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Au composite arrays and their enhanced photocatalytic properties
Journal of Colloid and Interface Science 432 (2014) 170–175
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Zinc Oxide nanorod/Au composite arrays and their enhanced photocatalytic properties Xueqin Liu, Zhen Li ⇑, Wen Zhao, Caixin Zhao, Jianbo Yang, Yang Wang Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 6 May 2014 Accepted 5 June 2014 Available online 17 June 2014 Keywords: ZnO/Au Photocatalysis Chemical adsorptivity Extended light absorption Efficient charge separation
a b s t r a c t In this paper, a high-performance photocatalyst of ZnO nanorod/Au composite arrays (ZAs) was synthesized via a facile low-temperature wet chemical method. The samples were characterized using field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) emission spectroscopy and ultraviolet–visible (UV–Vis) absorption. The unique nanostructured composite showed great adsorptivity of dyes, extended light absorption range, and efficient charge separation properties simultaneously. Hence, a significant enhancement in the photocatalytic properties in comparison with pure ZnO as demonstrated in photodegradation of methyl orange due to the incorporation of Au nanoparticles in ZnO nanorods. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction In the recent years, attention has been paid globally to hybrid nanomaterials that comprise two or more different components due to the possibility of combination and integration of material properties together from the viewpoint of technique requirements, which are not usually attainable in single-component nanocrystals [1–3]. Generally, there are three kinds of hybrid nanomaterials: blending, alloy and core/shell nanostructures. Various materials have been prepared into such nanostructures with distinctive properties. Among these nanostructures, semiconductor/metal composites are mostly attractive and well-studied due to their unique structure and remarkable optical, electrical, magnetic and chemical properties [4,5]. ZnO, as an important wide and direct band-gap semiconductor, has been extensively investigated as a promising photocatalyst because of its high catalytic efficiency, low cost, and environmental sustainability [6–8]. However, pure ZnO usually exhibit low photoenergy conversion efficiency probably because of their relatively low charge separation efficiency and fast recombination of charge carriers. Noble metal nanoparticles have been shown to increase the photocatalytic properties of ZnO by three factors, that is, increasing the efficiency of charge carrier separation [9–11], and extending light absorption and facilitating creation of electron/ hole pairs induced by the surface plasmon resonance (SPR) effect ⇑ Corresponding author. Fax: +86 27 678 83732. E-mail address: [email protected] (Z. Li). http://dx.doi.org/10.1016/j.jcis.2014.06.008 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
(especially for Au and Ag) [12–14]. Additionally, ZnO nanorods often achieved a better performance than nanoparticles because of its high crystallinity and elongated shape [15]. In this paper, we demonstrated a simple method to fabricate the ZnO nanorod/Au composite arrays with a unique nanostructure under mild condition. By growing Au nanoparticles on the surface of ZnO nanorods, they exhibit shift in the Fermi level to more negative potential, resulting in the enhancement of the efficiency of interfacial charge-transfer process. The obtained composites displayed enhanced photocatalytic properties compared to bare ZnO as demonstrated in the applications of degradation of methyl orange (MO). Moreover, the detail mechanism for the enhancement was suggested. The present results could highlight the importance of designing semiconductor/metal composite nanostructures for highly efficient photocatalysts. 2. Experimental 2.1. Synthesis All chemicals were of analytical reagent grade and purchased from the Shanghai Chemical Reagents Company, China, and used without further purification. ZnO nanorod arrays over alloy substrate (Fe–Co–Ni) were prepared by a wet chemical methods reported by Liu et al. [16]. In a typical procedure, an alloy substrate (20 mm 20 mm 0.15 mm) was cleaned in the ultrasonic bath with absolute ethanol and deionized water to remove adsorbed dust and surface contamination.
X. Liu et al. / Journal of Colloid and Interface Science 432 (2014) 170–175
Then, the alloy substrate was suspended in 200 mL of aqueous solution containing 0.035 M Zn(NO3)2 and 0.65 M NH3H2O in a sealed beaker followed by heating at a constant temperature of 70 °C for 24 h. Finally, the substrate was taken out of the solution and rinsed several times with deionized water, and then blew dried in air at room temperature. The prepared ZnO nanorod arrays over alloy substrate were carefully immersed into 40 mL of ethanol solution containing 0.05 g of SnCl2 with vigorous stirring for about 30 min at room temperature to obtain the activated ZnO nanoarrays. Then, the activated ZnO nanoarrays were immersed into a HAuCl4 solution with a certain concentration and reaction time. After that, the obtained ZAs were taken out from the solution and washed with deionized water for many times. Finally, the products were dried in a vacuum at 50 °C for one night. 2.2. Characterization The products were characterized by the X-ray photoelectron spectroscopy (XPS) which was collected on the ESCALab MKII X-ray photoelectron spectrometer (VG Multilab 2000). The size and morphology of the samples were investigated by field emission scanning electron microscopy (FESEM; JEOL-6300F) coupled with an energy dispersive X-ray (EDX) spectrometer. UV–Vis absorption spectra were recorded on a Lambda 35 UV–Vis spectrometer. The photoluminescent (PL) properties were measured on an F-4500 fluorescence spectrophotometer at room temperature using Xe lamp with a wavelength of 365 nm as the excitation source. 2.3. Photocatalytic measurements The obtained products were used as catalyst for the oxidation and decoloration of the methyl orange (MeO) dye. In a typical photocatalytic experiment, the as-prepared products (ZnO nanorods and ZAs) were added into 40 mL 20 mg/L MeO solution to form a
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mixture. Before irradiation, the mixture was magnetically stirred in the dark for 1 h to ensure the establishment of the adsorption/ desorption equilibrium of the dye onto the surface of photocatalysts. Afterward, the solution was exposed to a 500 W xenon lamp with a UV-cutoff filter (k > 400 nm). At given time intervals, the solution was sampled (2 mL), diluted (4 mL). Then the solution was put into a quartz cell, and the absorption spectrums were measured with a UV-2401 spectrophotometer. 2.4. Photoelectrochemical measurements The photocurrent measurements were taken in a dark box and measured in 0.1 M Na2SO4 aqueous solution under the illumination of 500 W Xe lump. The electric properties of the samples were performed with an electrochemical workstation (CHI 660D, CH Instrument Company, China). All electrochemical analyses were executed using a conventional three-electrode system. The as-prepared samples (1 cm2), a Pt foil (1 cm2) and Ag/AgCl electrode were used as working electrode, counter electrode and reference electrode, respectively. Electrochemical impedance spectra (EIS) were recorded at 0.0 V (with reference to the SCE). A sinusoidal ac perturbation of 5 mV was applied to the electrode in the frequency range of 50 mHz to 100 kHz. 3. Results and discussion 3.1. FESEM images Fig. 1 exhibits the typical SEM images of as-synthesized ZnO nanorod arrays and ZAs which reveal that the obtained structures are grown in very high density and multiply Au decorated ZnO nanorods. Fig. 1A shows the top-view SEM images of ZnO nanorods on Fe–Co–Ni alloy. It can be seen that the nanorods are wellaligned and uniform over a large scale with a diameter range of 80–120 nm. Moreover, the density of nanorods was estimated to
Fig. 1. FESEM images of pure ZnO (A and B) and ZnO nanorod/Au composite arrays (C and D). The insets are EDX spectrum of ZnO nanorod/Au composite arrays.
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Fig. 2. XPS spectra of the as-prepared ZnO nanorod/Au composite arrays: (A) XPS full spectra, (B) O 1s spectra, (C) Zn 2p spectra and (D) Au 4f spectra.
be about 20 per lm2. The length of ZnO nanorods was about 2 lm, which can be clearly seen from the representative cross-sectional SEM images of ZnO nanorods (Fig. 1B). Fig. 1C and D show the morphologies of the ZAs. Obviously, the surfaces of the ZnO nanorods become rough after the deposition of Au nanoparticles, as can be seen in Fig. 1C. The more detailed structural characteristics of the heteroarchitecture can be revealed from the SEM image with high magnification (Fig. 1D), where small Au nanoparticles with diameters mostly at 30–50 nm are distributed on the whole side surface of the rod-like ZnO nanoarray. To further confirm it is Au nanoparticles loaded on the surface of ZnO nanorods, the energy-disperse X-ray spectrum (EDX) measurement was undertook on the composites, as shown in the inset figure in Fig. 1D. The results further reveals that the samples contain elements of O, Zn and Au, which indicate that the Au nanoparticles were indeed coated on the ZnO nanorods.
3.2. X-ray photoelectron spectra To obtain the chemical states of elements of the composites after the addition of HAuCl4, the obtained composites were characterized by XPS. In Fig. 2A, all of the peaks on the curve are ascribed to O, Zn, Au and C elements and no peaks of other elements are observed. The presence of C comes mainly from pump oil due to vacuum treatment before the XPS test. Therefore, it is concluded that the sample is composed of three elements, O, Zn and Au,
which is in good agreement with the above EDS result. The O1s peak is shown in Fig. 2B, and the peak at about 531.5 eV can be attributed mainly to the binding of O to Zn in ZnO nanorods [17]. The binding energies of Zn 2p3/2 and Zn 2p1/2 locate at 1021.9 eV and 1045.1 eV, respectively, as shown in Fig. 2C. The binding energy of Zn 2p3/2 for the ZAs shifts to the higher binding energy compared with the corresponding value of pure ZnO. The shift of the binding energy indicates the strong interactions between ZnO multipods and the supporting Au nanoparticles [18]. Additionally, the peaks observed at 87.2 and 83.5 eV, as shown in Fig. 2D, can be attributed to Au 4f5/2 and Au 4f7/2 of the metallic Au, respectively, proving the formation of Au nanoparticles on the surface of ZnO nanorods after the addition of the HAuCl4. During the formation of ZnO nanorod/Au composite arrays, Sn2+ serves as the reducing agent that induces the formation of Au from Au3+ according to the redox reaction: ZnO–Sn2+ + HAuCl4 ? CuO–Au + Sn4+. Au precursor can be interacted directly with the Sn2+ ions, and subsequently transformed into Au nanoparticles deposited on the ZnO nanorods [19].
3.3. Photocatalytic activities The photocatalytic degradation of MO has been chosen as a model reaction to evaluate the photocatalytic activities of the obtained ZAs under visible light. To demonstrate the enhancement of the photocatalytic efficiency of ZnO/Au nanocomposite, the
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Fig. 3. (A) The time-dependent absorption spectra of MO solution in the presence of ZnO/Au composites, and the inset is the color-change sequence of MO solution during this process. (B) Photodegradation, (C) the ln(C/C0) versus time curves of photodegradation of MO over photocatalyst-free solution, P25, pure ZnO and ZnO/Au and (D) cycling runs in the photocatalytic degradation of MO in the presence of ZnO/Au composites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. UV–vis spectra (A) and curve of (ahv)1/2 versus photon energy (B) of the pure ZnO nanorods and ZnO nanorod/Au composite arrays.
degradation curves of MO by pure ZnO nanorods array, P25 and the blank (without any catalyst added) were also presented, as shown in Fig 3. Fig. 3A displays the absorption spectra of MO solution in
the presence of ZAs. The characteristic absorption of MO at 464 nm decreases progressively by increasing irradiation time, and almost disappears after about 210 min. The color change
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3.4. Mechanism of enhanced photoactivities
Fig. 5. Bar plot showing the remaining MO in solution: initial and equilibrated with ZnO and ZnO/Au in the dark after 30 min stirring.
sequence in the MO solution during the photocatalytic degradation process is shown in the inset of Fig. 3A, from which it is clear that the intense orange color of the initial solution was almost degraded completely within 210 min. The systematic data of the photodegradation by the ZnO/Au, ZnO and no catalyst under the visible irradiations are plotted in Fig. 3B. It can be seen that the photodegradation of the dye does not occur in the absence of a catalyst. The photocatalytic activity of P25 and ZnO nanorods is low under visible light. Only about 11% and 57% of the dye could be degraded within 210 min, respectively. However, the degradation rate of MO is clearly enhanced upon utilizing the ZAs as catalysts. A simple calculation of the first-order degradation rate constant, k = ln(C/C0)/t, gives a quantitative measure of this difference in photocatalytic performance, here, C0 and C are the initial concentration of MO and the concentration of exposure time t, which correspond well to the absorbance of MO at 464 nm, respectively. k is the degradation constant. In our experiment, as shown in Fig. 3C, k was found to be 0.00022, 0.00058, 0.00405 and 0.01458 min 1, for no catalyst, P25, pure ZnO and ZAs, respectively. It is obvious that adding Au provides a 300% increase in degradation rate, compared to the pure ZnO. The stability of the photocatalyst is important for its application. Thus, to investigate the stability of photocatalytic performance, the ZAs was used to degrade MO dye in five repeated cycles, as shown in Fig. 3D. It was noteworthy that after five recycles for the photodegradation of MO, the photocatalyst did not exhibit any significant loss of activity, which indicates that the ZnO/Au photocatalyst has high stability and does not photocorrode during the photocatalytic oxidation of the model pollutant molecules.
During the photocatalysis, three factors are crucial, those are, the light absorption, the adsorption of contaminant molecules, and the charge transportation and separation [20,21]. Fig. 4 displays the absorption spectra of pure ZnO nanorods and the ZAs. As mentioned above, the absorption range of light plays an important role in the photocatalysis, especially for the visible light photodegradation of contaminants. The ZnO nanorod arrays exhibit UV absorption at 380 nm, as shown in Fig. 4A. In the case of the Au/ ZnO, an intense peak is clearly observed at 550 nm, which is due to the surface plasma resonance effect of gold nanoparticles, and a redshift of 30 nm in the absorption edge can be obviously observed. This result indicated that the narrowing of the band gap of ZnO occurred with the Au introduction. While, the band gap of photocatalyst can be calculated according to the plot of (ahv)1/2 ahv, where a = (1 R)2/2R, R = 10 A and A is an optical absorption. As shown in Fig. 4B, the band gap of pure ZnO is 3.27 eV, whereas the band gap of the ZnO/Au composite has been slightly reduced to 3.02 eV. Hence, a more efficient utilization of the solar spectrum could be achieved. As a consequent, the improvement of photocatalytic performances can be attributed to the enhanced absorption of visible-light and reduced band gap of ZAs. As we know, the enhanced adsorptivity is a prerequisite for good photocatalytic activity. Fig. 5 summarizes the data obtained from ZnO/Au (or ZnO) dispersions equilibrated with the dye in the dark. Remarkably, only about 10% of the initial dye was removed from the aqueous solution by adsorption on the ZnO surfaces after 30 min equilibration. While, as to ZnO/Au, more than 30% of the initial dye had been adsorbed. Compared to the smooth ZnO nanorod arrays, the rough surface of ZAs can offer more active adsorption sites and photocatalytic reaction centers, which could result in an enhanced photocatalytic activity. Photocatalytic activity is closely related to the lifetime of photogenerated electrons and holes. PL signals result from the recombination of photoinduced charge carriers, so PL measurements were carried out to confirm the charge separation behavior and efficiency in ZAs. The emission properties of the pure ZnO nanorods and ZAs have been studied at room temperature by using a PL spectrum excited with a 325 nm, as shown in Fig. 6A. The UV emission band centered at 385 nm is ascribed to the radiative recombination, which occurs due to recombination between the electrons in a conduction band and the holes in a valence band. It can be seen that the PL spectra of the ZAs were very similar to ZnO without Au nanoparticles. However, ZnO/Au composite structures exhibit much lower emission intensity than that of pure ZnO, indicating a quenching of PL emissions due to the incorporation of Au nanoparticles on ZnO surface [22]. It is generally believed that a lower excitonic PL intensity means an enhanced separation and
Fig. 6. PL spectra (A), photoresponse curves (B) and EIS changes (C) of ZnO and ZnO nanorod/Au composite arrays.
X. Liu et al. / Journal of Colloid and Interface Science 432 (2014) 170–175
transfer of photoinduced electrons. Therefore, with efficient electron transfer from ZnO nanorods to Au nanoparticles, charge recombination is reduced within the ZnO nanorods, resulting in the enhancement of the photocatalytic activity. Additionally, the photoelectrochemical performance such as photocurrent spectra as well as electrochemical impedance spectroscopy (EIS) was employed to evaluate the efficiency of photogenerated charge interface separation for enhanced photocatalytic performance. The photoresponse of pure ZnO and ZAs was recorded using potentiostatic (current versus time, I–t) measurements under white light (xenon lamp, ca. 5 mW/cm2), as shown in Fig. 6B. As is well-known that the xenon lamp has a similar energy distribution with sunlight in spectrum, the majority of which consists of the visible and infrared light and only 3–5% of UV light, the electrodes with notable visible light-induced photocurrent suggested high light conversion efficiency and showed much more significant photocurrent because of the effective utilization of visible light [23]. For the pure ZnO nanorods, it only exhibited a slightly lifted photocurrent when the visible light source was turned on (1 lA/cm2). This is due to the fact that the band gap of ZnO is too wide to produce photo-induced electrons effectively by visible light. While, the photocurrent density of the ZAs electrode is 7.8 lA/cm2, which is about 8 times as high as that of the pure ZnO, indicating that the introduction of Au effectively improves the separation efficiency of photoinduced electron–hole pairs of ZnO nanorods so that the photocatalytic performance can be greatly enhanced. EIS is an effective tool for studying the interface charge separation efficiency and recombination of photogenerated electrons and holes of surface-modified electrodes. Fig. 6C represents the EIS Nyquist plots of pure ZnO and ZAs measured under dark conditions with a forward bias of 0.6 V. The radius of the arc on the EIS spectra reflects the reaction rate occurring at the surface of the electrode [24,25]. Generally, the smaller arc radius on the EIS Nyquist plot indicates an effective separation of the photogenerated electron–hole pairs and a fast interfacial charge transfer to the electron donor and/or electron acceptor. It can be observed that the ZAs show depressed semicircles at high frequencies compared with the pure ZnO, which suggests the ZnO/Au electrode displays greater separation efficiency of photogenerated electron–hole pairs and faster charge transfer than the ZnO electrode at the solid–liquid interface. Namely, the integration of ZnO nanorods with Au improves the transfer of photogenerated charge carriers, which is important to the enhancement of photocatalytic property. 4. Conclusions In summary, high-quality ZnO nanorod/Au composite arrays were successfully synthesized via a facile chemical deposition
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route at low temperature and its use for the photodegradation of organic dye from water under visible light was investigated. This composite possessed great adsorptivity of dyes, extended photoresponding range, and enhanced charge separation and transportation properties simultaneously. Thus, the photocatalytic efficiency for ZnO nanorod/Au composite arrays is much higher than that of pure ZnO nanorod arrays. These promising results would make this plasmonic photocatalyst an exceptional choice for the removal of toxic pollutants from aqueous solution. This approach also provides a new insight into a new class of semiconductors/metal composites with possible applications in energy-conversion devices and biofunctionalized materials.
References [1] B.X. Li, T.X. Liu, Y.F. Wang, Z.F. Wang, J Colloid Interface Sci. 377 (2012) 114– 121. [2] L. Gao, W.B. Yue, S.S. Tao, L.Z. Fan, Langmuir 29 (2013) 957–964. [3] H. Tada, T. Soejima, S. Ito, H. Kobayashi, J. Am. Chem. Soc. 126 (2004) 15952– 15953. [4] Q.P. Lu, Z.D. Lu, Y.Z. Lu, L.F. Lv, Y. Ning, H.X. Yu, Y.B. Hou, Y.D. Yin, Nano Lett. 13 (2013) 5698–5702. [5] X.Q. Liu, Z. Li, C.X. Zhao, W. Zhao, J.B. Yang, Y. Wang, F. Li, J. Colloid Interface Sci. 419 (2014) 9–16. [6] G.X. Zhu, Y.J. Liu, H. Xu, Y. Chen, X.P. Shen, Z. Xu, CrystEngComm 14 (2012) 719–725. [7] W. Wu, S.F. Zhang, X.H. Xiao, J. Zhou, F. Ren, L.L. Sun, C.Z. Jiang, ACS Appl. Mater. Interfaces 4 (2012) 3602–3609. [8] Y.Z. Chen, D.Q. Zeng, K. Zhang, A.L. Lu, L.S. Wang, D.L. Peng, Nanoscale 6 (2014) 874–881. [9] Z.F. Bian, T. Tachikawa, W. Kim, W. Choi, T. Majima, J. Phys. Chem. C 116 (2012) 25444–25453. [10] J. Lee, H.S. Shim, M. Lee, J.K. Song, D. Lee, J. Phys. Chem. Lett. 2 (2011) 2840– 2845. [11] W.W. He, H.K. Kim, W.G. Wamer, D. Melka, J.H. Callahan, J.J. Yin, J. Am. Chem. Soc. 136 (2014) 750–757. [12] P. Li, Z. Wei, T. Wu, Q. Peng, Y.D. Li, J. Am. Chem. Soc. 133 (2011) 5660–5663. [13] N. Udawatte, M. Lee, J. Kim, D. Lee, ACS Appl. Mater. Interfaces 3 (2011) 4531– 4538. [14] S. Mubeen, J. Lee, N. Singh, S. Kramer, G.D. Stucky, M. Moskovits, Nat. Nanotechnol. 8 (2013) 247–251. [15] S.S. Li, C.P. Chang, C.C. Lin, Y.Y. Lin, C.H. Chang, J.R. Yang, M.W. Chu, C.W. Chen, J. Am. Chem. Soc. 133 (2011) 11614–11620. [16] J.P. Liu, X.T. Huang, Y.Y. Li, X.X. Ji, Z.K. Li, X. He, F.L. Sun, J. Phys. Chem. C 111 (2007) 4990–4997. [17] W. Song, X. Han, L. Chen, Y.M. Yang, B. Tang, W. Ji, W.D. Ruan, W.Q. Xu, B. Zhao, Y. Ozaki, J. Raman Spectrosc. 41 (2010) 907–913. [18] G.Y. Shan, M.Y. Zhong, S. Wang, Y.J. Li, Y.C. Liu, J. Colloid Interface Sci. 326 (2008) 392–395. [19] J. Sittikun, Y. Boonyongmaneerat, P. Weerachawanasak, P. Praserthdam, J. Panpranot, Reac. Kinet. Mech. Cat. 111 (2014) 123–135. [20] L.W. Zhang, H.B. Fu, Y.F. Zhu, Adv. Funct. Mater. 18 (2008) 2180–2189. [21] H. Zhang, X.J. Lv, Y.M. Li, Y. Wang, J.H. Li, ACS Nano 4 (2010) 380–386. [22] M. De Lourdes Ruiz Peralta, U. Pal, R. Sa´nchez Zeferino, ACS Appl. Mater. Interfaces 4 (2012) 4807–4816. [23] H. Zhang, G. Wang, D. Chen, X.J. Lv, J.H. Li, Chem. Mater. 20 (2008) 6543–6549. [24] Y. Wang, X. Bai, C. Pan, J. He, Y. Zhu, J. Mater. Chem. 22 (2012) 11568–11573. [25] Y.J. Wang, F.M. Wang, J. He, Nanoscale 5 (2013) 11291–11297.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Zinc Oxide nanorod/Au composite arrays and their enhanced photocatalytic properties Xueqin Liu, Zhen Li ⇑, Wen Zhao, Caixin Zhao, Jianbo Yang, Yang Wang Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 6 May 2014 Accepted 5 June 2014 Available online 17 June 2014 Keywords: ZnO/Au Photocatalysis Chemical adsorptivity Extended light absorption Efficient charge separation
a b s t r a c t In this paper, a high-performance photocatalyst of ZnO nanorod/Au composite arrays (ZAs) was synthesized via a facile low-temperature wet chemical method. The samples were characterized using field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) emission spectroscopy and ultraviolet–visible (UV–Vis) absorption. The unique nanostructured composite showed great adsorptivity of dyes, extended light absorption range, and efficient charge separation properties simultaneously. Hence, a significant enhancement in the photocatalytic properties in comparison with pure ZnO as demonstrated in photodegradation of methyl orange due to the incorporation of Au nanoparticles in ZnO nanorods. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction In the recent years, attention has been paid globally to hybrid nanomaterials that comprise two or more different components due to the possibility of combination and integration of material properties together from the viewpoint of technique requirements, which are not usually attainable in single-component nanocrystals [1–3]. Generally, there are three kinds of hybrid nanomaterials: blending, alloy and core/shell nanostructures. Various materials have been prepared into such nanostructures with distinctive properties. Among these nanostructures, semiconductor/metal composites are mostly attractive and well-studied due to their unique structure and remarkable optical, electrical, magnetic and chemical properties [4,5]. ZnO, as an important wide and direct band-gap semiconductor, has been extensively investigated as a promising photocatalyst because of its high catalytic efficiency, low cost, and environmental sustainability [6–8]. However, pure ZnO usually exhibit low photoenergy conversion efficiency probably because of their relatively low charge separation efficiency and fast recombination of charge carriers. Noble metal nanoparticles have been shown to increase the photocatalytic properties of ZnO by three factors, that is, increasing the efficiency of charge carrier separation [9–11], and extending light absorption and facilitating creation of electron/ hole pairs induced by the surface plasmon resonance (SPR) effect ⇑ Corresponding author. Fax: +86 27 678 83732. E-mail address: [email protected] (Z. Li). http://dx.doi.org/10.1016/j.jcis.2014.06.008 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
(especially for Au and Ag) [12–14]. Additionally, ZnO nanorods often achieved a better performance than nanoparticles because of its high crystallinity and elongated shape [15]. In this paper, we demonstrated a simple method to fabricate the ZnO nanorod/Au composite arrays with a unique nanostructure under mild condition. By growing Au nanoparticles on the surface of ZnO nanorods, they exhibit shift in the Fermi level to more negative potential, resulting in the enhancement of the efficiency of interfacial charge-transfer process. The obtained composites displayed enhanced photocatalytic properties compared to bare ZnO as demonstrated in the applications of degradation of methyl orange (MO). Moreover, the detail mechanism for the enhancement was suggested. The present results could highlight the importance of designing semiconductor/metal composite nanostructures for highly efficient photocatalysts. 2. Experimental 2.1. Synthesis All chemicals were of analytical reagent grade and purchased from the Shanghai Chemical Reagents Company, China, and used without further purification. ZnO nanorod arrays over alloy substrate (Fe–Co–Ni) were prepared by a wet chemical methods reported by Liu et al. [16]. In a typical procedure, an alloy substrate (20 mm 20 mm 0.15 mm) was cleaned in the ultrasonic bath with absolute ethanol and deionized water to remove adsorbed dust and surface contamination.
X. Liu et al. / Journal of Colloid and Interface Science 432 (2014) 170–175
Then, the alloy substrate was suspended in 200 mL of aqueous solution containing 0.035 M Zn(NO3)2 and 0.65 M NH3H2O in a sealed beaker followed by heating at a constant temperature of 70 °C for 24 h. Finally, the substrate was taken out of the solution and rinsed several times with deionized water, and then blew dried in air at room temperature. The prepared ZnO nanorod arrays over alloy substrate were carefully immersed into 40 mL of ethanol solution containing 0.05 g of SnCl2 with vigorous stirring for about 30 min at room temperature to obtain the activated ZnO nanoarrays. Then, the activated ZnO nanoarrays were immersed into a HAuCl4 solution with a certain concentration and reaction time. After that, the obtained ZAs were taken out from the solution and washed with deionized water for many times. Finally, the products were dried in a vacuum at 50 °C for one night. 2.2. Characterization The products were characterized by the X-ray photoelectron spectroscopy (XPS) which was collected on the ESCALab MKII X-ray photoelectron spectrometer (VG Multilab 2000). The size and morphology of the samples were investigated by field emission scanning electron microscopy (FESEM; JEOL-6300F) coupled with an energy dispersive X-ray (EDX) spectrometer. UV–Vis absorption spectra were recorded on a Lambda 35 UV–Vis spectrometer. The photoluminescent (PL) properties were measured on an F-4500 fluorescence spectrophotometer at room temperature using Xe lamp with a wavelength of 365 nm as the excitation source. 2.3. Photocatalytic measurements The obtained products were used as catalyst for the oxidation and decoloration of the methyl orange (MeO) dye. In a typical photocatalytic experiment, the as-prepared products (ZnO nanorods and ZAs) were added into 40 mL 20 mg/L MeO solution to form a
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mixture. Before irradiation, the mixture was magnetically stirred in the dark for 1 h to ensure the establishment of the adsorption/ desorption equilibrium of the dye onto the surface of photocatalysts. Afterward, the solution was exposed to a 500 W xenon lamp with a UV-cutoff filter (k > 400 nm). At given time intervals, the solution was sampled (2 mL), diluted (4 mL). Then the solution was put into a quartz cell, and the absorption spectrums were measured with a UV-2401 spectrophotometer. 2.4. Photoelectrochemical measurements The photocurrent measurements were taken in a dark box and measured in 0.1 M Na2SO4 aqueous solution under the illumination of 500 W Xe lump. The electric properties of the samples were performed with an electrochemical workstation (CHI 660D, CH Instrument Company, China). All electrochemical analyses were executed using a conventional three-electrode system. The as-prepared samples (1 cm2), a Pt foil (1 cm2) and Ag/AgCl electrode were used as working electrode, counter electrode and reference electrode, respectively. Electrochemical impedance spectra (EIS) were recorded at 0.0 V (with reference to the SCE). A sinusoidal ac perturbation of 5 mV was applied to the electrode in the frequency range of 50 mHz to 100 kHz. 3. Results and discussion 3.1. FESEM images Fig. 1 exhibits the typical SEM images of as-synthesized ZnO nanorod arrays and ZAs which reveal that the obtained structures are grown in very high density and multiply Au decorated ZnO nanorods. Fig. 1A shows the top-view SEM images of ZnO nanorods on Fe–Co–Ni alloy. It can be seen that the nanorods are wellaligned and uniform over a large scale with a diameter range of 80–120 nm. Moreover, the density of nanorods was estimated to
Fig. 1. FESEM images of pure ZnO (A and B) and ZnO nanorod/Au composite arrays (C and D). The insets are EDX spectrum of ZnO nanorod/Au composite arrays.
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Fig. 2. XPS spectra of the as-prepared ZnO nanorod/Au composite arrays: (A) XPS full spectra, (B) O 1s spectra, (C) Zn 2p spectra and (D) Au 4f spectra.
be about 20 per lm2. The length of ZnO nanorods was about 2 lm, which can be clearly seen from the representative cross-sectional SEM images of ZnO nanorods (Fig. 1B). Fig. 1C and D show the morphologies of the ZAs. Obviously, the surfaces of the ZnO nanorods become rough after the deposition of Au nanoparticles, as can be seen in Fig. 1C. The more detailed structural characteristics of the heteroarchitecture can be revealed from the SEM image with high magnification (Fig. 1D), where small Au nanoparticles with diameters mostly at 30–50 nm are distributed on the whole side surface of the rod-like ZnO nanoarray. To further confirm it is Au nanoparticles loaded on the surface of ZnO nanorods, the energy-disperse X-ray spectrum (EDX) measurement was undertook on the composites, as shown in the inset figure in Fig. 1D. The results further reveals that the samples contain elements of O, Zn and Au, which indicate that the Au nanoparticles were indeed coated on the ZnO nanorods.
3.2. X-ray photoelectron spectra To obtain the chemical states of elements of the composites after the addition of HAuCl4, the obtained composites were characterized by XPS. In Fig. 2A, all of the peaks on the curve are ascribed to O, Zn, Au and C elements and no peaks of other elements are observed. The presence of C comes mainly from pump oil due to vacuum treatment before the XPS test. Therefore, it is concluded that the sample is composed of three elements, O, Zn and Au,
which is in good agreement with the above EDS result. The O1s peak is shown in Fig. 2B, and the peak at about 531.5 eV can be attributed mainly to the binding of O to Zn in ZnO nanorods [17]. The binding energies of Zn 2p3/2 and Zn 2p1/2 locate at 1021.9 eV and 1045.1 eV, respectively, as shown in Fig. 2C. The binding energy of Zn 2p3/2 for the ZAs shifts to the higher binding energy compared with the corresponding value of pure ZnO. The shift of the binding energy indicates the strong interactions between ZnO multipods and the supporting Au nanoparticles [18]. Additionally, the peaks observed at 87.2 and 83.5 eV, as shown in Fig. 2D, can be attributed to Au 4f5/2 and Au 4f7/2 of the metallic Au, respectively, proving the formation of Au nanoparticles on the surface of ZnO nanorods after the addition of the HAuCl4. During the formation of ZnO nanorod/Au composite arrays, Sn2+ serves as the reducing agent that induces the formation of Au from Au3+ according to the redox reaction: ZnO–Sn2+ + HAuCl4 ? CuO–Au + Sn4+. Au precursor can be interacted directly with the Sn2+ ions, and subsequently transformed into Au nanoparticles deposited on the ZnO nanorods [19].
3.3. Photocatalytic activities The photocatalytic degradation of MO has been chosen as a model reaction to evaluate the photocatalytic activities of the obtained ZAs under visible light. To demonstrate the enhancement of the photocatalytic efficiency of ZnO/Au nanocomposite, the
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Fig. 3. (A) The time-dependent absorption spectra of MO solution in the presence of ZnO/Au composites, and the inset is the color-change sequence of MO solution during this process. (B) Photodegradation, (C) the ln(C/C0) versus time curves of photodegradation of MO over photocatalyst-free solution, P25, pure ZnO and ZnO/Au and (D) cycling runs in the photocatalytic degradation of MO in the presence of ZnO/Au composites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. UV–vis spectra (A) and curve of (ahv)1/2 versus photon energy (B) of the pure ZnO nanorods and ZnO nanorod/Au composite arrays.
degradation curves of MO by pure ZnO nanorods array, P25 and the blank (without any catalyst added) were also presented, as shown in Fig 3. Fig. 3A displays the absorption spectra of MO solution in
the presence of ZAs. The characteristic absorption of MO at 464 nm decreases progressively by increasing irradiation time, and almost disappears after about 210 min. The color change
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3.4. Mechanism of enhanced photoactivities
Fig. 5. Bar plot showing the remaining MO in solution: initial and equilibrated with ZnO and ZnO/Au in the dark after 30 min stirring.
sequence in the MO solution during the photocatalytic degradation process is shown in the inset of Fig. 3A, from which it is clear that the intense orange color of the initial solution was almost degraded completely within 210 min. The systematic data of the photodegradation by the ZnO/Au, ZnO and no catalyst under the visible irradiations are plotted in Fig. 3B. It can be seen that the photodegradation of the dye does not occur in the absence of a catalyst. The photocatalytic activity of P25 and ZnO nanorods is low under visible light. Only about 11% and 57% of the dye could be degraded within 210 min, respectively. However, the degradation rate of MO is clearly enhanced upon utilizing the ZAs as catalysts. A simple calculation of the first-order degradation rate constant, k = ln(C/C0)/t, gives a quantitative measure of this difference in photocatalytic performance, here, C0 and C are the initial concentration of MO and the concentration of exposure time t, which correspond well to the absorbance of MO at 464 nm, respectively. k is the degradation constant. In our experiment, as shown in Fig. 3C, k was found to be 0.00022, 0.00058, 0.00405 and 0.01458 min 1, for no catalyst, P25, pure ZnO and ZAs, respectively. It is obvious that adding Au provides a 300% increase in degradation rate, compared to the pure ZnO. The stability of the photocatalyst is important for its application. Thus, to investigate the stability of photocatalytic performance, the ZAs was used to degrade MO dye in five repeated cycles, as shown in Fig. 3D. It was noteworthy that after five recycles for the photodegradation of MO, the photocatalyst did not exhibit any significant loss of activity, which indicates that the ZnO/Au photocatalyst has high stability and does not photocorrode during the photocatalytic oxidation of the model pollutant molecules.
During the photocatalysis, three factors are crucial, those are, the light absorption, the adsorption of contaminant molecules, and the charge transportation and separation [20,21]. Fig. 4 displays the absorption spectra of pure ZnO nanorods and the ZAs. As mentioned above, the absorption range of light plays an important role in the photocatalysis, especially for the visible light photodegradation of contaminants. The ZnO nanorod arrays exhibit UV absorption at 380 nm, as shown in Fig. 4A. In the case of the Au/ ZnO, an intense peak is clearly observed at 550 nm, which is due to the surface plasma resonance effect of gold nanoparticles, and a redshift of 30 nm in the absorption edge can be obviously observed. This result indicated that the narrowing of the band gap of ZnO occurred with the Au introduction. While, the band gap of photocatalyst can be calculated according to the plot of (ahv)1/2 ahv, where a = (1 R)2/2R, R = 10 A and A is an optical absorption. As shown in Fig. 4B, the band gap of pure ZnO is 3.27 eV, whereas the band gap of the ZnO/Au composite has been slightly reduced to 3.02 eV. Hence, a more efficient utilization of the solar spectrum could be achieved. As a consequent, the improvement of photocatalytic performances can be attributed to the enhanced absorption of visible-light and reduced band gap of ZAs. As we know, the enhanced adsorptivity is a prerequisite for good photocatalytic activity. Fig. 5 summarizes the data obtained from ZnO/Au (or ZnO) dispersions equilibrated with the dye in the dark. Remarkably, only about 10% of the initial dye was removed from the aqueous solution by adsorption on the ZnO surfaces after 30 min equilibration. While, as to ZnO/Au, more than 30% of the initial dye had been adsorbed. Compared to the smooth ZnO nanorod arrays, the rough surface of ZAs can offer more active adsorption sites and photocatalytic reaction centers, which could result in an enhanced photocatalytic activity. Photocatalytic activity is closely related to the lifetime of photogenerated electrons and holes. PL signals result from the recombination of photoinduced charge carriers, so PL measurements were carried out to confirm the charge separation behavior and efficiency in ZAs. The emission properties of the pure ZnO nanorods and ZAs have been studied at room temperature by using a PL spectrum excited with a 325 nm, as shown in Fig. 6A. The UV emission band centered at 385 nm is ascribed to the radiative recombination, which occurs due to recombination between the electrons in a conduction band and the holes in a valence band. It can be seen that the PL spectra of the ZAs were very similar to ZnO without Au nanoparticles. However, ZnO/Au composite structures exhibit much lower emission intensity than that of pure ZnO, indicating a quenching of PL emissions due to the incorporation of Au nanoparticles on ZnO surface [22]. It is generally believed that a lower excitonic PL intensity means an enhanced separation and
Fig. 6. PL spectra (A), photoresponse curves (B) and EIS changes (C) of ZnO and ZnO nanorod/Au composite arrays.
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transfer of photoinduced electrons. Therefore, with efficient electron transfer from ZnO nanorods to Au nanoparticles, charge recombination is reduced within the ZnO nanorods, resulting in the enhancement of the photocatalytic activity. Additionally, the photoelectrochemical performance such as photocurrent spectra as well as electrochemical impedance spectroscopy (EIS) was employed to evaluate the efficiency of photogenerated charge interface separation for enhanced photocatalytic performance. The photoresponse of pure ZnO and ZAs was recorded using potentiostatic (current versus time, I–t) measurements under white light (xenon lamp, ca. 5 mW/cm2), as shown in Fig. 6B. As is well-known that the xenon lamp has a similar energy distribution with sunlight in spectrum, the majority of which consists of the visible and infrared light and only 3–5% of UV light, the electrodes with notable visible light-induced photocurrent suggested high light conversion efficiency and showed much more significant photocurrent because of the effective utilization of visible light [23]. For the pure ZnO nanorods, it only exhibited a slightly lifted photocurrent when the visible light source was turned on (1 lA/cm2). This is due to the fact that the band gap of ZnO is too wide to produce photo-induced electrons effectively by visible light. While, the photocurrent density of the ZAs electrode is 7.8 lA/cm2, which is about 8 times as high as that of the pure ZnO, indicating that the introduction of Au effectively improves the separation efficiency of photoinduced electron–hole pairs of ZnO nanorods so that the photocatalytic performance can be greatly enhanced. EIS is an effective tool for studying the interface charge separation efficiency and recombination of photogenerated electrons and holes of surface-modified electrodes. Fig. 6C represents the EIS Nyquist plots of pure ZnO and ZAs measured under dark conditions with a forward bias of 0.6 V. The radius of the arc on the EIS spectra reflects the reaction rate occurring at the surface of the electrode [24,25]. Generally, the smaller arc radius on the EIS Nyquist plot indicates an effective separation of the photogenerated electron–hole pairs and a fast interfacial charge transfer to the electron donor and/or electron acceptor. It can be observed that the ZAs show depressed semicircles at high frequencies compared with the pure ZnO, which suggests the ZnO/Au electrode displays greater separation efficiency of photogenerated electron–hole pairs and faster charge transfer than the ZnO electrode at the solid–liquid interface. Namely, the integration of ZnO nanorods with Au improves the transfer of photogenerated charge carriers, which is important to the enhancement of photocatalytic property. 4. Conclusions In summary, high-quality ZnO nanorod/Au composite arrays were successfully synthesized via a facile chemical deposition
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route at low temperature and its use for the photodegradation of organic dye from water under visible light was investigated. This composite possessed great adsorptivity of dyes, extended photoresponding range, and enhanced charge separation and transportation properties simultaneously. Thus, the photocatalytic efficiency for ZnO nanorod/Au composite arrays is much higher than that of pure ZnO nanorod arrays. These promising results would make this plasmonic photocatalyst an exceptional choice for the removal of toxic pollutants from aqueous solution. This approach also provides a new insight into a new class of semiconductors/metal composites with possible applications in energy-conversion devices and biofunctionalized materials.
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