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shell nanorods
Materials Chemistry and Physics xxx (2015) 1e8
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Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/shell nanorods Chengjun Dong, Xuechun Xiao, Gang Chen, Hongtao Guan, Yude Wang* Department of Materials Science & Engineering, Yunnan University, 650091 Kunming, People's Republic of China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
1D Co3O4/ZnO core/shell NRs were synthesized on nickel foil by a twostep synthetic strategy. The thickness of ZnO coating is determined to be about 20 nm. Co3O4/ZnO NRs exhibited better photocatalytic performance for MB degradation under UV irradiation. The formation of p-n junctions confirmed by PL is a critical factor for photocatalytic enhancement. The working mechanism for Co3O4/ ZnO NRs as photocatalyst was proposed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2014 Received in revised form 29 December 2014 Accepted 10 January 2015 Available online xxx
One-dimension (1D) Co3O4/ZnO core/shell nanorods (NRs) were synthesized on nickel foil substrate by means of a two-step synthetic strategy. Co3O4 NRs were initially fabricated by a facile hydrothermal reaction and then ZnO was coated via a simple thermal decomposition. The results verified that the surface of the p-type Co3O4 core was uniformly assembled by the n-type ZnO nanoparticles with approximate 20 nm thickness. Compared with pristine Co3O4 NRs, Co3O4/ZnO core/shell NRs was exhibited to have a much higher photocatalytic properties in the decomposition of a model dye compound, methylene blue (MB), under ultraviolet irradiation. As confirmed by Photoluminescence (PL) spectra, the formation of p-n junction heterostructures gives rise to the enhanced photocatalystic performance of Co3O4/ZnO core/shell NRs. This study provides a general and effective method in the fabrication of 1D composition NRs with sound heterojunctions that show remarkable enhancement of photocatalytic performance. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Oxides Composite materials Heterostructures
1. Introduction As an ideal green technology, the semiconductor photocatalysts have been extensively recognized and documented, due to their potential applications in environmental cleaning and solar energy
* Corresponding author. E-mail address: [email protected] (Y. Wang).
utilization. It is generally accepted that the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalysts. The higher specific surface area of the catalysts can lead to a more unsaturated surface coordination sites to greatly increase the photocatalytic reaction sites [1]. 1D nanostructures including NRs, nanowires, nanotubes, nanofibers, have received increasing attention because of their unique properties associated with their highly anisotropic geometry and size
http://dx.doi.org/10.1016/j.matchemphys.2015.01.033 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: C. Dong, et al., Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/ shell nanorods, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.01.033
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C. Dong et al. / Materials Chemistry and Physics xxx (2015) 1e8
confinement [2]. The high specific surface area for reactant molecules facilitates transport of reagents to the active sites during the catalytic reaction. Therefore, many oxidations with 1D or quasi-1D nanostructures have been widely used as catalysts or additive to catalysts or support of catalysts in a variety of reactions [2e4]. Photocatalysis, in general, occurs based on the reaction between adsorbed molecules (oxygen, surface hydroxyls groups) or water and photogenerated electron/hole pairs excited by photon with equal or higher energy than the band gap of the semiconductor. As a result, the high oxidizing radicals are yielded, thus the organic molecules present in the solution are readily oxidized into carbon dioxide and water [5]. However, the electron/hole recombination is blamed for the low quantum yields, which is still a big obstacle for the photocatalytic activity improvement. For further improvement of photocatalytic activity, numerous efforts such as doping, formation of p-n heterojunctions and so forth, have been attempted over the past decades. In particular, research found that the p-n junctions formed in combination with both p-type and n-type semiconductors can effectively reduce the recombination rate of photogenerated electro/hole pairs, which subsequently enhances the photocatalytic activity [6,7]. Co3O4 is acceptable as an important p-type semiconductor which has been widely utilized. Up until now, 1D based on Co3O4 systems have been synthesized and showed high performance in CO oxidations [8], lithium-ion batteries [9] and supercapacitor [10]. Unfortunately, there are still no such applications for photocatalysis on the 1D Co3O4 basis. Apart from 1D nanostructure, significant attentions have been directed toward development of Co3O4 based heterojunction catalyst, such as Co3O4/BiVO4 [11], NiO/Co3O4 and Fe2O3/Co3O4 [12]. As an n-type semiconductor, ZnO has been considered as one of the best photocatalytic materials because of its nontoxic nature, low cost and high photochemical reactivity. The combination of p-type Co3O4 and n-type ZnO leads to the build-up of an inner electric field at the p-n junction interface that can be advantageous for photocatalytic, optoelectronic, and gas sensing applications. A diode consisting of p-type Co3O4 nanoplate/n-type ZnO NRs showing reasonable electrical performance has been reported [13]. Room temperature prepared Co3O4/ZnO systems exhibited catalytic behavior in the production of glycerol carbonate [14]. The p- Co3O4/n-ZnO composites were synthesized by a twostep PECVD process, indicating attractive performances in the detection of reducing (CH3CH2OH, CH3COCH3 and oxidizing (NO2) gases [15]. To the best of our knowledge, to date, the fabrication strategy to form p-type Co3O4-core/n-type ZnO-shell heterostructure NRs and its photocatalytic performance has rarely been explored. In our previous work, we reported the direct synthesis of long 1D Co3O4 NRs by a facile hydrothermal method on Ni foil substrate [16]. To combine the advantages of the obtained Co3O4 NRs and the ZnO material and thus to further improve the performance of the combination as heterogeneous catalysts, a nanorod-based p-n junction core/shell structure is being designed and used in this work. We have successfully synthesized uniform p-type Co3O4core/n-type ZnO-shell heterostructure NRs. Its photocatalytic performance has been investigated by taking methylene blue (MB) as an example under UV light irradiation. The Co3O4/ZnO core/shell NRs exhibit enhancement of photocatalytic activity in degradation of MB than single Co3O4 NRs component. In particular, nanorod shape of Co3O4/ZnO core/shell structure can provide more photocatalytic sites and accelerate the surface electron transfer rate because of its higher specific surface area. Additionally, the built-up inner electric field in the interface of p-n Co3O4/ZnO heterojunction can efficiently reduce the recombination of the photogenerated electron-hole to highly drive the photocatalytic reaction.
2. Experimental 2.1. Materials As substrate, nickel foils (99.9%) with 0.1 mm thickness were used. All the chemical reagents and solvents, which were purchased from commercial sources, were analytical grade and used as received without any further purification. The aqueous solutions were freshly prepared with high purity water (18 MU cm at 25 C) throughout all experiments. 2.2. Photocatalyst preparation The Co3O4/ZnO NRs were prepared by a simple two step method on a Ni foil (5 cm 2 cm). In the first step, Co3O4 NRs were prepared according to our previous reported procedure [16]. Briefly, a grounded Ni foil was cleaned sequentially by acetone and deionized water and then put into a volume of 80 mL Teflon at an angle against the wall. A pink color solution was obtained after dissolving 10 mmol Co(NO3)2 in 30 mL of H2O and gradually turned into black followed by adding 20 mL of 28 wt% ammonia solution under magnetically stirring in 30 min. Before transferring to the Teflon, 10 mmol NaNO3 were dissolved into the above solution and the hydrothermal reaction was conducted at 120 C for 12 h. The black Co3O4 NRs on Ni foil were formed via thermal treatment of the collected product at 250 C for 1 h in the air atmosphere. A half of the Ni foil (2.5 cm 2 cm) with highly dense Co3O4 NRs was cut to make next ZnO composition. In the second step, a droplet of 5 mM zinc acetate in N, N-Dimethylformamide (DMF) were dropped onto the synthesized Co3O4 NRs and dried in air. The above composited procedure was repeated by ten times. Finally, the dried zinc acetate loaded Co3O4 NRs were calcined in an oven in air at 350 C with the heating rate of 5 C/min for 30 min to form Co3O4-core/ZnO-shell NRs heterostructure. 2.3. Characterizatrion The X-ray diffraction (XRD) measurements, which were used to characterize the crystalline phase, phase composition of all asprepared samples, were carried out by an X-ray diffractometer (Rigaku TTRIII) in the 2q range of 10e80 using CuKa radiation (l ¼ 1.5406 Å). The accelerating voltage and applied current were 40 kV and 200 mA, respectively. The Raman spectra were recorded on a Raman imaging microscope (Ranishaw Model 2000). A 514.5 nm Arþ laser line with a power output of 20 mW was used for excitation. Scanning electronmicroscopy (SEM) and energydispersive X-ray spectroscopy (EDX) were carried out by a FEI QUANTA 200 equipped with an EDX attachment at an accelerating voltage of 30 kV. Transmission electron microscopy (TEM) and Highresolution transmission electron microscopy (HRTEM) images were performed on a JEOL JEM-2100 at the acceleration voltage of 200 kV. The TEM samples were prepared by carefully scratching the products from Ni foil, followed by dispersing in ethanol and casting them on the copper grids. Fourier transformed infrared (FTIR) spectra, in the range of 4000e500 cm1, were recorded on an infrared spectrophotometer. Photoluminescence (PL) spectra were performed on Horiba Jobin Yvon iHR320 imaging spectrometer. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the surface chemistries of the samples in ESCALAB system with AlKa X-ray radiation at 15 kV. All XPS spectra were accurately calibrated by the C 1s peak at 284.6 eV to compensate for the charge effect. 2.4. Photocatalytic evaluation The photocatalytic activity of the as-prepared Co3O4 NRs and
Please cite this article in press as: C. Dong, et al., Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/ shell nanorods, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.01.033
C. Dong et al. / Materials Chemistry and Physics xxx (2015) 1e8
Co3O4/ZnO core/shell NRs for the degradation of MB in aqueous solution under ultraviolet light was evaluated by measuring the absorbance of the irradiated solution. A 30 W low-pressure mercury lamp was used as a light source with a main emission wavelength of 253.7 nm at a 10 cm distance. The Ni foil (2.5 cm 2 cm) covered with photocatalysts was placed into 50 ml of MB aqueous solutions with a concentration of 10 mg L1 in a quartz beaker. Before illumination, the solutions were kept in the dark for 2 h to reach the adsorptionedesorption equilibrium between the MB and the photocatalyst. At time intervals of 24 h, about 4 mL solutions were extracted for concentration check. The changes of concentration of the extracted solutions were measured by a UV-2401 PC spectrophotometer. The concentrations of the MB were monitored immediately by checking the absorbance at 665 nm during the photodegradation process by using a spectrophotometer (Model No. JH722N). All experiments were carried out under ambient conditions.
3. Results and discussion 3.1. Characterization of the catalysts Fig. 1 displays the XRD patterns of the pure Co3O4 NRs (a) and Co3O4/ZnO core/shell NRs. The diffraction peaks of the Ni foil substrate located at 2q of 51.76 and 76.43 are observed in both samples. It is found that all the identical peaks of the pristine Co3O4 NRs match perfectly to a cubic spinel phase of Co3O4 (JCPDS Card No. 42-1467). The previous reports show that the Co3O4 phase derived from the Co(OH)2 via thermal treatment at 250 C for 1 h [10,16]. By comparing the curves of Fig. 1(b) with (a), three additional weak peaks appear along with Co3O4 phase at 2q ¼ 31.76 , 34.42 , and 36.25 for Co3O4/ZnO core/shell NRs, respectively. These three peaks are attributed to (100), (002), and (101) planes of hexagonal ZnO (JCPDS Card No. 36-1451). The broad and weakness of the ZnO peaks, shown as inset in Fig. 1(b), can be explained by the small crystallite sizes of the as-coated ZnO materials. Besides, no diffraction peaks of impurities are investigated in the XRD patterns. The above analysis significantly confirms that the ZnO has been successfully composited on Co3O4 NRs by the experimental
Fig. 1. XRD patterns for pristine Co3O4 NRs (a) and Co3O4/ZnO core/shell NRs (b), respectively. The diffraction peaks of ZnO are highlighted with red and enlarged in inset for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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strategy which is also used in CuO-core/ZnO-shell NWs preparation [17]. The intensity of Co3O4 remarkably decreases in the case of Co3O4/ZnO core/shell NRs due to the coating of Co3O4 NRs by ZnO, which will be strongly evident in following morphology characterizations. Basing also on comparison results, no apparent peak shifts are observed for Co3O4 phase after ZnO loading, suggesting that the ZnO only loaded on the surface of Co3O4 NRs instead of doping Zn atoms into Co3O4 lattice. A typical room temperature Raman spectra (Fig. 2) of the Co3O4 NRs and Co3O4/ZnO core/shell NRs were examined using a Raman imaging microscope under 514.5 nm excitation with a power output of 20 mW. As we all known, Co3O4 has a cubic spinel structure with Co2þ and Co3þ positioned at tetrahedral and octahedral sites, respectively, which belongs to the space group of Fd3m [18]. Obtained Raman spectra of the Co3O4 NRs consist five Ramanactive modes of A1g þ3F2g þ Eg, which fits well with the theoretical and experimental predicated modes [18,19]. Such an observation is also in good agreement with the above XRD measurement. In the case of Co3O4/ZnO core/shell NRs, two additional peaks were appeared at 435.17 and 574.83 cm-1 respectively. These two peaks are assigned to the E2 (high) and A1 (high) modes of ZnO phase [20]. Again, this result is further evidence of ZnO composition with Co3O4. All the peaks are summarized in Table 1 for the purpose of clarity. Remarkably, all the modes of Co3O4 phase shift towards higher values of the wave number by an amount of 3e9 cm-1 after ZnO loading. This shift suggests that the ZnO, to some extent, affects the Co3O4 phonon structure. The morphologies of the pristine Co3O4 NRs and Co3O4/ZnO core/shell NRs were characterized by SEM, as shown in Fig. 3(a) and (b). From the low magnification SEM images, the whole surface of the Ni foil substrate was covered with uniform and highly dense Co3O4 NRs consisting of bundle NRs and dispersed NRs. The Ni foil covered with Co3O4 NRs is suitable for photocatalytic degradation of dyes in wastewater by taking advantage of reutilization. It is discovered that the difference in terms of both lattice and thermal coefficient between Ni foil and Co3O4 NRs is the reason to the formation of bundle NRs and dispersed NRs [16]. After composited with ZnO, no big changes occurred for the morphology checking via SEM in Fig. 3(b). The chemical composition was analyzed by an energy dispersive X-ray spectroscopy (EDX) for bundle NRs and dispersed NRs marked in black dash rectangle in Fig. 3(b), as further
Fig. 2. Room temperature Raman spectra of pristine Co3O4 NRs (a) and Co3O4/ZnO core/shell NRs recorded under 514.5 nm excitation. (The modes of ZnO are highlighted with red color.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Peak positions of the Raman active phonon modes of Co3O4 NRs and ZnO (unit: cm1). Samples
F2g
Eg
F2g
F2g
A1g
E2 (high) A1 (high)
191.72 473.91 515.87 610.72 680.60 Co3O4 NRs Co3O4/ZnO NRs 195.11 480.72 521.08 618.91 689.47 435.17 Dshift 3.39 6.81 5.21 8.19 8.8
574.83
thickness of the ZnO layer can be easily varied. This data supports the successful coating of ZnO on the surface of Co3O4 NRs, and makes perfect sense for the above uniform constituent of Zn distribution based on EDX analysis. Fig. 4(f) further shows that the ZnO particles are standing on the surface of the Co3O4 cores in broken Co3O4/ZnO NRs. Fig. 4(g) depicts the HRTEM image of a Co3O4/ZnO core/shell NRs. As can be observed, the lattice spacing of 0.285 nm
Fig. 3. Morphologies of the pristine Co3O4 NRs (a) and Co3O4/ZnO core/shell NRs (b) with featured bundle NRs and dispersed NRs. (c) and (d) are the EDX composition analysis of the Co3O4/ZnO core/shell NRs for featured bundle NRs and dispersed NRs (marked with black dash rectangles in (b)), respectively.
shown in Fig. 3(c) and (d), respectively. It is revealed that the Co3O4/ ZnO core/shell NRs are composed of Co, Zn, and O elements, further confirming the formation of ZnO species on the Co3O4 NRs. Specifically, the constituent of Zn are estimated to be 3.9 at % and 4.75 at % in the selected parts for bundle NRs and dispersed NRs as illustrated in inset table in Fig. 3(c) and (d), indicating a uniform distributions of ZnO NPs on the surface of Co3O4 NRs as following TEM analysis. To get further insight into the detailed structure, the products were slightly peeled off from Ni foil to perform TEM and HRTEM measurements. Fig. 4 presents a series of low and high magnification TEM images and HRTEM images for the as-synthesized Co3O4 NRs and Co3O4/ZnO core/shell NRs. As depicted in Fig. 4(a) and (b), the long isolated rods are about 1 mm in diameter with rough side walls and terminating in a circular solid end. Revealed by the highmagnification TEM image in Fig. 4(c), the nanorod is stacked layer by layer from single grain subunits with sizes of about 50e200 nm. Fig. 4(d) and (e) are the TEM images of a representative single Co3O4/ZnO core/shell NRs. It is verified that the n-tpye ZnO shell is interconnected and quite uniformly grown on the surface of the ptpye Co3O4 core, forming an attractive coaxial heterostructure. The measured thickness of the ZnO shell is approximate 20 nm. By controlling the concentration and the droplet of zinc acetate, the
is indexed to the (220) plane of the Co3O4. Meanwhile, the spacing of 0.256 nm and 0.281 nm are corresponded to the (002) and (100) planes of ZnO. The HRTEM image of the Co3O4/ZnO NRs gives the direct evidence for the formation of heterojunction between Co3O4 core NRs and ZnO NPs shell. XPS survey was performed to investigate the chemical binding states of the Co3O4 NRs, and Co3O4/ZnO core/shell NRs. Fig. 5(a) illustrates the determined oxidation states of Co from Co 2p XPS spectra. The Co 2p spectra of the Co3O4 NRs shows two main 2p3/2 and 2p1/2 spineorbit lines at 779.5 and 795.4 eV, respectively, consistent with other reports [21]. In the aspects of Co3O4/ZnO core/shell NRs, however, only a weak Co 2p3/2 peak is clear in big noise background due to the perfect coating of about 20 nm ZnO shell on the surface of Co3O4 core NRs supported by former TEM results. Conversely, the Zn 2p spectrum obviously appears in Co3O4/ ZnO core/shell NRs. The peak energies of 1020.9 and 1044.1 eV in Fig. 5(b), have been identified as the binding energies of the Zn 2p3/ 2 and the Zn 2p1/2 of ZnO, respectively, which is in line with other survey [22]. The O 1s regions for the Co3O4 NRs, and Co3O4/ZnO core/shell NRs are shown in Fig. 5(c) and (d). Both O1s spectra have been deconvoluted into two LorentzianeGaussain peaks using a Shirley background. Two peaks are clearly pronounced at 529.83 eV and 531.18 eV for O 1s spectrum of the Co3O4 NRs (Fig. 5(c)). In view
Please cite this article in press as: C. Dong, et al., Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/ shell nanorods, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.01.033
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Fig. 4. (a) and (b) are the typical low-magnification TEM images of isolated Co3O4 NRs. (c) High-magnification TEM image of the Co3O4 NR. (d) and (e) are the high-magnification TEM images of the representative Co3O4/ZnO core/shell NRs. (f) The grains in broken Co3O4/ZnO core/shell NRs. (g) HRTEM image of the Co3O4/ZnO core/shell NRs.
Fig. 5. XPS spectra of (a) Co 2p, (b) Zn 2p, (c) and (d) O1s in Co3O4 NRs, and Co3O4/ZnO core/shell NRs, respectively. Both O1s spectra have been deconvoluted into two LorentzianeGaussain peaks using a Shirley background.
of Co3O4/ZnO core/shell NRs (Fig. 5(d)), two corresponding peaks positioned at 529.90 eV and 531.41 eV for O 1s spectrum are detected. The main low-energy oxygen peaks can be assigned to lattice O2, as has been previously found for Co3O4 [21]. The second peaks at higher binding energy are comparable to that reported for surface hyfroxyls, and chemisorbed oxygen [21,23]. No significant difference is observed for the both deconvoluted peaks.
3.2. Photocatalytic properties Based on the crystal structures, morphologies, and microstructures, the interphases formation of ZneCoeO ternary phases through solid state reaction between Co3O4 and ZnO [24] could be reasonably ruled out in the Co3O4/ZnO core/shell NRs, since the low temperature and short time for ZnO formation. Additionaly, the
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Co3O4 NRs grown on the surface of Ni foil substrate are free from agglomeration, and hence keep the high specific surface area of NRs and the stability of the backbone. Therefore, the Co3O4 NRs and Co3O4/ZnO core/shell NRs are used for the photocatalytic degradation experiments. The photocatalytic activity of the as-prepared samples was evaluated by the degradation of MB dye in water under UV illumination (l ¼ 253.7 nm) at room temperature. All experiments were conducted under the following conditions in brief: Ni foil with catalysts in size 2.5 cm 2 cm, and 50 ml MB solutions with a concentration of 10 mg L1. The characteristic absorption of MB at 665 nm was used to monitor the photocatalytic degradation process. The temporal UVevis spectral changes of MB aqueous solutions in the process of photodegradation are displayed in Fig. 6(a) and (b). As can be seen, three UVevis absorbance peaks centered at 246, 293, and 665 nm from the spectra of MB starting solution. With increasing irradiation time, the major absorbance in the visible and UV regions clearly decreased. After photocatalytic reaction, the original black color of Co3O4 NRs and Co3O4/ZnO core/ shell NRs is not varied by eye. As can be observed from FTIR spectra in Fig. 7, no MB corresponding absorption peaks except for Co3O4 [19] are found from both catalysts after photocatalytic reaction, demonstrating that the chromophores of MB have been destroyed instead of being simply decolorized by adsorption. The adsorption peaks of the dye in UVevis region have completely disappeared after 96 h in the presence of Co3O4/ZnO core/shell NRs (Fig. 6(b)).
Fig. 7. FTIR spectra of MB, Co3O4 NRs and Co3O4/ZnO core/shell NRs after photocatalytic reaction.
On the contrary, it costs 144 h to completely degrade 50 ml MB with a concentration of 10 mg L1 in the presence of Co3O4 NRs
Fig. 6. UVevis spectral changes of MB aqueous solutions as a function of irradiation time in the presence of (a) Co3O4 NRs, (b) Co3O4/ZnO core/shell NRs, (c) Photocatalytic activities of as-prepared Co3O4 NRs and Co3O4/ZnO core/shell NRs for MB (50 ml with a concentration of 10 mg L1) degradation under UV-light irradiation, and (d) The pseudo-first-order kinetics for adsorption of MB at different time intervals for Co3O4 NRs and Co3O4/ZnO core/shell NRs.
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(Fig. 6(a)), indicating the improvement of photocatalytic activity by coupling with ZnO. Fig. 6(c) displays the changes of MB relative concentrations as a function of irradiation time (C0 and C are the concentration of MB at initial and any time). For comparative purpose, an identical experiment in the dark was conducted in the presence of Co3O4 NRs and Co3O4/ZnO core/shell NRs, respectively, which show negligible activity. Without catalysts, MB degradation under UV light illumination is relatively slow. However, with catalysts and UV light irradiation under the same experimental conditions, the rate of MB degradation is significantly increased. The as-prepared Co3O4 NRs and Co3O4/ZnO core/shell NRs show completely degradation for 50 ml MB solutions with a concentration of 10 mg L1 in 144 h and 96 h, respectively. The photocatalytic degradation kinetic was also investigated. The linear simulation of degradation of dye concentration can be accounted for by a pseudo first-order model, namely the LangmuireHinshelwood (L-H) model. The L-H model is well established for heterogeneous photocatalysis at low dye concentration [25], and the equation can be expressed as following: ln(C/C0) ¼ kt þ b
(1)
here b, k and t are a constant, rate constant and reaction time, respectively. A plot of -ln(C/C0) will yield a slope of k summarized in Fig. 6(d) with the relative correlative coefficient (R2) by lineal fitting of the experimental data. It is clearly demonstrated that the adsorption reactions follow the pseudo-first-order kinetics process. For the experiment without catalysts, MB degradation under UV light illumination is relatively slow, with an apparent reaction rate constant k ¼ 0.00565 min1. With catalysts under the same experimental conditions, however, the reaction rate is dramatically promoted. It is illustrated in Fig. 6(d) that the Co3O4 NRs exhibits the reaction rate constant with k ¼ 0.02624 min1 while Co3O4/ZnO core/shell NRs delivers better photoreactivity of k ¼ 0.04206 min1. Note that the coated p-type Co3O4 NRs with n-type ZnO provides a high photocatalytic performance catalyst. 3.3. Discussion Taken together, as for the higher photocatalytic activity of the Co3O4/ZnO core/shell NRs, several reasons may account for this. Firstly, the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst. 1D Co3O4 NRs serve as the backbone for ZnO NPs with a high specific surface
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area to provide more unsaturated surface coordination sites and maintain the stability of the nanostructure, which hence endows the heterojunction with an enhancement of photocatalytic activity [26]. Secondly, ZnO itself possess high catalytic activity [27], promoting the photocatalytic performance after coupling with Co3O4 NRs. Moreover, the p-n Co3O4/ZnO core/shell NRs heterojunctions have retarded the recombination of the photogenerated electrons and holes, as verified by the PL emission in Fig. 8(a). The Photoluminescence spectra of the Co3O4 NRs and Co3O4/ZnO core/shell NRs are recorded under 500 nm excitation at room temperature. Co3O4 NRs has an obvious peak at 762 nm in the PL spectrum, which is very close to that of indirect band gap of 1.60 eV [28]. The intensity of the PL decreases significantly but a peak at 762 nm still can be observed for Co3O4/ZnO core/shell NRs. The difference between Co3O4/ZnO and Co3O4 in intensity at 762 nm can be attributed to the reduction of the recombination of photoinduced electrons and holes and prolong the lifetime of the electron pairs, as reported for other heterojunctions such as Ag/ZnO [29] and NiO/ ZnO in the literature [30]. The results clearly show that the recombination of photogenerated charge carrier was greatly suppressed in the Co3O4/ZnO core/shell NRs composite semiconductors. This is a critical factor for the high photocatalystic performance of Co3O4/ZnO core/shell NRs. On the basis of the above experimental results and theory analysis, a possible photocatalysis mechanism is proposed schematically in Fig. 8(b). Co3O4 is a p-type semiconductor with direct band gap of 2.10 eV and indirect band gap of 1.60 eV [27], while ZnO is an n-type semiconductor with band gap of 3.30 eV [31]. When ZnO NPs are coated onto the Co3O4 NRs surface, then p-n nanoheterojunctions are formed at the interface and electrons transfer occurred from Co3O4 to ZnO until their Fermi levels align. After equilibrium, the p-type Co3O4 has a negative charge while the ntype ZnO regions have a positive charge. Consequently, an inner electrostatic field is created at the junction interface. Under UV light illumination, both Co3O4 and ZnO are simultaneously excited to yield electrons and holes. During the photocatalysis process, the photogenerated electrons can easily transfer from the conductive band of Co3O4 to that of ZnO shell. The photogenerated electrons react with adsorbed oxygen molecules (O2) and H2O on the surface of the Co3O4/ZnO core/shell NRs to produce superoxide radical anions such as $O 2 , OOH, and OH . Meanwhile, the excited holes can be trapped by H2O and OH to further yield OH, and OH species. The separation of photogenerated electron/hole pairs can be enhanced based on the p-type Co3O4 core/n-type ZnO shell
Fig. 8. (a) Photoluminescence spectra of the Co3O4 NRs and Co3O4/ZnO core/shell NRs recorded under 500 nm excitation. (b) Schematic of the postulate carrier transferring mechanism of MB photodegradation with Co3O4/ZnO core/shell NRs under UV light.
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heterojunction interface and then produced powerful superoxide radical as well as oxidizing agent via chemical reactions to decompose the organic dyes [32]. The Co3O4/ZnO core/shell NRs plays a critical role in accelerating the surface electron transfer rate, and the inner electric field in the interface acted as a potential barrier to retard the recombination of photogenerated electron and hole pairs. Hence, the photocatalytic activity is greatly enhanced in Co3O4/ZnO core/shell NRs than single Co3O4 NRs component. 4. Conclusions In summary, p-type Co3O4/n-type ZnO core/shell NRs was successfully synthesized by coating ZnO NPs on the surface of Co3O4 NRs prepared via a hydrothermal method on the Ni foil substrate. A uniform distribution of ZnO shell with thickness of about 20 nm has been illustrated. In the presence of catalysts, the reaction rate to degraded MB is dramatically promoted under UV light irradiation. Compared with a single component of Co3O4 NRs, the Co3O4/ZnO core/shell NRs proves to be an enhanced photocatalytic performance because of the efficient separation of photogenerated electronehole pairs at the p-n junction. By linear simulation of degradation process with the LangmuireHinshelwood model, the reaction rate constant k ¼ 0.04206 min1 was obtained in term of Co3O4/ZnO core/shell NRs, which is better than k ¼ 0.02624 min1 for only Co3O4 NRs. It is thought that the p-n junction heterostructure NRs can render a promising method to modify the Co3O4 NRs physical properties, and thereby widens the range of its applications, e.g., in developing new types of catalysts and gas sensors. Acknowledgments This work was supported by the Department of Science and Technology of Yunnan Province via the Key Project for the Science and Technology (Grant No. 2011FA001), National Natural Science Foundation of China (Grant No. 51262029), the Key Project of the Department of Education of Yunnan Province (ZD2013006), and the Applied Basic Research Program of Science and Technology Foundation of Yunnan Province (Grant No. 2013FB006).
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Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/shell nanorods Chengjun Dong, Xuechun Xiao, Gang Chen, Hongtao Guan, Yude Wang* Department of Materials Science & Engineering, Yunnan University, 650091 Kunming, People's Republic of China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
1D Co3O4/ZnO core/shell NRs were synthesized on nickel foil by a twostep synthetic strategy. The thickness of ZnO coating is determined to be about 20 nm. Co3O4/ZnO NRs exhibited better photocatalytic performance for MB degradation under UV irradiation. The formation of p-n junctions confirmed by PL is a critical factor for photocatalytic enhancement. The working mechanism for Co3O4/ ZnO NRs as photocatalyst was proposed.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2014 Received in revised form 29 December 2014 Accepted 10 January 2015 Available online xxx
One-dimension (1D) Co3O4/ZnO core/shell nanorods (NRs) were synthesized on nickel foil substrate by means of a two-step synthetic strategy. Co3O4 NRs were initially fabricated by a facile hydrothermal reaction and then ZnO was coated via a simple thermal decomposition. The results verified that the surface of the p-type Co3O4 core was uniformly assembled by the n-type ZnO nanoparticles with approximate 20 nm thickness. Compared with pristine Co3O4 NRs, Co3O4/ZnO core/shell NRs was exhibited to have a much higher photocatalytic properties in the decomposition of a model dye compound, methylene blue (MB), under ultraviolet irradiation. As confirmed by Photoluminescence (PL) spectra, the formation of p-n junction heterostructures gives rise to the enhanced photocatalystic performance of Co3O4/ZnO core/shell NRs. This study provides a general and effective method in the fabrication of 1D composition NRs with sound heterojunctions that show remarkable enhancement of photocatalytic performance. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Oxides Composite materials Heterostructures
1. Introduction As an ideal green technology, the semiconductor photocatalysts have been extensively recognized and documented, due to their potential applications in environmental cleaning and solar energy
* Corresponding author. E-mail address: [email protected] (Y. Wang).
utilization. It is generally accepted that the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalysts. The higher specific surface area of the catalysts can lead to a more unsaturated surface coordination sites to greatly increase the photocatalytic reaction sites [1]. 1D nanostructures including NRs, nanowires, nanotubes, nanofibers, have received increasing attention because of their unique properties associated with their highly anisotropic geometry and size
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C. Dong et al. / Materials Chemistry and Physics xxx (2015) 1e8
confinement [2]. The high specific surface area for reactant molecules facilitates transport of reagents to the active sites during the catalytic reaction. Therefore, many oxidations with 1D or quasi-1D nanostructures have been widely used as catalysts or additive to catalysts or support of catalysts in a variety of reactions [2e4]. Photocatalysis, in general, occurs based on the reaction between adsorbed molecules (oxygen, surface hydroxyls groups) or water and photogenerated electron/hole pairs excited by photon with equal or higher energy than the band gap of the semiconductor. As a result, the high oxidizing radicals are yielded, thus the organic molecules present in the solution are readily oxidized into carbon dioxide and water [5]. However, the electron/hole recombination is blamed for the low quantum yields, which is still a big obstacle for the photocatalytic activity improvement. For further improvement of photocatalytic activity, numerous efforts such as doping, formation of p-n heterojunctions and so forth, have been attempted over the past decades. In particular, research found that the p-n junctions formed in combination with both p-type and n-type semiconductors can effectively reduce the recombination rate of photogenerated electro/hole pairs, which subsequently enhances the photocatalytic activity [6,7]. Co3O4 is acceptable as an important p-type semiconductor which has been widely utilized. Up until now, 1D based on Co3O4 systems have been synthesized and showed high performance in CO oxidations [8], lithium-ion batteries [9] and supercapacitor [10]. Unfortunately, there are still no such applications for photocatalysis on the 1D Co3O4 basis. Apart from 1D nanostructure, significant attentions have been directed toward development of Co3O4 based heterojunction catalyst, such as Co3O4/BiVO4 [11], NiO/Co3O4 and Fe2O3/Co3O4 [12]. As an n-type semiconductor, ZnO has been considered as one of the best photocatalytic materials because of its nontoxic nature, low cost and high photochemical reactivity. The combination of p-type Co3O4 and n-type ZnO leads to the build-up of an inner electric field at the p-n junction interface that can be advantageous for photocatalytic, optoelectronic, and gas sensing applications. A diode consisting of p-type Co3O4 nanoplate/n-type ZnO NRs showing reasonable electrical performance has been reported [13]. Room temperature prepared Co3O4/ZnO systems exhibited catalytic behavior in the production of glycerol carbonate [14]. The p- Co3O4/n-ZnO composites were synthesized by a twostep PECVD process, indicating attractive performances in the detection of reducing (CH3CH2OH, CH3COCH3 and oxidizing (NO2) gases [15]. To the best of our knowledge, to date, the fabrication strategy to form p-type Co3O4-core/n-type ZnO-shell heterostructure NRs and its photocatalytic performance has rarely been explored. In our previous work, we reported the direct synthesis of long 1D Co3O4 NRs by a facile hydrothermal method on Ni foil substrate [16]. To combine the advantages of the obtained Co3O4 NRs and the ZnO material and thus to further improve the performance of the combination as heterogeneous catalysts, a nanorod-based p-n junction core/shell structure is being designed and used in this work. We have successfully synthesized uniform p-type Co3O4core/n-type ZnO-shell heterostructure NRs. Its photocatalytic performance has been investigated by taking methylene blue (MB) as an example under UV light irradiation. The Co3O4/ZnO core/shell NRs exhibit enhancement of photocatalytic activity in degradation of MB than single Co3O4 NRs component. In particular, nanorod shape of Co3O4/ZnO core/shell structure can provide more photocatalytic sites and accelerate the surface electron transfer rate because of its higher specific surface area. Additionally, the built-up inner electric field in the interface of p-n Co3O4/ZnO heterojunction can efficiently reduce the recombination of the photogenerated electron-hole to highly drive the photocatalytic reaction.
2. Experimental 2.1. Materials As substrate, nickel foils (99.9%) with 0.1 mm thickness were used. All the chemical reagents and solvents, which were purchased from commercial sources, were analytical grade and used as received without any further purification. The aqueous solutions were freshly prepared with high purity water (18 MU cm at 25 C) throughout all experiments. 2.2. Photocatalyst preparation The Co3O4/ZnO NRs were prepared by a simple two step method on a Ni foil (5 cm 2 cm). In the first step, Co3O4 NRs were prepared according to our previous reported procedure [16]. Briefly, a grounded Ni foil was cleaned sequentially by acetone and deionized water and then put into a volume of 80 mL Teflon at an angle against the wall. A pink color solution was obtained after dissolving 10 mmol Co(NO3)2 in 30 mL of H2O and gradually turned into black followed by adding 20 mL of 28 wt% ammonia solution under magnetically stirring in 30 min. Before transferring to the Teflon, 10 mmol NaNO3 were dissolved into the above solution and the hydrothermal reaction was conducted at 120 C for 12 h. The black Co3O4 NRs on Ni foil were formed via thermal treatment of the collected product at 250 C for 1 h in the air atmosphere. A half of the Ni foil (2.5 cm 2 cm) with highly dense Co3O4 NRs was cut to make next ZnO composition. In the second step, a droplet of 5 mM zinc acetate in N, N-Dimethylformamide (DMF) were dropped onto the synthesized Co3O4 NRs and dried in air. The above composited procedure was repeated by ten times. Finally, the dried zinc acetate loaded Co3O4 NRs were calcined in an oven in air at 350 C with the heating rate of 5 C/min for 30 min to form Co3O4-core/ZnO-shell NRs heterostructure. 2.3. Characterizatrion The X-ray diffraction (XRD) measurements, which were used to characterize the crystalline phase, phase composition of all asprepared samples, were carried out by an X-ray diffractometer (Rigaku TTRIII) in the 2q range of 10e80 using CuKa radiation (l ¼ 1.5406 Å). The accelerating voltage and applied current were 40 kV and 200 mA, respectively. The Raman spectra were recorded on a Raman imaging microscope (Ranishaw Model 2000). A 514.5 nm Arþ laser line with a power output of 20 mW was used for excitation. Scanning electronmicroscopy (SEM) and energydispersive X-ray spectroscopy (EDX) were carried out by a FEI QUANTA 200 equipped with an EDX attachment at an accelerating voltage of 30 kV. Transmission electron microscopy (TEM) and Highresolution transmission electron microscopy (HRTEM) images were performed on a JEOL JEM-2100 at the acceleration voltage of 200 kV. The TEM samples were prepared by carefully scratching the products from Ni foil, followed by dispersing in ethanol and casting them on the copper grids. Fourier transformed infrared (FTIR) spectra, in the range of 4000e500 cm1, were recorded on an infrared spectrophotometer. Photoluminescence (PL) spectra were performed on Horiba Jobin Yvon iHR320 imaging spectrometer. X-ray photoelectron spectroscopy (XPS) was utilized to investigate the surface chemistries of the samples in ESCALAB system with AlKa X-ray radiation at 15 kV. All XPS spectra were accurately calibrated by the C 1s peak at 284.6 eV to compensate for the charge effect. 2.4. Photocatalytic evaluation The photocatalytic activity of the as-prepared Co3O4 NRs and
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Co3O4/ZnO core/shell NRs for the degradation of MB in aqueous solution under ultraviolet light was evaluated by measuring the absorbance of the irradiated solution. A 30 W low-pressure mercury lamp was used as a light source with a main emission wavelength of 253.7 nm at a 10 cm distance. The Ni foil (2.5 cm 2 cm) covered with photocatalysts was placed into 50 ml of MB aqueous solutions with a concentration of 10 mg L1 in a quartz beaker. Before illumination, the solutions were kept in the dark for 2 h to reach the adsorptionedesorption equilibrium between the MB and the photocatalyst. At time intervals of 24 h, about 4 mL solutions were extracted for concentration check. The changes of concentration of the extracted solutions were measured by a UV-2401 PC spectrophotometer. The concentrations of the MB were monitored immediately by checking the absorbance at 665 nm during the photodegradation process by using a spectrophotometer (Model No. JH722N). All experiments were carried out under ambient conditions.
3. Results and discussion 3.1. Characterization of the catalysts Fig. 1 displays the XRD patterns of the pure Co3O4 NRs (a) and Co3O4/ZnO core/shell NRs. The diffraction peaks of the Ni foil substrate located at 2q of 51.76 and 76.43 are observed in both samples. It is found that all the identical peaks of the pristine Co3O4 NRs match perfectly to a cubic spinel phase of Co3O4 (JCPDS Card No. 42-1467). The previous reports show that the Co3O4 phase derived from the Co(OH)2 via thermal treatment at 250 C for 1 h [10,16]. By comparing the curves of Fig. 1(b) with (a), three additional weak peaks appear along with Co3O4 phase at 2q ¼ 31.76 , 34.42 , and 36.25 for Co3O4/ZnO core/shell NRs, respectively. These three peaks are attributed to (100), (002), and (101) planes of hexagonal ZnO (JCPDS Card No. 36-1451). The broad and weakness of the ZnO peaks, shown as inset in Fig. 1(b), can be explained by the small crystallite sizes of the as-coated ZnO materials. Besides, no diffraction peaks of impurities are investigated in the XRD patterns. The above analysis significantly confirms that the ZnO has been successfully composited on Co3O4 NRs by the experimental
Fig. 1. XRD patterns for pristine Co3O4 NRs (a) and Co3O4/ZnO core/shell NRs (b), respectively. The diffraction peaks of ZnO are highlighted with red and enlarged in inset for clarity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
3
strategy which is also used in CuO-core/ZnO-shell NWs preparation [17]. The intensity of Co3O4 remarkably decreases in the case of Co3O4/ZnO core/shell NRs due to the coating of Co3O4 NRs by ZnO, which will be strongly evident in following morphology characterizations. Basing also on comparison results, no apparent peak shifts are observed for Co3O4 phase after ZnO loading, suggesting that the ZnO only loaded on the surface of Co3O4 NRs instead of doping Zn atoms into Co3O4 lattice. A typical room temperature Raman spectra (Fig. 2) of the Co3O4 NRs and Co3O4/ZnO core/shell NRs were examined using a Raman imaging microscope under 514.5 nm excitation with a power output of 20 mW. As we all known, Co3O4 has a cubic spinel structure with Co2þ and Co3þ positioned at tetrahedral and octahedral sites, respectively, which belongs to the space group of Fd3m [18]. Obtained Raman spectra of the Co3O4 NRs consist five Ramanactive modes of A1g þ3F2g þ Eg, which fits well with the theoretical and experimental predicated modes [18,19]. Such an observation is also in good agreement with the above XRD measurement. In the case of Co3O4/ZnO core/shell NRs, two additional peaks were appeared at 435.17 and 574.83 cm-1 respectively. These two peaks are assigned to the E2 (high) and A1 (high) modes of ZnO phase [20]. Again, this result is further evidence of ZnO composition with Co3O4. All the peaks are summarized in Table 1 for the purpose of clarity. Remarkably, all the modes of Co3O4 phase shift towards higher values of the wave number by an amount of 3e9 cm-1 after ZnO loading. This shift suggests that the ZnO, to some extent, affects the Co3O4 phonon structure. The morphologies of the pristine Co3O4 NRs and Co3O4/ZnO core/shell NRs were characterized by SEM, as shown in Fig. 3(a) and (b). From the low magnification SEM images, the whole surface of the Ni foil substrate was covered with uniform and highly dense Co3O4 NRs consisting of bundle NRs and dispersed NRs. The Ni foil covered with Co3O4 NRs is suitable for photocatalytic degradation of dyes in wastewater by taking advantage of reutilization. It is discovered that the difference in terms of both lattice and thermal coefficient between Ni foil and Co3O4 NRs is the reason to the formation of bundle NRs and dispersed NRs [16]. After composited with ZnO, no big changes occurred for the morphology checking via SEM in Fig. 3(b). The chemical composition was analyzed by an energy dispersive X-ray spectroscopy (EDX) for bundle NRs and dispersed NRs marked in black dash rectangle in Fig. 3(b), as further
Fig. 2. Room temperature Raman spectra of pristine Co3O4 NRs (a) and Co3O4/ZnO core/shell NRs recorded under 514.5 nm excitation. (The modes of ZnO are highlighted with red color.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Peak positions of the Raman active phonon modes of Co3O4 NRs and ZnO (unit: cm1). Samples
F2g
Eg
F2g
F2g
A1g
E2 (high) A1 (high)
191.72 473.91 515.87 610.72 680.60 Co3O4 NRs Co3O4/ZnO NRs 195.11 480.72 521.08 618.91 689.47 435.17 Dshift 3.39 6.81 5.21 8.19 8.8
574.83
thickness of the ZnO layer can be easily varied. This data supports the successful coating of ZnO on the surface of Co3O4 NRs, and makes perfect sense for the above uniform constituent of Zn distribution based on EDX analysis. Fig. 4(f) further shows that the ZnO particles are standing on the surface of the Co3O4 cores in broken Co3O4/ZnO NRs. Fig. 4(g) depicts the HRTEM image of a Co3O4/ZnO core/shell NRs. As can be observed, the lattice spacing of 0.285 nm
Fig. 3. Morphologies of the pristine Co3O4 NRs (a) and Co3O4/ZnO core/shell NRs (b) with featured bundle NRs and dispersed NRs. (c) and (d) are the EDX composition analysis of the Co3O4/ZnO core/shell NRs for featured bundle NRs and dispersed NRs (marked with black dash rectangles in (b)), respectively.
shown in Fig. 3(c) and (d), respectively. It is revealed that the Co3O4/ ZnO core/shell NRs are composed of Co, Zn, and O elements, further confirming the formation of ZnO species on the Co3O4 NRs. Specifically, the constituent of Zn are estimated to be 3.9 at % and 4.75 at % in the selected parts for bundle NRs and dispersed NRs as illustrated in inset table in Fig. 3(c) and (d), indicating a uniform distributions of ZnO NPs on the surface of Co3O4 NRs as following TEM analysis. To get further insight into the detailed structure, the products were slightly peeled off from Ni foil to perform TEM and HRTEM measurements. Fig. 4 presents a series of low and high magnification TEM images and HRTEM images for the as-synthesized Co3O4 NRs and Co3O4/ZnO core/shell NRs. As depicted in Fig. 4(a) and (b), the long isolated rods are about 1 mm in diameter with rough side walls and terminating in a circular solid end. Revealed by the highmagnification TEM image in Fig. 4(c), the nanorod is stacked layer by layer from single grain subunits with sizes of about 50e200 nm. Fig. 4(d) and (e) are the TEM images of a representative single Co3O4/ZnO core/shell NRs. It is verified that the n-tpye ZnO shell is interconnected and quite uniformly grown on the surface of the ptpye Co3O4 core, forming an attractive coaxial heterostructure. The measured thickness of the ZnO shell is approximate 20 nm. By controlling the concentration and the droplet of zinc acetate, the
is indexed to the (220) plane of the Co3O4. Meanwhile, the spacing of 0.256 nm and 0.281 nm are corresponded to the (002) and (100) planes of ZnO. The HRTEM image of the Co3O4/ZnO NRs gives the direct evidence for the formation of heterojunction between Co3O4 core NRs and ZnO NPs shell. XPS survey was performed to investigate the chemical binding states of the Co3O4 NRs, and Co3O4/ZnO core/shell NRs. Fig. 5(a) illustrates the determined oxidation states of Co from Co 2p XPS spectra. The Co 2p spectra of the Co3O4 NRs shows two main 2p3/2 and 2p1/2 spineorbit lines at 779.5 and 795.4 eV, respectively, consistent with other reports [21]. In the aspects of Co3O4/ZnO core/shell NRs, however, only a weak Co 2p3/2 peak is clear in big noise background due to the perfect coating of about 20 nm ZnO shell on the surface of Co3O4 core NRs supported by former TEM results. Conversely, the Zn 2p spectrum obviously appears in Co3O4/ ZnO core/shell NRs. The peak energies of 1020.9 and 1044.1 eV in Fig. 5(b), have been identified as the binding energies of the Zn 2p3/ 2 and the Zn 2p1/2 of ZnO, respectively, which is in line with other survey [22]. The O 1s regions for the Co3O4 NRs, and Co3O4/ZnO core/shell NRs are shown in Fig. 5(c) and (d). Both O1s spectra have been deconvoluted into two LorentzianeGaussain peaks using a Shirley background. Two peaks are clearly pronounced at 529.83 eV and 531.18 eV for O 1s spectrum of the Co3O4 NRs (Fig. 5(c)). In view
Please cite this article in press as: C. Dong, et al., Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/ shell nanorods, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.01.033
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Fig. 4. (a) and (b) are the typical low-magnification TEM images of isolated Co3O4 NRs. (c) High-magnification TEM image of the Co3O4 NR. (d) and (e) are the high-magnification TEM images of the representative Co3O4/ZnO core/shell NRs. (f) The grains in broken Co3O4/ZnO core/shell NRs. (g) HRTEM image of the Co3O4/ZnO core/shell NRs.
Fig. 5. XPS spectra of (a) Co 2p, (b) Zn 2p, (c) and (d) O1s in Co3O4 NRs, and Co3O4/ZnO core/shell NRs, respectively. Both O1s spectra have been deconvoluted into two LorentzianeGaussain peaks using a Shirley background.
of Co3O4/ZnO core/shell NRs (Fig. 5(d)), two corresponding peaks positioned at 529.90 eV and 531.41 eV for O 1s spectrum are detected. The main low-energy oxygen peaks can be assigned to lattice O2, as has been previously found for Co3O4 [21]. The second peaks at higher binding energy are comparable to that reported for surface hyfroxyls, and chemisorbed oxygen [21,23]. No significant difference is observed for the both deconvoluted peaks.
3.2. Photocatalytic properties Based on the crystal structures, morphologies, and microstructures, the interphases formation of ZneCoeO ternary phases through solid state reaction between Co3O4 and ZnO [24] could be reasonably ruled out in the Co3O4/ZnO core/shell NRs, since the low temperature and short time for ZnO formation. Additionaly, the
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Co3O4 NRs grown on the surface of Ni foil substrate are free from agglomeration, and hence keep the high specific surface area of NRs and the stability of the backbone. Therefore, the Co3O4 NRs and Co3O4/ZnO core/shell NRs are used for the photocatalytic degradation experiments. The photocatalytic activity of the as-prepared samples was evaluated by the degradation of MB dye in water under UV illumination (l ¼ 253.7 nm) at room temperature. All experiments were conducted under the following conditions in brief: Ni foil with catalysts in size 2.5 cm 2 cm, and 50 ml MB solutions with a concentration of 10 mg L1. The characteristic absorption of MB at 665 nm was used to monitor the photocatalytic degradation process. The temporal UVevis spectral changes of MB aqueous solutions in the process of photodegradation are displayed in Fig. 6(a) and (b). As can be seen, three UVevis absorbance peaks centered at 246, 293, and 665 nm from the spectra of MB starting solution. With increasing irradiation time, the major absorbance in the visible and UV regions clearly decreased. After photocatalytic reaction, the original black color of Co3O4 NRs and Co3O4/ZnO core/ shell NRs is not varied by eye. As can be observed from FTIR spectra in Fig. 7, no MB corresponding absorption peaks except for Co3O4 [19] are found from both catalysts after photocatalytic reaction, demonstrating that the chromophores of MB have been destroyed instead of being simply decolorized by adsorption. The adsorption peaks of the dye in UVevis region have completely disappeared after 96 h in the presence of Co3O4/ZnO core/shell NRs (Fig. 6(b)).
Fig. 7. FTIR spectra of MB, Co3O4 NRs and Co3O4/ZnO core/shell NRs after photocatalytic reaction.
On the contrary, it costs 144 h to completely degrade 50 ml MB with a concentration of 10 mg L1 in the presence of Co3O4 NRs
Fig. 6. UVevis spectral changes of MB aqueous solutions as a function of irradiation time in the presence of (a) Co3O4 NRs, (b) Co3O4/ZnO core/shell NRs, (c) Photocatalytic activities of as-prepared Co3O4 NRs and Co3O4/ZnO core/shell NRs for MB (50 ml with a concentration of 10 mg L1) degradation under UV-light irradiation, and (d) The pseudo-first-order kinetics for adsorption of MB at different time intervals for Co3O4 NRs and Co3O4/ZnO core/shell NRs.
Please cite this article in press as: C. Dong, et al., Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/ shell nanorods, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.01.033
C. Dong et al. / Materials Chemistry and Physics xxx (2015) 1e8
(Fig. 6(a)), indicating the improvement of photocatalytic activity by coupling with ZnO. Fig. 6(c) displays the changes of MB relative concentrations as a function of irradiation time (C0 and C are the concentration of MB at initial and any time). For comparative purpose, an identical experiment in the dark was conducted in the presence of Co3O4 NRs and Co3O4/ZnO core/shell NRs, respectively, which show negligible activity. Without catalysts, MB degradation under UV light illumination is relatively slow. However, with catalysts and UV light irradiation under the same experimental conditions, the rate of MB degradation is significantly increased. The as-prepared Co3O4 NRs and Co3O4/ZnO core/shell NRs show completely degradation for 50 ml MB solutions with a concentration of 10 mg L1 in 144 h and 96 h, respectively. The photocatalytic degradation kinetic was also investigated. The linear simulation of degradation of dye concentration can be accounted for by a pseudo first-order model, namely the LangmuireHinshelwood (L-H) model. The L-H model is well established for heterogeneous photocatalysis at low dye concentration [25], and the equation can be expressed as following: ln(C/C0) ¼ kt þ b
(1)
here b, k and t are a constant, rate constant and reaction time, respectively. A plot of -ln(C/C0) will yield a slope of k summarized in Fig. 6(d) with the relative correlative coefficient (R2) by lineal fitting of the experimental data. It is clearly demonstrated that the adsorption reactions follow the pseudo-first-order kinetics process. For the experiment without catalysts, MB degradation under UV light illumination is relatively slow, with an apparent reaction rate constant k ¼ 0.00565 min1. With catalysts under the same experimental conditions, however, the reaction rate is dramatically promoted. It is illustrated in Fig. 6(d) that the Co3O4 NRs exhibits the reaction rate constant with k ¼ 0.02624 min1 while Co3O4/ZnO core/shell NRs delivers better photoreactivity of k ¼ 0.04206 min1. Note that the coated p-type Co3O4 NRs with n-type ZnO provides a high photocatalytic performance catalyst. 3.3. Discussion Taken together, as for the higher photocatalytic activity of the Co3O4/ZnO core/shell NRs, several reasons may account for this. Firstly, the catalytic process is mainly related to the adsorption and desorption of molecules on the surface of the catalyst. 1D Co3O4 NRs serve as the backbone for ZnO NPs with a high specific surface
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area to provide more unsaturated surface coordination sites and maintain the stability of the nanostructure, which hence endows the heterojunction with an enhancement of photocatalytic activity [26]. Secondly, ZnO itself possess high catalytic activity [27], promoting the photocatalytic performance after coupling with Co3O4 NRs. Moreover, the p-n Co3O4/ZnO core/shell NRs heterojunctions have retarded the recombination of the photogenerated electrons and holes, as verified by the PL emission in Fig. 8(a). The Photoluminescence spectra of the Co3O4 NRs and Co3O4/ZnO core/shell NRs are recorded under 500 nm excitation at room temperature. Co3O4 NRs has an obvious peak at 762 nm in the PL spectrum, which is very close to that of indirect band gap of 1.60 eV [28]. The intensity of the PL decreases significantly but a peak at 762 nm still can be observed for Co3O4/ZnO core/shell NRs. The difference between Co3O4/ZnO and Co3O4 in intensity at 762 nm can be attributed to the reduction of the recombination of photoinduced electrons and holes and prolong the lifetime of the electron pairs, as reported for other heterojunctions such as Ag/ZnO [29] and NiO/ ZnO in the literature [30]. The results clearly show that the recombination of photogenerated charge carrier was greatly suppressed in the Co3O4/ZnO core/shell NRs composite semiconductors. This is a critical factor for the high photocatalystic performance of Co3O4/ZnO core/shell NRs. On the basis of the above experimental results and theory analysis, a possible photocatalysis mechanism is proposed schematically in Fig. 8(b). Co3O4 is a p-type semiconductor with direct band gap of 2.10 eV and indirect band gap of 1.60 eV [27], while ZnO is an n-type semiconductor with band gap of 3.30 eV [31]. When ZnO NPs are coated onto the Co3O4 NRs surface, then p-n nanoheterojunctions are formed at the interface and electrons transfer occurred from Co3O4 to ZnO until their Fermi levels align. After equilibrium, the p-type Co3O4 has a negative charge while the ntype ZnO regions have a positive charge. Consequently, an inner electrostatic field is created at the junction interface. Under UV light illumination, both Co3O4 and ZnO are simultaneously excited to yield electrons and holes. During the photocatalysis process, the photogenerated electrons can easily transfer from the conductive band of Co3O4 to that of ZnO shell. The photogenerated electrons react with adsorbed oxygen molecules (O2) and H2O on the surface of the Co3O4/ZnO core/shell NRs to produce superoxide radical anions such as $O 2 , OOH, and OH . Meanwhile, the excited holes can be trapped by H2O and OH to further yield OH, and OH species. The separation of photogenerated electron/hole pairs can be enhanced based on the p-type Co3O4 core/n-type ZnO shell
Fig. 8. (a) Photoluminescence spectra of the Co3O4 NRs and Co3O4/ZnO core/shell NRs recorded under 500 nm excitation. (b) Schematic of the postulate carrier transferring mechanism of MB photodegradation with Co3O4/ZnO core/shell NRs under UV light.
Please cite this article in press as: C. Dong, et al., Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/ shell nanorods, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.01.033
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heterojunction interface and then produced powerful superoxide radical as well as oxidizing agent via chemical reactions to decompose the organic dyes [32]. The Co3O4/ZnO core/shell NRs plays a critical role in accelerating the surface electron transfer rate, and the inner electric field in the interface acted as a potential barrier to retard the recombination of photogenerated electron and hole pairs. Hence, the photocatalytic activity is greatly enhanced in Co3O4/ZnO core/shell NRs than single Co3O4 NRs component. 4. Conclusions In summary, p-type Co3O4/n-type ZnO core/shell NRs was successfully synthesized by coating ZnO NPs on the surface of Co3O4 NRs prepared via a hydrothermal method on the Ni foil substrate. A uniform distribution of ZnO shell with thickness of about 20 nm has been illustrated. In the presence of catalysts, the reaction rate to degraded MB is dramatically promoted under UV light irradiation. Compared with a single component of Co3O4 NRs, the Co3O4/ZnO core/shell NRs proves to be an enhanced photocatalytic performance because of the efficient separation of photogenerated electronehole pairs at the p-n junction. By linear simulation of degradation process with the LangmuireHinshelwood model, the reaction rate constant k ¼ 0.04206 min1 was obtained in term of Co3O4/ZnO core/shell NRs, which is better than k ¼ 0.02624 min1 for only Co3O4 NRs. It is thought that the p-n junction heterostructure NRs can render a promising method to modify the Co3O4 NRs physical properties, and thereby widens the range of its applications, e.g., in developing new types of catalysts and gas sensors. Acknowledgments This work was supported by the Department of Science and Technology of Yunnan Province via the Key Project for the Science and Technology (Grant No. 2011FA001), National Natural Science Foundation of China (Grant No. 51262029), the Key Project of the Department of Education of Yunnan Province (ZD2013006), and the Applied Basic Research Program of Science and Technology Foundation of Yunnan Province (Grant No. 2013FB006).
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Please cite this article in press as: C. Dong, et al., Synthesis and photocatalytic degradation of methylene blue over p-n junction Co3O4/ZnO core/ shell nanorods, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.01.033