalysis property [19–22]. However, the incongruous position of energy band and low efficiency remain an enormous challenge. It's worth noting that the n-type ZnO is attractive and utilized in catalytic field and energy storage, and the appropriate band gap would regulate the recombination of photogenerated electron-hole pairs after compositing with Co3O4. Nonetheless, there have been a number of high profile cases compromising to indicate that the normal synthetic methods of metal oxides invariably generate a low specific area [24], limited exposure of active sites by the poor surface area would restrict the catalytic efficiency [25]. Furthermore, the direct ZnO/Co3O4 heterojunction without the linkage is unbeneficial for the rapid charge transfer via the boundary. Hence, it's urgent to take measures to further advance photocatalytic performance. Recent developments of metal organic frameworks (MOFs) have attracted much attention because of its abundant porous and large specific area, which make it have potential utilization in energy storages, light harvesting, catalysis and drug delivery [26–28].
Abbreviations: MOFs, metal-organic frameworks; ZnCo12, ZnO/Co3O4 1:2; ZnCo11, ZnO/Co3O4 1:1; ZnCo21, ZnO/Co3O4 2:1; MB, methylene blue ⁎ Corresponding author at: Key Laboratory of Microsystems and Microstructures Manufacturing, Ministry of Education, Harbin Institute of Technology, Harbin 150001, China. E-mail address: [email protected] (Z. Li). https://doi.org/10.1016/j.mssp.2018.01.004 Received 5 November 2017; Received in revised form 18 December 2017; Accepted 2 January 2018 1369-8001/ © 2018 Elsevier Ltd. All rights reserved.
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Jobin Yvon iHR320 imaging spectrometer.
Moreover, the carbon doping metal oxides ramifications derived from MOFs also continue to be a great capacity. In addition to this, the morphology of ramifications could be tailored and epitaxial prepared by the unique structure of MOF precursors, which provides a fresh route for nonmetal doping of metal oxides synthesis [29–31]. In this study, we design and synthesize hollow C doping ZnO/Co3O4 (ZnO/Co3O4/C) dodecahedron heterojunctions with MOF precursors. Although the ZnO/Co3O4 hybrid composites were mentioned in previous reports, to the best of our knowledge, the hollow ternary compound involved amorphous carbon is generated firstly with MOFs precursor as photocatalysts. Importantly, the as-obtained sample simultaneously possesses initial MOF morphology (porous structure with connected nanoparticles) along with abundant C doping, which promote the charge transfer and separation [32]. Therefore, the sufficient physical integration of nanoparticle and C doping is a promising approach to obtain superior performance photocatalysts. To sum up, The as-prepared ZnO/Co3O4/C composites reveal several extensional advantages: 1) large specific area of MOF based materials is benefit for exposure of active sites; 2) C doping can serve as a potentially vital pathway to increase the charge mobility of photocatalysts most efficiently; 3) the in situ synthesized composite could furnish more interoperable interface than ex situ process; 4) the composite was prepared with a simple method, which avoids the complex dislodge template process of hollow structure; 5) the hollow structures further magnify the contact of catalyst and dye. Moreover, The ZnO/Co3O4/C composite displays an enhancement of inherent absorbance than the pure ZnO for visible light.
2.4. Catalytic activity measurements The activities of the catalysts were evaluated in a 200 mL reaction vessel containing MB as a model organic contaminant with the initial concentration of 10 mg/L−1. A 300 W Xe lamp provided a visible light source with a 400 nm cutoff filter at 15 cm distance. The process temperature was maintained via using circulation condensed water. In each typical experiment, 100 mL of MB aqueous solution containing 50 mg of catalytic amount was first kept in the dark under magnetical stirring for 1.5 h to establish the adsorption-desorption equilibrium. Then, at each 20 min interval of irradiation time, 3–5 mL of suspensions was taken and centrifuged (13,000 rpm, 1 min). Soon afterwards, the liquid supernatants was collected to monitor the change process of MB concentration by using the UV–vis spectrophotometer at 664 nm [31]. 3. Results and discussion 3.1. Characterization of the as-prepared samples The typical synthesis process of the materials was exhibited in Scheme 1. The MOFs were prepared by dropping 2-methylimidazole into metallic solution and aged for 24 h，and the metallic oxides were synthesized with one-step carbonization process of the obtained MOFs. The molar radio of ZnO to Co3O4 in the preparation of different heterojunctions was approximately 1:2 for ZnCo12, 1:1 for ZnCo11, and 2:1 for ZnCo21. And the bare ZnO/C and Co3O4/C were also prepared with singular metallic solution as contrast. The understanding of crystal phases for ZnO/Co3O4/C composites supported by the XRD analysis was revealed in Fig. 1a. The three samples showed the existing of both crystal ZnO (JCPDS No. 76-0704) and Co3O4 (JCPDS No. 74-2120), which meant that the crystalline had no change by adjusting the metal ratios. However, the peak intensity of each oxide in the three simples was completely different. The peaks of ZnO turned weakness with the increment of Co3O4, this phenomenon indicated the oxides had the same crystal state but different concentration in different composites. Thermogravimetric analysis (TG) was carried out for the sake of clearness the process of synthesizing ZnCo12 by MOFs precursor (Fig. 1b). First weight loss before 200 °C corresponded to the removal of water molecules, and pyrolysis occurred at 310 °C, that generated carbon and metal oxide, and then quality rapidly reduced at 520 °C due to the oxidization of carbon in the material. Until 600 °C, weightlessness was to stop and the carbon in the material remove. Experiment showed that ZnO, Co3O4 and carbon species existed at the same time with calcination temperature of 400 °C. The morphologies of the simples were characterized by SEM and TEM images. As revealed in Fig. 2a, the MOFs had a normative regular dodecahedron structure with a diameter of approximately 400 nm, while after calcination, the simples of oxide turned into a sunken hollow dodecahedron structure. For the ZnO/Co3O4/C composites
2. Materials and methods 2.1. Preparation of MOFs In a typical synthesis process, 2.97 g Zn(NO3)2·6H2O (10 mmol) and 1.45 g Co(NO3)2·6H2O (5 mmol) were dissolved in 150 mL methyl alcohol and stirred for 30 min to form a solution A. 4.97 g 2-methylimidazole was added into 50 mL methyl alcohol to form a clear solution B. Then solution B was poured into solution A slowly with continuous stirring for 15 min. The resulted solution was aged for 24 h at room temperature. And the MOF after centrifugation was dried at 60 °C for 12 h. The different MOFs were prepared with different molar ratios of Zn(NO3)2·6H2O and Co(NO3)2·6H2O. 2.2. Preparation of metallic oxide The ZnO/Co3O4/C was synthesized by one-step calcination process. The as-prepared MOFs were transferred into a crucible and heated to 400 °C for 100 min. The ZnO/C and Co3O4/C were obtained with different MOF precursors. 2.3. Characterization The crystalline phase and phase composition analysis of all the prepared samples were carried out by the X-ray diffractometer (XRD) using Cu Kα (λ = 1.5406 Å) radiation source. The morphologies of all the samples were performed by Transmission electron microscopy (TEM) on a JEOL JEM-2100 at the acceleration voltage of 200 kV and Scanning electron microscopy (SEM), moreover, High resolution transmission electron microscopy (HRTEM) and elemental mapping were also used to characterize the samples. The surface chemistries of the as-prepared samples were measured by X-ray photoelectron spectroscopy (XPS) in ESCALAB250Xi system with Mg Kα X-ray sources. The C1s peak at 284.6 eV was utilized to calibrate the whole of XPS spectra. The specific surface area was detected by BET N2 adsorptiondesorption isotherms. UV–vis absorption spectra were acquired with the use of a UV-2550, Shimadzu, Japan spectrophotometer. The photoluminescence (PL) spectra analyses were conducted with Horiba
Scheme 1. Synthesis scheme of ZnO/C, Co3O4/C and ZnO/Co3O4/C.
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Fig. 1. XRD patterns of ZnCo12, ZnCo11, and ZnCo21.
observation from the XRD pattern. The exposure of active sites is proportional to the surface area of catalysts. The BET measurements revealed that the MOF had large specific area of 1273 m2/g (Fig. 3a). While after oxidation in the air, the specific area of ZnCo21, ZnCo11 and ZnCo12 was 43, 91 and 93 m2/g (Fig. 3b), and the pore volume of ZnCo21, ZnCo11 and ZnCo12 was 0.08, 0.20 and 0.24 cm3/g, respectively. The different specific area and pore volume identified with the SEM images, the hollow ZnCo12 displayed highest specific area and the solid ZnCo21 only showed half of the area to ZnCo12. In spite of this, the obtained catalysts revealed higher specific area than the most oxides, which were beneficial to enhance the photocatalytic activity. The surface chemical composition and bonding environment were
exhibited in Fig. 2d–f, the morphology of surfaces changed into more sinking with the decrease of Co3O4 content. The ZnCo12 had an ecumenical roughly surface, and the surface of ZnCo11 turned into a reticular structure, while the ZnCo21 displayed a considerable shrunken dodecahedron construction. The phenomenon explained the process of topotactic transformation not only depended on the morphology of precursors, but also was influenced by the species and content of reagents. The elemental mapping of ZnCo12 in Fig. 2g illustrated the Zn, Co, O and C elements were uniformly distributed in the composite. The hollow structure of ZnCo12 was further confirmed by the TEM results in Fig. 2h, and the HRTEM image in Fig. 2i exhibited lattice fringes of ZnCo12, which revealed the heterojunction of the (111) plane of Co3O4 and (101) plane of ZnO in ZnCo12. The results were consistent with the
Fig. 2. SEM images of (a) MOF, (b) ZnO/C, (c) Co3O4/C, (d) ZnCo12, (e) ZnCo11 and (f) ZnCo21. (g) Elemental mapping of Zn, Co and O in ZnCo12. (h) TEM and (i) HRTEM image of ZnCo12.
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Fig. 3. N2 adsorption–desorption isotherms of as-prepared (a) ZnCo MOF, (b) ZnCo11, ZnCo12 and ZnCo21 composites.
Fig. 4. XPS spectrum of ZnCo21, ZnCo11 and ZnCo12: (a) Zn 2p, (b) Co 2p and (c) O 1s.
heterojunctions were of importance factor for the improvement of photocatalytic performance. A pseudo first-order kinetic model was used to calculate the rate constant (k) of degradation of dye concentration. As shown in Fig. 5b, the k value of the ZnCo12 optimal activity equaled to 0.00741 min−1, 8.4 times of pure ZnO/C particle (0.00088 min−1), 5.6 times of Co3O4/C particle (0.00132 min−1), confirming the crucial role of p-n heterojunctions for highly enhanced catalysis activity. The photocatalytic degradation process was monitored at the peak of 664 nm (the characteristic absorption of MB) by the temporal UV–vis spectral changes of MB solutions as a function of irradiation time, in the presence of the ZnCo12 nanocomposite (Fig. 5c). With time passing, the major absorbance syllabify decreased. After 180 min illumination, corresponding absorption peaks faded away, illustrating that the change process of MB concentration was chromophores split, but being merely achromatized by adsorption [23,38]. Whether photocatalysts can be applied for depollution of environment, the stability is one of the most pivotal factors. Herein, the circulation experiments of the degradation of MB were carried out five runs to estimate the stability of as-synthesized samples. After the completion of each cycling test, the photocatalyst was centrifuged and collected, then washed with water and absolute alcohol for three times, the recycled sample was dried at 70 °C for 12 h for the next cycle of photocatalysis. It can be observed from Fig. 5d, the photocatalyst maintained reliable activity with imperceptible deactivation after successive cycles, indicating the value of practical application [39]. The XRD pattern of the recycled ZnCo12 sample exhibited in Fig. 6a disclosed there was no discernible change in the phase and structure after five cycles, and the morphology of recycled ZnCo12 identified with the fresh catalyst (Fig. 6b), which was in favor of the stability of the ZnO/ Co3O4/C dodecahedron nanostructure photocatalyst in the same way. To explore the optimal amount of photocatalyst for the degradation
investigated by XPS in Fig. 4. Fig. 4a illustrated the determined oxidation states of Zn from Zn 2p XPS spectrum, and the peak energies of 1019.8 and 1042.9 eV were attributed to the binding energies of the Zn 2p3/2 and the Zn 2p1/2 of ZnO. The XPS spectrum of Co 2p were exhibited in Fig. 4b, the peaks of 778.3 and 793.8 eV identified Co3+ 2p3/ 3+ 2p1/2, and the fitting peaks of 779.3, 787.7 and 803.2 eV 2 and Co were due to the existence of Co2+, and the difference between the peaks of Co3+ 2p1/2 and Co3+ 2p3/2 was 15.5 eV, which demonstrated the presence of the Co3O4 phase [11]. The XPS spectrum of O 1s consisting of three peaks (Fig. 4c) around 528.2, 530.0 and 531.2 eV were related to the oxygen in the Co3O4, ZnO crystal lattice and the surface chemisorbed oxygen, respectively. And the peak intensity was in agreement with the radio of ZnO and Co3O4. Moreover, there were no significant difference for the peak energies among the three samples, revealing a similar bonding environment but different content [33]. Fig. 4d shown the XPS spectrum of C 1s, the two peaks of 283.7 and 287.6 eV represented the presence of C-C and C-O bonds, respectively.
3.2. Photocatalytic activity The photocatalytic performance of the as-prepared ZnO/C, Co3O4/C and ZnO/Co3O4/C compounds was estimated via recording photocatalytic degradation process of MB under visible light irradiation, which had been seen as a representative dye [34]. Fig. 5a demonstrated ZnO/C and Co3O4/C had slight photocatalytic activities due to wide band gap resulting in the impossibility on absorbing visible light as well as high recombination of photogeneration electrons and holes, respectively [35–37]. However, the ZnO/Co3O4/C nanostructures showed remarkable enhancement of the degradation of MB. Among them, the dye degradation of the ZnCo12 photocatalysis reached up to 95% in 180 min, while the ZnCo11 and the ZnCo21 only 80% and 66% in the same procedure,which indicated that cobalt oxides in formational p-n 27
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Fig. 5. (a) The photocatalytic activities of as-prepared samples for MB degradation under visible-light. (b) The pseudo-first-order reaction kinetics apparent rate constants for MB degradation. (c) UV–visible absorption spectra of ZnCo12 photocatalyst under visible light irradiation. (d)Degradation efficiency of ZnCo12 with increasing number of catalytic cycles.
of organic pollutants, we prepared the samples with varying the dosage of 30 mg, 40 mg, 50 mg and 60 mg to study the effect of degrading MB under visible light irradiation (Fig. 7). The results indicated that photocatalytic performance enhanced with percent of catalyst in solution increased, which could be assigned to the fact that an increase in the number of samples improved the number of photons absorbed and active sites. However, with the dosage increased in excess of 50 mg, the photoactivity suffered fading over the course of 180 min time. It can be attributed to the fact the addition of a substantial amount of dark catalyst caused shading effect and had an impact on the penetration of light, which resulted in a loss of light harvesting and a decline of the photocatalytic performance. Hence, the optimum amount of catalyst is 50 mg for the MB degradation.
Fig. 7. (a) XRD pattern and (b) SEM image of ZnCo12 after photocatalytic runs.
Fig. 6. Photocatalytic activities of different amounts as-prepared ZnCo12 for MB degradation under visible-light.
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Fig. 9. Trapping experiments using different active species trappers for photodegradation of MB over ZnCo12 under visible light illumination.
Fig. 8. PL emission spectra of pure ZnO/C, pure Co3O4/C, ZnCo11, ZnCo21 and ZnCo12.
ZnCO12 catalyst reveals a higher degradation rate compared with reported Co3O4 based heterojunction [40], the shorter irradiation time and higher catalytic performance confirm the improvement of photocatalytic activity by the unique structure. It is generally acknowledged that, in a certain excitation energy, the electrons and holes generate and then shift to the surface of the semiconductor participating in redox reactions which lead to generate the reactive species such as the hydroxyl radical (•OH) and superoxide radical (•O2−) during the photocatalytic degradation process. So the species trapping tests were executed to insight the photocatalytic mechanism. As shown in Fig. 9, compared with the bare ZnCo12 remove dye under visible irradiation, the adding of the isopropyl alcohol (IPA, the hydroxyl radicals scavenger) and ammonium oxalate (AO, the holes scavenger) inhibited the degradation rate of MB (70% and 80%), deducing that the •OH radicals and the holes were the imperative active species. In addition, the adding of the benzoquinone (BQ, the superoxide radicals scavenger) caused trivial decline of the degradation efficiency, indicating the •O2− radicals had some help to some extent. Furthermore, when the AgNO3 (the electrons scavenger) was added into reaction system, the rate of the degradation was accelerated slightly, the probable reason was that the elimination of electrons decreased the recombination rate of the electrons and holes. In line accordance with above obtained experiment information and literature report, a possible photocatalytic mechanism for the MB photodegradation over the ZnO/Co3O4/C structures is put forward (Scheme 2). The higher photocatalytic activity of the hybrid composition can be ascribed to the synergistic effect of ZnO, Co3O4 and carbon. Co3O4 is a p-type semiconductor, while ZnO is an n-type semiconductor [41]. The p-type Co3O4 and n-type ZnO heterojunctions are formed in situ after the calcination of the ZnCo MOFs. At the interface of the heterostructures, the electrons transfer turn up from Co3O4 to ZnO until equilibrium owing to the difference of Fermi levels, then the p-type
3.3. Photocatalytic mechanism Further experiment was investigated for the enhancing photocatalytic mechanism. Photoluminescence emission spectra (PL) served as assessment system for the recombination and migration capacity of photoelectrons and holes (Fig. 8). As we can expect, the PL intensity of single semiconductor sample at 475 nm was higher than the ZnO/ Co3O4/C nanoparticle, which was relevant to their direct band gap structure leading to low separating efficiency of charge carriers. In the presence of p-n type ZnO/Co3O4 heterostructures, especially C doping hollow dodecahedron, the lower intensity of PL emission manifested that the design of ZnO/Co3O4/C system can impede the recombination rate of photoinduced electrons-holes, accelerate charge transfer, as well as give rise to photocatalytic activity [40]. As illustrated in Table 1, the
Table 1 Photocatalytic performance of ZnO/Co3O4/C compared with Co3O4 based heterojunction. Materials
Experimental conditions
Photodegradation efficiency
Reference
Co3O4 sheets
Catalyst = 0.5 g/L MB = 20 mg/L Irradiation time: 180 min Catalyst = 3 g/L Phenol = 18 mg/L Irradiation time: 300 min Catalyst = 1 g/L MO = 10 mg/L Irradiation time: 180 min Catalyst = 1 g/L Orange II = 10 mg/ L Irradiation time: 300 min Catalyst = 0.2 g/L MB = 1.2 ×10−5 M Irradiation time: 300 min Catalyst = 1 g/L RhB = 10 μM Irradiation time: 400 min Catalyst = 1 g/L 2,4-DCP = 20 mg/L Irradiation time: 150 min Catalyst = 0.5 g/L MB = 10 mg/L Irradiation time: 180 min
80%
Ref. [12]
95%
Ref. [10]
100%
Ref. [19]
84.7%
Ref. [20]
85%
Ref. [43]
89%
Ref. [47]
91%
Ref. [44]
95%
This work
Co3O4/BiVO4
Co3O4-g-C3N4
Bi2O3/Co3O4
ZnO/Co3O4
Co3O4-ZnO
ZnO/Co3O4graphene
ZnO/Co3O4/C
Scheme 2. The photocatalytic degradation mechanism scheme of ZnO/Co3O4/C heterojunction.
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Co3O4 regions bring negative charges and the n-type ZnO possess positive charges. Whereafter, the joint interface shapes an inner electrostatic field. When visible light beat down on as-synthesized samples, Co3O4 are excited to yield photoinduced electrons from valence band (VB) to conduction band (CB), on the impact of the decreased potential energy, the CB electrons of Co3O4 can spontaneously transfer to the CB of ZnO, thus improve the reduction efficiency of the recombination of the photogenerated electron-hole pairs [42]. The physical connection between carbon and heterojunctions would be regarded as a continuous path for rapid excited electrons transfer from CB to carbon matrix and further captured by adsorbed oxygen molecular (O2) to generate the superoxide radical anions (•O2-), and proceed to the next step to form the hydroxyl radicals (•OH). In the meantime, the excited holes left behind in the VB are trapped by H2O of catalysis surface and so forth to produce the •OH species, participating in the photodegradation process of organic dye [43–47]. Therefore, it comes to a conclusion that the formation of in-situ ZnO/Co3O4/C system can improve the separation and transfer of photoinduced charge carriers, display high-efficiency photocatalytic capacity.
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