J. Phys. Chem. C 2008, 112, 10773–10777
10773
Photocatalytic Activity of Ag/ZnO Heterostructure Nanocatalyst: Correlation between Structure and Property Yuanhui Zheng,*,† Chongqi Chen,† Yingying Zhan,† Xingyi Lin,† Qi Zheng,*,† Kemei Wei,† and Jiefang Zhu‡ National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou UniVersity, Gongye Road 523, Fuzhou 350002, Fujian, China, Department of Applied Physics, Chalmers UniVersity of Technology, Fysikgrand 3, Goteborg SE 412 96, Sweden
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ReceiVed: March 30, 2008; ReVised Manuscript ReceiVed: May 5, 2008
Ag/ZnO heterostructure nanocatalysts with Ag content of 1 wt % are successfully prepared through three different simple methods, where chemical reduction and photolysis reaction are adopted to fabricate the heterostructure. The dispersity of Ag clusters and/or nanoparticles in Ag/ZnO nanocatalyst is investigated by EDX mapping and XPS techniques. The experimental results show that deposition-precipitation is an efficient method to synthesize Ag/ZnO nanocatalyst with highly dispersed Ag clusters and/or nanoparticles; the photocatalytic activity of Ag/ZnO photocatalysts mainly depends on the dispersity of metallic Ag in Ag/ZnO nanocatalyst; the higher the dispersity of metallic Ag in Ag/ZnO nanocatalyst is, the higher the photocatalytic activity of Ag/ZnO photocatalyst should be. In addition, it is also found that the dispersity of Ag/ZnO photocatalyst in the dye solution is another key factor for liquid-phase photocatalysis due to the UV-light utilizing efficiency. The higher the UV-light utilizing efficiency is, the higher the photocatalytic activity of Ag/ZnO heterostructure photocatalyst should be. 1. Introduction Semiconductor-based heterostructures with desired compositions and/or morphologies can modulate the properties of materials and find potential applications in biomedicine, photocatalysis, and nanodevices.1–12 Stimulated by these applications, significant advances have been made to design various kinds of semiconductor-based heterostructures, such as core/ shell and anisotropic (e.g., dimer/trimer-type and hierarchical composite materials) heterostructures13–17 in recent years. So far, metal/semiconductor is one of the most popular heterostructures and has been extensively studied because of its excellent catalytic activity. For example, recently, Ag/ZnO heterostructure photocatalyst with high catalytic activity has attracted much research attention.18–21 However, the relationship between the structure and the photocatalytic property of Ag/ ZnO heterostructure photocatalyst is still not clear. Considering the relatively high price of Ag species and the promising application of Ag/ZnO photocatalyst, we expect to find an effective method to decrease its cost (i.e., decrease the Ag content of Ag/ZnO photocatalyst) and understand the effect of the dispersity of Ag clusters and/or nanoparticles in Ag/ZnO photocatalyst on its photocatalytic performance. It is well-known that material properties are determined by structure, such as size, morphology, pore, defect, and composition.18,22–25 With the photocatalytic performance of semiconductor-based heterostructure photocatalyst as an example, it is mainly dependent on the concentration of heterostructure interface and defect which can increase the separation efficiency of photogenerated electron-hole pairs. As an example, in our previous study,18 it is found that Ag nanoparticles and oxygen * Corresponding authors. E-mails:
[email protected] and
[email protected], Phone: +86-591-8373-1234-8112, Fax: +86-5918373-8808. † Fuzhou University. ‡ Chalmers University of Technology.
SCHEME 1: Photogenerated Electron Transfer in Ag/ZnO Nanocatalyst during the Catalytic Processa
a ø: work function; E : Fermi level; V · · : oxygen vacancy; CB: f o conduction band; VB: valence band; vac: vacuum level; m: metal; and s: semiconductor.18
vacancy defects (Vo · · ) on the surface of ZnO nanocrystals benefit the separation of photogenerated electron-hole pairs, thus enhancing the photocatalytic activity. It has also been reported that there are two different pathways to transfer the photogenerated electrons from ZnO semiconductor to the dye for Ag/ZnO photocatalyst (see Scheme 1) in this literature, when the catalyst is irradiated by UV light. As shown in Scheme 1, the photogenerated electrons can be transferred to the dye through the Ag-ZnO interface (pathway I) and the oxygen vacancy defects on the surface of the ZnO semiconductor (pathway II). As to pathway I, the photogenerated electrons should reach the surface of Ag nanopaticles via the Ag-ZnO interface before they are transferred to the dye. So, it is believed that the dispersity of Ag clusters and/or nanoparticles in Ag/ ZnO nanocrystals (i.e., the concentration of Ag-ZnO interface)
10.1021/jp8027275 CCC: $40.75 2008 American Chemical Society Published on Web 06/26/2008
10774 J. Phys. Chem. C, Vol. 112, No. 29, 2008 should affect the photocatalytic activity of the Ag/ZnO heterostructure photocatalyst: the higher the dispersity of Ag clusters and/or nanoparticles on the surface of ZnO nanocrystals is, the higher the photocatalytic activity of the Ag/ZnO heterostructure photocatalyst should be. Therefore, it is necessary to improve the dispersion of metallic Ag in Ag/ZnO photocatalyst and use simple techniques to characterize the dispersion of metallic Ag in the catalyst. So far, metal dispersion in the catalysts has been extensively measured by chemisorption,26–30 and only a little work has been done by energy-dispersive X-ray (EDX) mapping.30,31 In this paper, three facile synthetic methods are employed to fabricate the Ag/ZnO heterostructure photocatalyst with Ag content of 1.0 wt %. EDX mapping and X-ray photoelectron spectroscopy (XPS) are adopted to investigate the dispersity of Ag clusters and/or nanoparticles in Ag/ZnO photocatalyst (i.e., the dispersion of metallic Ag on the surface of ZnO nanocrystals). The relationship between the structure and the photocatalytic performance of these catalysts is investigated in detail. The experimental results show that the photocatalytic activity of Ag/ZnO photocatalysts depends not only on the dispersity of Ag clusters and/or nanoparticles in Ag/ZnO photocatalyst but also on Ag/ZnO photocatalyst in the dye solution. 2. Experimental Section 2.1. Preparation of Ag/ZnO Heterostructure Nanocatalysts. 2.1.1. Materials. Zinc acetate, silver acetate, sodium hydroxide, and alcohol are all analytical grades and used without further purification. 2.1.2. Synthesis. Ag/ZnO heterostructure nanocatalysts have been synthesized through the following three methods: For the deposition-precipitation (DP) method, 0.037 mmol of CH3COOAg and 5 mmol of fresh ZnO nanocrystals (named Z5 in ref25) synthesized through the solvothermal method25 were dispersed in 150 mL of ethanol. Then, 1 mL of NaOH ethanol solution (0.17 M) was added into the above mixture and stirred under natural light at room temperature for two days. For the coprecipitation (CP) method, 150 mL of NaOH ethanol solution (0.17 M) was added into the mixture of 5 mmol of Zn (Ac)2 · 2H2O and 0.037 mmol of CH3COOAg with agitation under natural light at room temperature for two days. For the solvothermal (ST) method, 150 mL of NaOH ethanol solution (0.17 M) was added into the mixture of 5 mmol of Zn(Ac)2 · 2H2O and 0.037 mmol of CH3COOAg with agitation for 30 min in the dark, then turned into five Teflon-lined stainless steel autoclaves of 50 mL capacity equally. The sealed tanks were put into an oven and heated at 160 °C for one day. The precipitates were collected by filtration, washed with deionized water and ethanol several times, and finally dried in the air at 60 °C for 10 h. The obtained samples with Ag content of 1.0 wt % were named Ag/ZnO-DP, Ag/ZnO-CP, and Ag/ZnO-ST, respectively. 2.2. Characterizations. The powder X-ray diffraction (XRD) patterns of the samples were recorded by a Panalytical X′Pert Pro diffractometer using Co KR radiation (λ ) 0.179 nm) at a scanning rate of 0.12°/min. The surface areas were measured by the Brunauer-Emmett-Teller (BET) method by using N2 adsorption at 77 K on a Quantachrome NOVA 4200e apparatus. The dispersity of Ag element in the as-synthesized samples was characterized by a field emission scanning electron microscope (SEM, JSM-6700F) equipped with an energy-dispersive X-ray spectroscopy (EDXS) system at 15 kV (penetration depth of electrons: ∼2.2 µm). The X-ray photoelectron spectroscopy (XPS) measurements were performed on a Phi Quantum 2000 spectrophotometer with Al KR radiation (1486.6 eV) (penetra-
Zheng et al. tion depth of X-ray: Ag/ZnO-CP > Ag/ZnO-ST), indicating that the uniformity of metallic Ag in the as-synthesized samples decreases from Ag/ZnO-DP to Ag/ZnO-ST. This means that the concentration of Ag-ZnO interface decreases from Ag/ZnODP to Ag/ZnO-ST. A typical HRTEM image of an individual Ag/ZnO heterostructure (the inset in Figure 2a,c) reveals that the metallic Ag nanoparticle attaches on the surface of ZnO nanocrystal (i.e., the formation of a dimer-type heterostructure). The composition of the as-synthesized samples is further characterized by EDXS and the results are shown in Figure 2d. All of the peaks on the curves are ascribed to Zn, Ag, O, Cu, and C elements, and no peaks of other elements are observed. The copper and carbon signals are from the sample holder with conductive tape. Therefore, it is concluded that the assynthesized samples are composed of Zn, Ag, and O elements, which is in good agreement with the above XRD result. In addition, EDXS analysis reveals that the Ag content of the assynthesized samples (listed in Figure 2d) approximates the theoretical value (1.0 wt %). The surface Ag content of the as-prepared samples is also investigated by XPS, and the results are shown in Figure 3. The binding energies in the XPS spectra presented in Figure 3 are calibrated by C1s (284.8 eV). The peaks appearing in Figure 3 are attributed to Ag 3d5/2 (367.2 eV) and Ag 3d3/2 (373.2 eV), respectively. It is worthwhile to mention that the peak positions of Ag 3d shift remarkably to the lower binding energy compared with that of bulk Ag (Ag 3d5/2, 368.3 eV; and Ag 3d3/2, 374.3 eV32), which is ascribed to the electron transfer from metallic Ag to ZnO nanocrystals (i.e., the formation of monovalent Ag).18
J. Phys. Chem. C, Vol. 112, No. 29, 2008 10775 The shift of the binding energy of metallic Ag indicates that there is a strong interaction between metallic Ag and ZnO nanocrystals. In addition, it is found that the intensity of Ag 3d peaks decreases from Ag/ZnO-DP to Ag/ZnO-ST and the calculated surface Ag contents of the corresponding samples are 0.89, 0.68, and 0.35 wt %, implying the decrease of the content of surface Ag from Ag/ZnO-DP to Ag/ZnO-ST. In combination with the EDX mapping results, it can be concluded that Ag/ZnO-DP possesses the highest dispersity of metallic Ag among the as-synthesized samples. 3.4. Optical Properties of Ag/ZnO Heterostructure Nanocrystals. The UV-vis diffuse-reflectance and photoluminescence (PL) spectra of the as-synthesized Ag/ZnO heterostructure nanocrystals are shown in Figure 4. In Figure 4a, two prominent absorption bands are observed in the range 200-800 nm. The former is assigned to the absorption of the ZnO semiconductor, and the corresponding absorption edge locates at around 375 nm. The absorption edge of ZnO nanocrystals for Ag/ZnO-CP is slightly smaller than that of ZnO nanocrystals for Ag/ZnODP and Ag/ZnO-ST, indicating that the size of ZnO nanocrystals in Ag/ZnO-CP is smaller, which is in good agreement with the above XRD result. The latter is attributed to the characteristic absorption of surface plasmon resulting from the metallic Ag clusters and/or nanoparticles in the Ag/ZnO heterostructures.18 The appearance of two kinds of characteristic absorption bands also confirms that the as-synthesized samples are composed of zerovalent Ag and ZnO. Comparing with the position and intensity of the metallic Ag absorption in Figure 4a, one can see that the position blue-shifts for Ag/ZnO-DP (highlighted by an arrow), indicating that this sample has the smallest size of Ag clusters and/or nanoparticles. It is also found that for the Ag/ZnO-ST sample the intensity is much lower, which should result from the worst Ag distributions in the sample. In Figure 4b, there is a broad emission peak in the range 400-800 nm for all samples. It is obvious that the emission peak of the assynthesized samples shifts from 527 to 585 nm (Ag/ZnO-ST f Ag/ZnO-DP f Ag/ZnO-CP: red shift) and the corresponding intensity also changes. On the basis of our previous study,25 the green and orange emissions at about 527 and 605 nm should be ascribed to oxygen vacancy (Vo · · ) and interstitial oxygen (Oi′′) defects, respectively. The red shift of emission peak indicates that the concentration of Oi′′ defects increase from Ag/ ZnO-ST to Ag/ZnO-CP (the concentration of Oi′′ defects: Ag/ ZnO-CP > Ag/ZnO-DP > Ag/ZnO-ST). Moreover, the concentration of surface oxygen defects (including Vo · · and Oi′′ defects) of Ag/ZnO-DP and Ag/ZnO-CP is lowest and highest among that of the as-synthesized samples (the concentration of surface oxygen defects: Ag/ZnO-CP > Ag/ZnO-ST > Ag/ZnODP).18 3.5. Photocatalytic Activities of Ag/ZnO Heterostructure Nanocatalysts. MO is presently adopted as a representative organic pollutant to evaluate the photocatalytic performance of the as-synthesized Ag/ZnO heterostructure nanocatalysts. In the experiments, the commercial TiO2 (Degussa P-25) is used as a photocatalytic reference to qualitatively understand the photocatalytic activity of Ag/ZnO catalysts. The photocatalytic activities of the as-prepared samples with Ag content of 1.0 wt % and Degussa P-25 are shown in Figure 5. C0 and C in Figure 5 are the initial concentration after the equilibrium adsorption and the reaction concentration of MO, respectively. As seen in Figure 5, the photocatalytic activity of Ag/ZnO-DP and Ag/ZnO-ST is much higher than that of P-25, for example, and the required time for an entire decolorization of MO over Ag/ZnO-DP and Ag/ZnO-ST (Figure 5a,c) is less than or
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Figure 2. Low-magnification SEM images and EDX mapping of the as-synthesized samples: (a) Ag/ZnO-DP, (b) Ag/ZnO-CP and (c) Ag/ZnOST; (d) EDXS spectra recorded from the corresponding rectangular region of the as-synthesized samples in a-c. The coverage of blue color in a-c reflects Ag distributions in the as-synthesized samples, and the inset in a and c is the corresponding HRTEM image of a single Ag/ZnO heterostructure nanocrystal.
Figure 3. Ag 3d XPS spectra of the as-synthesized samples: (a) Ag/ ZnO-DP, (b) Ag/ZnO-CP, and (c) Ag/ZnO-ST.
equal to 40 min, much shorter than the corresponding value of P-25 catalyst. However, the MO degradation efficiency is about 60% for Ag/ZnO-CP at 40 min (Figure 5b), which is much smaller than that of P-25 (92%). Moreover, it is obvious that Ag/ZnO-DP shows the highest photocatalytic activity among all these samples. The photograph of the as-synthesized samples (the inset of Figure 5) in MO solution shows that Ag/ZnO-DP and Ag/ZnO-ST samples are much more easily dispersed than Ag/ZnO-CP sample. It has been reported that the higher the concentration of oxygen defects on the surface of ZnO nanocrystals, the higher
the photocatalytic activity should be.18 However, based on the above results, despite lowest concentration of oxygen defects on the surface of ZnO nanocrystals for Ag/ZnO-DP sample, its photocatalytic activity is remarkably better than that of the other two samples (Ag/ZnO-CP and Ag/ZnO-ST). According to the EDX mapping and XPS results, this phenomenon should be attributed to the highest dispersity of metallic Ag on the surface of ZnO nanocrystals for Ag/ZnO-DP sample. It has been reported that Ag clusters and/or nanoparticles on the surface of ZnO nanocrystals act as a sink for the electrons, promote interfacial charge-transfer kinetics between the metal and the semiconductor, improve the separation of photogenerated electron-hole pairs, and thus enhance the photocatalytic activity of Ag/ZnO photocatalysts.18 Therefore, the higher the dispersity of Ag clusters and/or nanoparticles on the surface of ZnO nanocrystals is, the higher the photocatalytic activity of Ag/ ZnO heterostructure photocatalyst should be. However, as presented in Figure 5, the photocatalytic efficiency of Ag/ZnOCP is much lower than that of Ag/ZnO-ST, which should be related to the dispersity of Ag/ZnO photocatalyst in the dye solution. It is found that Ag/ZnO-CP cannot be well-dispersed in the dye solution (the inset of Figure 5), so the UV-light utilizing efficiency of this sample is very low. The higher the UV-light utilizing efficiency is, the higher the photocatalytic activity of the Ag/ZnO heterostructure photocatalyst should be.33 Considering all the above factors, it is reasonable that Ag/ZnODP and Ag/ZnO-CP exhibit the highest and lowest photocatalytic activity, respectively. Finally, the experimental results presented in this paper should be valuable for the synthesis of other metal/semiconductor catalysts with high catalytic activity.
Photocatalytic Activity of Ag/ZnO Nanocatalyst
J. Phys. Chem. C, Vol. 112, No. 29, 2008 10777 efficient method to synthesized Ag/ZnO nanocatalyst with highly dispersed metallic Ag on the surface of ZnO nanocrystals; the photocatalytic activity of Ag/ZnO photocatalysts depends on the dispersity of Ag clusters and/or nanoparticles in Ag/ZnO photocatalyst and Ag/ZnO photocatalyst in the dye solution. The higher the dispersities of metallic Ag in Ag/ZnO photocatalyst and Ag/ZnO catalyst in the dye solution are, the higher the photocatalytic activity of Ag/ZnO heterostructure photocatalyst should be. Acknowledgment. The authors acknowledge the financial support from the Department of Science of the People’s Republic of China (20771025), and the Department of Science & Technology of Fujian Province (2005H201-2). References and Notes
Figure 4. (a) UV-vis diffuse-reflectance and (b) PL spectra of the as-synthesized Ag/ZnO heterostructure nanocrystals.
Figure 5. Photodegradation of MO by the as-synthesized samples: (a) Ag/ZnO-DP, (b) Ag/ZnO-CP, and (c) Ag/ZnO-ST. The inset is the photograph of the as-synthesized samples dispersed in MO solution (the concentration of the catalysts is 1.25 mg/mL).
4. Conclusion Ag/ZnO heterostructure nanocatalysts were successfully prepared through three simple methods, where chemical reduction and photolysis reaction are adopted to fabricate the heterostructure. It is found that deposition-precipitation is an
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