ZnO Self-Assembled 3D Hollow

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J. Phys. Chem. C 2008, 112, 16792–16800

One-Pot Synthesis of Ag/ZnO Self-Assembled 3D Hollow Microspheres with Enhanced Photocatalytic Performance Weiwei Lu,† Shuyan Gao,‡ and Jianji Wang*,‡ College of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou, Gansu 730000, People’s Republic of China and School of Chemistry and EnVironmental Science, Henan Key Laboratory of EnVironmental Pollution Control, Henan Normal UniVersity, Xinxiang, Henan 453007, People’s Republic of China ReceiVed: April 26, 2008; ReVised Manuscript ReceiVed: August 16, 2008

In this paper, three-dimensional (3D) Ag/ZnO hollow microspheres with different Ag contents were prepared through a facile one-pot hydrothermal method assisted by sodium alginate. The samples were structurally characterized by X-ray diffraction, field emission scanning electron microscope, high resolution transmission electron microscope, and X-ray photoelectron spectroscopy. It was shown that all samples were composed of metallic Ag and wurtzite ZnO; the 3D Ag/ZnO hollow microspheres were constructed from self-assembled 1D Ag/ZnO nanorods; the surface O species can be categorized to surface hydroxyl oxygen (OH) and crystal lattice oxygen (OL), and the ratio between them varies with different Ag loadings. The photocatalytic performance for the degradation of Orange G was also evaluated. The results show that such hierarchical Ag/ZnO hollow microspheres exhibit significantly enhanced photocatalytic efficiency. Investigation of the relationship of photoluminescence (PL) spectra and surface structure of the samples with their photocatalytic performance indicated that optimized amount of Ag deposits not only acted as electron sinks to enhance the separation of photoinduced electrons from holes, but also elevated the amount of the surface hydroxyl, leading to the formation of more hydroxyl radicals ( · OH) and then the higher photodegradation efficiency. 1. Introduction Semiconductor photocatalysts (e.g., TiO2 and ZnO) have been applied to a variety of environmental interests, such as remediation of contaminants and destruction of microorganisms due to their electronic structure characterized by a filled valence band (VB) and an empty conduction band (CB).1-4 The general scheme for the photocatalytic destruction of organic compounds involves the following three steps: (i) when the energy hυ of a photon is equal to or higher than the band gap (Eg) of the semiconductor, an electron is excited to CB, with simultaneous generation of a hole in the VB; (ii) then the photogenerated electrons and holes can be trapped, generally by the oxygen and surface hydroxyl, respectively, to ultimately produce the hydroxyl radicals ( · OH), which are known as the primary oxidizing species; and (iii) the hydroxyl radicals unselectively attack and commonly mineralize the adsorbed organic substances. However, in step (i), the photoinduced electrons and holes can also recombine to decrease the available photocatalytic efficiency. Originally, to overcome this limitation, one efficient way was to deposit the semiconductor with noble metals (for example, gold,5-9 platinum,10-13 silver14-17 and palladium12,18). Then the electrons in the CB can transfer to the metal deposits, which act as electron sinks due to the Schottky barrier at the metal-semiconductor interface,3,6 while the photoinduced holes can remain on the semiconductor surface. However, except for the improvement of segregation of the electrons and holes, the positive effect of metal particles on photocatalysis has also been explained in other ways. Some * To whom correspondence should be addressed. Tel: 86-373-3325805. Fax: 86-373-3325805. E-mail: [email protected]. † Lanzhou University. ‡ Henan Normal University.

researchers proposed that surface metallization may enhance the organic adsorption on the catalyst surface and thus elevated the degradation rate.15 Others suggested that the metal particles may even alter the photocatalytic reaction mechanism by providing catalytic sites19,20 and/or the noble metal itself can act as a catalyst to degrade the contaminants.21 Recently, two groups also reported that the noble metal modification can change the concentrations of defects in the semiconductor so as to enhance the photocatalytic efficiency, although the defect sites were considered to play opposite role in the individual work.22,23 Meanwhile, it is known that the photocatalytic redox reaction mainly takes place on the surface of the photocatalysts, so the surface properties, such as surface area24-28 and hydrophilicity,29,30 also have significant influences on the efficiency of photocatalysts. Furthermore, it has been proven that, for a photocatalytic oxidation process, surface hydroxyls of the photocatalyst play a significant role. Their presence on the surface not only can easily capture holes to form active · OH radicals,2,3,31,32 but also can enhance O2 adsorption to trap electrons and produce more · OH radicals.33-35 However, the influence of metal modification on the surface property, especially on the amounts of surface hydroxyl of the photocatalysts, is seldom investigated. While TiO2 is widely employed, ZnO is also an ideal photocatalyst because of their similar band gap energies, and thus the similar phototocatalytic mechanism and capacity. However, some studies have confirmed that ZnO exhibits better efficiency than TiO2 in photocatalytic degradation of some dyes, even in aqueous solution in some cases.36-39 Moreover, ZnO photocatalysts on the nanometer-scale have become more and more attractive because of their unique physical and chemical properties from the bulk.40,41 However, nanometer-scaled materials, such as nanoparticles and nanorods, with high surface-

10.1021/jp803654k CCC: $40.75  2008 American Chemical Society Published on Web 10/01/2008

One-Pot Synthesis of Ag/ZnO Microspheres

J. Phys. Chem. C, Vol. 112, No. 43, 2008 16793 transferred into a 50-mL Teflon-lined autoclave and heated at 393 K for 8 h. When the reactions were completed, the precipitates were collected, washed with deionized water and ethanol several times, and finally dried in air naturally. 2.2. Sample Characterizations. The phase and purity of the sample was determined by XRD on a Bruker D8A X-ray diffractionmeter with Cu KR radiation (λ ) 0.154056 nm) at 2θ ranging from 25° to 75°. The morphologies of the asprepared samples were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4800). The TEM and HRTEM were performed on JEOL 2100 at 200 kV. The surface property of all as-prepared samples was determined by XPS on a PHI Quantum 2000 Scanning XPS Microprobe with Al KR radiation (1486.6 eV). As an internal reference for the absolute binding energies, the C 1s peak of hydrocarbon contamination was used. The fitting of XPS curves was analyzed with the software Origin 7.0. Using the atomic sensitivity factors, the actual content of Ag for each sample was calculated by the relation:

Figure 1. XRD patterns of the ZnO and Ag/ZnO samples with various silver contents.

to-volume ratios tend to aggregate during the preparation and photocatalysis process, which results in the reduction of the photocatalytic efficiency. An available way to prevent the nanoparticles from aggregation and maintain the high photocatalytic efficiency is to organize these nanometer-scaled materials into a hierarchical structure.42 Therefore, in this work, first, the Ag-modified threedimensional (3D) ZnO hollow microspheres, which were constructed from the self-assembled of one-dimensional (1D) nanorods with the assistance of sodium alginate, were prepared through a facile one-step hydrothermal method. Then the asprepared samples were structurally characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). Especially, the influence of deposited Ag on the electronic structure and surface property of ZnO has been emphasized. Finally, the photocatalytic performance of the prepared Ag/ZnO samples with different Ag contents for degradation of Orange G was systematically explored. The relationship between the structure of the samples and their photocatalytic performance shows that optimal Ag deposits can significantly enhance the photocatalytic efficiency of ZnO hollow microspheres. For environmental application, it is expected that the prepared Ag/ZnO hollow microspheres may also have potential applications in the destruction of microorganisms because both Ag and ZnO are well-known inorganic antimicrobial agents. 2. Experimental Section 2.1. Preparation of 3D Ag/ZnO Hollow Microspheres. Sodium alginate was purchased from Acros, whereas all other chemicals of analytical grade were supplied by Beijing Chemicals Co. Ltd. and used as received. The synthesis procedure of Ag/ZnO hollow microsphere is a modified version of the one reported by our coauthor previously.43 Briefly, 0.55 mL of a 25.4 mM sodium alginate aqueous solution (calculated by the repeating unit), 30 mL of aqueous solution containing 1.5 mmol of zinc acetate dihydrate, and certain amount of silver nitrate, and 10 mL of ethanol were mixed with agitation in a beaker. Then, 1.0 mL of 28 wt.% ammonia aqueous solution was added dropwise to the above mixture. The resulting solution was

Ag at.% ) (IAg ⁄ σAg) ⁄ (IAg ⁄ σAg+IZn ⁄ σZn+IO ⁄ σO) where IAg, IZn, and IO are the area under the deconvoluted XPS curve of Ag 3d, Zn 2p3/2, and O 1s, respectively; σAg, σZn, and σO are the corresponding atomic sensitivity factors for Ag 3d (5.198), Zn 2p3/2, (3.354) and O 1s (0.711), respectively. The room-temperature photoluminescence (PL) spectra of the samples were obtained using a JASCO FP-6200 Fluorescence Spectrometer and the excitation wavelength used in the PL measurements was at 325 nm. 2.3. Photocatalytic Tests. Orange G (formula: C16H10N2O7S2Na2, molecular weight: 452), a widely used anionic azo dye, was employed as a representative dye pollutant to evaluate the photocatalytic performance of Ag/ZnO samples. A cylindrical photoreactor surrounded by a circulating water jacket to cool the reaction solution was used and the ultraviolet light was provided by a 250 W high-pressure fluorescent Hg lamp (Institute of Electrical Light Source, Beijing; the strongest emission at 365 nm). For a typical procedure, a mixture of 100 mL of 10 ppm Orange G aqueous solution and 100 mg of each Ag/ZnO photocatalyst was agitated in the dark for 30 min in photoreactor to ensure establishment of adsorption equilibrium of the dye on catalyst surface. After initiation of the reaction by irradiation of the reactor, 5 mL samples of the suspension were withdrawn at regular intervals and were immediately centrifuged and then filtered to completely remove catalyst particles. The degradation of Orange G and the decolorization of the solution were determined by measuring the absorbance of the solution samples on a TU-1810 UV-vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.). The total organic carbon (TOC) of the solution was determined by an Apollo 9000 TOC Analyzer (Tekmar-Dohrmann) with a NonDispersive Infra-Red (NDIR) detector. Some organic intermediates have been analyzed employing a gas chromatography-mass spectrometry (6890/5793N GC-MS, Agillent Corporation, USA) equipped with a HP-5 (Hewlett-Packard, USA, 30 m × 0.25 mm × 0.25 µm) capillary column. 3. Results and Discussion 3.1. Chemical Reaction. Metallic Ag modified 3D ZnO hollow microspheres were prepared through a facile one-pot hydrothermal method as described in the Experimental Section. At the initial stage of the reaction, in the presence of ammonia, Zn(NH3)42+ and Ag(NH3)2+ complexes were formed respectively, as shown by the following equations:

16794 J. Phys. Chem. C, Vol. 112, No. 43, 2008

Zn2++4NH3 f Zn(NH3)2+ 4

Lu et al.

(1)

Ag++2NH3 f Ag(NH3)+ 2

(2) 2+

Then under alkali thermal condition, Zn(NH3)4 and Ag(NH3)2+ were transformed to Zn(OH)42- and Ag(OH)2-, respectively. At the same time, intermolecular dehydrolysis between Zn(OH)42- and Ag(OH)2- may occur, leading to the formation of Ag2O/ZnO nuclei through a Zn-O-Ag bond.22 At the growth stage, ZnO nanorods were gradually formed due to the high tendency to grow along the c axis of ZnO. Also, Ag2O can be reduced by ethanol under this hydrothermal condition.44,45 So, a Zn-O...Ag bond can be formed between metallic Ag and ZnO nanocrystals, leading to the formation of Ag/ZnO metalsemiconductor nanocomposites. In this reaction procedure, the added ethanol acted not only as cosolvent but also as reducing agent to reduce ionic Ag+ into metallic Ag. Finally, with the assistance of biopolymer sodium alginate, the afore-formed nanorods self-assembled into 3D hollow microspheres. Therefore, the overall chemical reactions can be formulated as follows: 22

NH3+H2O f NH+ 4 +OH

(3)

+ 2Zn(NH3)2+ 4 + 2Ag(NH3)2 + 8OH f Zn(OH)4 + co - dehydrolysis

2Ag(OH)2 + 8NH3 98 Ag2O ⁄ ZnO Ag2O ⁄ ZnO + ethanol f Ag ⁄ ZnO

(4) (5)

It should be mentioned here that, in the whole preparation process, due to the complexation of Ag+ with ammonia in the

solution, the standard redox potential of Ag(NH3)2+/Ag (+ 0.38 eV) was much lower than that of the Ag+/Ag (+ 0.8 eV),46 so the Ag2O will be preferentially reduced by ethanol than Ag(NH3)2+ will be. And this leads to the formation of Ag nanoparticles on the ZnO surface, and that in the solution is prevented. 3.2. Crystal Structure and Microstructure of the asPrepared Ag/ZnO Hollow Microspheres. The XRD characterizations of the as-prepared Ag/ZnO samples with different contents of Ag (Figure 1) show that all the diffraction peaks can be categorized into two sets. Peaks marked with “*” match well with Bragg reflection of the standard wurtzite structure of ZnO (P63mc, a ) 3.25Å, c ) 5.21Å, JCPDF #36-1451), whereas others marked with “#” agree well with face-centered cubic (fcc) silver metal (JCPDF #04-0783). No characteristic peaks of impurities and other phases such as Zn(OH)2 and Ag2O are observed. In addition, negligible changes of all diffraction peak positions and lattice parameters of ZnO in all Ag/ZnO samples compared to that of pure ZnO suggest that Ag does not incorporate into the lattice of ZnO, but as metal deposit on the surface. The representative FESEM patterns of the sample with a Ag content of 1.62 at.% are shown in Figure 2a. This low magnified image indicates that the panoramic morphology of the asprepared sample is sphere-like with a diameter ranging from 3 to 5 µm. The highly magnified SEM image of a fragment of the broken sphere (Figure 2b) reveals that the architecture of the hollow microsphere is built from a single layer of oriented nanorods.

Figure 2. Representative FESEM images of the as-synthesized Ag/ZnO hollow microspheres with a Ag content of 1.62 at.%: (a) a low magnified panoramic view, (b) a fragment of one broken microsphere, (c) an individual microsphere, and (d) a high magnified FESEM image taken from the squared region.


J. Phys. Chem. C, Vol. 112, No. 43, 2008 16795

Figure 3. Typical TEM images of the as-synthesized Ag/ZnO hollow microspheres with a Ag content of 1.62 at.%: (a) low magnified image of a fragment of a broken microsphere, (b) high magnified image of an individual nanorod, (c) HRTEM image of the ZnO (the inset is the corresponding FFT pattern), and (d) HRTEM image of the Ag/ZnO (the inset is the corresponding FFT pattern of Ag)

The TEM/HRTEM images in Figure 3 corroborate the morphologies observed in the FESEM images. Figure 3a (a fragment of the broken microsphere) confirms that the wall of the hollow microsphere is composed of oriented nanorods, which are aligned with their growth axes perpendicular to the surface of the microsphere. A typical medium magnified TEM image of an individual Ag/ZnO nanorod is shown in Figure 3b. The 2D lattice fringes in the HRTEM image in Figure 3, parts c and d, signify the crystallinity of the ZnO nanorod. The distance between two fringes is about 0.528 nm, which is close to the d spacing of the (0001) plane, indicating that the direction (c axis) is the preferential growth direction of ZnO nanorods. And in Figure 3d, lattice fringes with interplanar spacing of 0.236 nm, corresponding to the (111) plane of polycrystalline Ag nanoparticles, are also observed. 3.3. Surface Structure of the As-Prepared Ag/ZnO Hollow Microspheres. The surface components and chemical states of the as-prepared samples with different Ag contents are investigated by XPS analysis, and the corresponding results are shown in Figure 4. Figure 4a shows the scan survey spectra for the representative Ag/ZnO sample with an Ag content of 1.62 at.%. All of the peaks on the curve can be ascribed to Ag, Zn, O, and C elements and no peaks of other elements were observed. The presence of C mainly comes from the hydrocarbon contaminants that commonly exist for XPS. Therefore, it can be concluded that the sample is composed of Ag, Zn, and O only, which is in agreement with the XRD results. The positions of Zn 2p3/2 peak for all Ag/ZnO samples (Figure 4b) are nearly at the same value of 1021.4 eV, which confirms that Zn element exists mainly in the form of Zn2+ chemical state on the sample surfaces. Figure 4c shows the Ag 3d XPS spectra for all of the samples. For comparison, Ag 3d XPS spectrum of the synthesized pure

metallic Ag under the same condition is also given. It is shown that the binding energy (BE) of Ag 3d5/2 for each Ag/ZnO sample shifts remarkably to the lower value compared with that of the prepared pure Ag nanoparticles and the bulk Ag° (the standard binding energy of Ag 3d5/2 for bulk Ag° is about 368.2 eV). This suggests that the electron density of Ag is decreased. The reduction of electron density of Ag nanoparticles may be due to the transfer of electrons from the Ag nanoparticle to ZnO nanorod.22 As shown in Figure 5a, the work function Φs of ZnO is about 5.2 eV, and the first electron affinity is about 4.3 eV, whereas the work function Φm of Ag is about 4.26 eV. Thus, the Fermi energy level of ZnO (Efs) is lower than that of Ag (Efm) because of the larger work function of ZnO. This results in the transfer of electrons from Ag to ZnO until the two systems attain equilibrium and form the new Fermi energy (Ef).47-49 It should be noted here that, in general, the BE value of zerovalence metallic atom is smaller than that of metal cation. However, Ag is exceptional50 (BE value of Ag° and Ag+ is about 368.2 and 367.2 eV, respectively). In our experiment, the BE value of Ag in all individual Ag/ZnO samples is even slightly lower than that of Ag+, indicating the significant positive charge of Ag because of the strong interaction with ZnO. Figure 4d shows the O 1s peaks for all of the samples under study. It can be seen that all of the O 1s peaks are somewhat asymmetric, suggesting that there are at least two kinds of oxygen species on the sample surface. The curve fitting patterns are also shown in Figure 4d. In all XPS curves, the hollow circles denote the original data, and the solid lines represent the fitted curves and the deconvoluted individual peaks of the two different oxygen species present on the surface. The deconvoluted peak marked with “R” is closely associated with the lattice oxygen (OL) of ZnO and its position is at about 530.2 eV, while the other peak marked with “β” is attributed to the


Lu et al.

Figure 4. (a) Representative XPS survey spectrum of Ag/ZnO sample with a Ag content of 1.62 at.%. The XPS full spectra of (b) Zn 2p3/2, (c) Ag 3d, and (d) O 1s of the as-prepared Ag/ZnO samples with various Ag contents (For comparison, the Ag 3d spectrum of pure Ag prepared under the same condition was also given in 4c).

oxygen of surface hydroxyl (OH) and its position is at about 531.7 eV.51-53 Since the surface hydroxyl plays a crucial role in trapping photoinduced electrons and holes, and then in the production of primary active hydroxyl radicals, the quantitative analysis of O 1s XPS data has been carried out and the results are summarized in Table 1. It can be seen that as the deposited Ag contents increase, the ratios between OH and OL increase first and then decrease, the maximum is at 1.62 at.% Ag loadings. On the basis of these data, it can be concluded that Ag deposition can modify the surface hydroxyl contents of ZnO microspheres. 3.4. Photocatalytic Performance. Orange G is selected as a representative organic pollutant to evaluate the photocatalytic performance of the Ag/ZnO hollow microspheres with various

Ag contents. The decolorization of Orange G is monitored by the UV-vis spectra, and the extent of the mineralization is determined by the TOC removal of the dye solution. As an example, the time-dependent absorbance spectra of the dye solution are shown in Figure 6 in the presence of Ag/ZnO microspheres with a Ag content of 1.62 at.%. It can be seen that the absorbance peaks at 248, 331, and 472 nm are reduced significantly, indicating the degradation of the dye molecules. It is also evident from Figure 6 that after the illumination time of 50 min, the initial Orange G solution can be totally decolorized. Moreover, the TOC data indicate that the initial dye and the formed intermediates can be completely mineralized within 60 min under the same condition (see Supporting Information, Figures S1-S2).



Figure 5. (a) The band structures of Ag and ZnO junction and the Fermi energy level equilibrium without UV irradiation.22,48,49 Because of the larger work function of ZnO, the Fermi energy level of ZnO (Efs) is lower than that of Ag (Efm), resulting in the transfer of electrons from Ag to ZnO until the two systems attain equilibrium and form the new Fermi energy (Ef). (b) the proposed charge separation process and the photocatalytic mechanism of as-prepared Ag/ZnO samples under UV irradiation. Due to that the energy level of CB for ZnO is higher than the Fermi energy level of Ag, the photoinduced electrons are transferred to the metallic Ag. Then the electrons in the Ag sinks can be trapped by the chemisorbed O2 and the hole can be captured by the surface hydroxyl.

TABLE 1: Calculation Results for O1s XPS Spectra of Ag/ZnO with Different Ag Contents sample ZnO 0.83 at.% Ag/ZnO 1.62 at.% Ag/ZnO 3.30 at.% Ag/ZnO 6.45 at.% Ag/ZnO

a

OL Eb (eV) fwhma (eV) Ri%b Eb (eV) fwhm (eV) Ri% Eb (eV) fwhm (eV) Ri% Eb (eV) fwhm (eV) Ri% Eb (eV) fwhm (eV) Ri%

530.2 1.38 77.3 530.2 1.38 69.1 530.3 1.43 52.5 530.4 1.50 59.2 530.1 1.40 67.4

OH 531.7 1.90 22.6 531.6 1.89 30.9 531.8 1.82 47.5 531.8 1.77 40.8 531.5 1.78 32.6 b

fwhm abbreviated for Full Width at Half Maximum; Ri%: the percent of the individual oxygen species calculated from the peak area.

Therefore, Ag nanoparticles, acting as electron sinks, reduce the recombination of photoinduced electrons and holes, and prolong the lifetime of the electron pairs. Subsequently, the electrons can be captured by the adsorbed O2, and the holes can be trapped by the surface hydroxyl, both resulting in the formation of hydroxyl radical species ( · OH).31,54,55 It is accepted that hydroxyl radical species show little selectivity for attacking dye molecules and are able to oxidize the pollutants due to their high oxidative capacity (reduction potential of · OH is 2.8 V). Thus, the possible mechanistic pathway of Ag/ZnO microspheres for degradation of Orange G can be proposed as follows: 22

+ ZnO + hV(UV) f ZnO(ecb + hvb) +

Ag f Ag + e

· O2H

H+

(6)

-

(7) e-

e- + O2(ads) f · O2 98 · O2H 98 O2 + H2O2 98 O2 + · OH + OH-

The photocatalytic experimental data can be converted to a linear pattern using pseudofirst kinetics model, and the results are shown in Figure 7. A control test without photocatalyst shows that the photoinduced self-sensitized photolysis can be neglected compared with the photocatalysis. Also, as a photocatalytic reference, commercial TiO2 (Degussa P-25) is used to evaluate the activity of Ag/ZnO samples, qualitatively. It can be seen from Figure 7 that the photocatalytic performance of ZnO microspheres can be significantly improved by depositing an appropriate amount of Ag nanoparticles. The positive effect of Ag deposits is commonly due to the fact that Ag nanoparticles on the semiconductor surface behave like electron sinks, which provide sites for the accumulation of photogenerated electrons, and then improve the separation of photogenerated electrons and holes. This can be understood based on the proposed charged separation of Ag/ZnO under UV irradiation as shown in Figure 5b. Because the bottom energy level of the CB of ZnO is higher than the new equilibrium Fermi energy level (Ef) of Ag/ZnO, the photoexcited electrons on the CB under UV irradiations can transfer from ZnO to Ag nanoparticles. It has been proposed that the charge separation is the outcome of a Schottky barrier formed at the metal-semiconductor interface.3,6

+

-

Ag + e f Ag h+ vb+OH f

· OH

· OH + Orange G f Degradation Products

(8) (9) (10) (11)

The PL spectrum has been proven to be an effective way to study the electronic structure, optical and photochemical properties of semiconductor materials, by which the information and efficiency of charge carrier trapping, immigration, and transfer can be obtained.56-58 Generally, one is interested in two extreme relaxation processes following the creation of photoinduced electron-hole pairs within the semiconductor particles. In the first, maximum photoluminescence is desired; that is, all charge carriers should recombinate radiatively, and no redox reactions should occur at the surface.56 At the other extreme, maximum chemical energy is expected to be extracted out of the illuminated particle; that is, all of the photoinduced electrons and holes should be totally separated to participate in the photocatalytic reaction.2 Therefore, the intrinsic relationship between PL spectra and photocatalysis is of significance and has been investigated in many literatures.6,7,22,23,41 However, it is far beyond clear due to the complication of PL and photocatalysis.


Lu et al.

Figure 6. Time-dependent UV-vis absorbance spectra of the Orange G solution in the presence of Ag/ZnO sample with a Ag content of 1.62 at.%.

Figure 7. The ln(C0/C) versus time curves of photodegradation of Orange G. C0 and C are the initial concentration after the adsorption equilibrium and the reaction concentration of Orange G, respectively. The experimental data are fitted using the pseudofirst-order kinetic equation: ln(C0/C) ) kt.

In our study, the effect of Ag nanoparticles on the separation of photoinduced electrons and holes in ZnO photocatalysts can be confirmed by the PL spectra. Figure 8 shows the PL spectra of the as-prepared ZnO and Ag/ZnO microspheres. It is found that all individual curves similarly show three emission bands, including a near UV emission at around 380-400 nm, a strong blue emission at about 450-470 nm, and a weak and broad green band at about 550 nm. It also can be seen that the PL intensity of each emission band of Ag/ZnO composites is lower than that of pure ZnO. This indicates that the Ag deposits act as electron sinks and hinder the recombination of photoinduced electrons and holes, accounting for the declined intensity of the PL emission. Moreover, it can be observed from Figure 8 that the PL intensity

of the Ag/ZnO microspheres decreases with the increase of Ag contents. The reason is that with the Ag content increasing, there are more metal sites to accept the electrons, so the separation effect for photoinduced electrons and holes increases correspondingly. From this point of view, the photocatalytic activity of the Ag/ZnO micropheres should increase with the increase of Ag loadings. However, it is found from Figure 7 that as the Ag content increases, the photocatalytic performance of the Ag/ZnO microspheres does not enhance monotonously. When the Ag content is relatively lower (1.62



Figure 8. PL spectra of the as-prepared ZnO and Ag/ZnO microspheres with various Ag contents.

Figure 9. Pseudofirst-order rate constant k and the surface hydroxyl content of Ag/ZnO as a function of Ag content. The value of rate constant k is equal to the corresponding slope value of the fitting line shown in Figure 7.

at.%), the photocatalytic activity of the Ag/ZnO decreases with the increase of Ag content (1.62 at.% Ag/ZnO > 3.30 at.% Ag/ZnO > 6.45 at.% Ag/ZnO). Thus, the optimal Ag content is approximately 1.62 at.%. Several groups have suggested that, at higher metal content than that optimized, the over accumulations of electron on metal deposits could attract the photogenerated holes to the metal sites. This may encourage the recombination of charge carriers and the metal deposits conversely behave as recombinant centers.14,17,59 In addition, higher surface loadings may decrease the catalytic efficiency of the semiconductor due to the reductive availability of semiconductor surface for light absorption and pollutant adsorption.9 In our study, this phenomenon can be attributed to the changes of the surface hydroxyl content of Ag/ZnO microspheres caused by different Ag loadings.

Figure 9 shows the variation of pseudofirst-order rate constant and the surface hydroxyl content of Ag/ZnO microspheres with the different Ag content. It can be seen that the photocatalytic performance of Ag/ZnO microspheres is in good agreement with the surface hydroxyl content, i.e., the more the surface hydroxyl content becomes, the more efficient the photocatalyst becomes. It has been proven that surface hydroxyls of the photocatalyst play a significant role in the photocatalysis process. Their presence on the surface not only can easily capture holes to form active · OH radicals,2,3,31,32 but also can enhance O2 adsorption to trap electrons and produce more · OH radicals.33-35 On the basis of the XPS characterization (Figure 4d and Table 1), we find that Ag nanoparticles as metal deposits can modify the surface hydroxyl content of ZnO microspheres. From this point, we can reasonably infer that various contents of surface

16800 J. Phys. Chem. C, Vol. 112, No. 43, 2008 hydroxyl on the Ag/ZnO microspheres caused by different Ag loadings also result in a difference in the photocatalytic activity. Therefore, the 1.62 at.% Ag/ZnO microspheres, which not only have the appropriate content of Ag to separate the photoinduced electrons and holes, but also have the most surface hydroxyls to capture them, exhibits the highest photocatalytic performance in our work. 4. Conclusions In summary, 3D Ag/ZnO hollow microspheres with enhanced photocatalytic efficiency and high stability against aggregation have been successfully synthesized through a facile one-pot hydrothermal method. On the basis of the structural characterizations and photocatalytic results, the effect of Ag deposition on the structure and photocatalytic performance of ZnO microspheres can be summarized as follows: (i) Ag nanoparticles on the ZnO microspheres act as electron sinks, improving the separation of photogenerated electrons and holes; (ii) appropriate contents of Ag deposition increase the surface hydroxyl contents of ZnO microspheres, facilitating the trapping of the photoinduced electrons and holes to form more active hydroxyl radicals, and thus, enhance the photocatalytic efficiency of ZnO. Acknowledgment. This work was supported financially by the National Natural Science Foundation of China (Grant No. 20573034) and the Innovation Scientists and Technicians Troop ConstructionProjectsofHenanProvince(GrantNo.084200510015). Supporting Information Available: The detailed study of the photodegradation process of Orange G. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Fox, M. A.; Dulay, M. T. Chem. ReV. 1993, 93, 341. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (3) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735. (4) Pirkanniemi, K.; Sillanpaa, M. Chemosphere 2002, 48, 1047. (5) Wang, C. Y.; Liu, C. Y.; Zheng, X.; Chen, J.; Shen, T. Colloids Surf. A 1998, 131, 271. (6) Li, X. Z.; Li, F. B. EnViron. Sci. Technol. 2001, 35, 2381. (7) Li, F. B.; Li, X. Z. Appl. Catal. A: Gen. 2002, 228, 15. (8) Subramanian, V.; Wolf, E.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 11439. (9) Arabatzis, I. M.; Stergiopoulos, T.; Andreeva, D.; Kitova, S.; Neophytides, S. G.; Falaras, P. J. Catal. 2003, 220, 127. (10) Zhao, W.; Chen, C.; Li, X.; Zhao, J.; Hidaka, H.; Serpone, N. J. Phys. Chem. B 2002, 106, 5022. (11) Lam, S. W.; Chiang, K.; Lim, T. M.; Amal, R.; Low, G. K. C. Appl. Catal. B 2007, 72, 363. (12) Jin, S.; Shiraishi, F. Chem. Eng. J. 2004, 97, 203. (13) Li, F. B.; Li, X. Z. Chemosphere 2002, 48, 1103. (14) Sclafani, A.; Herrmann, J. M. J. Photochem. Photobiol. A 1998, 113, 181. (15) Tada, H.; Teranishi, K.; Inubushi, Y.; Ito, S. Langmuir 2000, 16, 3304. (16) Vamathevan, V.; Amal, R.; Beydoun, D.; Low, G.; McEvoy, S. J. Photochem. Photobiol. A 2002, 148, 233. (17) You, X. F.; Chen, F.; Zhang, J. L.; Anpo, M. Catal. Lett. 2005, 102, 247. (18) Wang, C. M.; Heller, A.; Gerischer, H. J. Am. Chem. Soc. 1992, 114, 5230. (19) Kim, S.; Choi, W. J. Phys. Chem. B 2002, 106, 13311. (20) Lee, J. S.; Choi, W. Y. EnViron. Sci. Technol. 2004, 38, 4026.

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