2.1

Catalyst Synthesis

All the chemicals used in this work were of analytical reagent grade and used as received without further purification. In a typical synthesis, 0.02 mol zinc acetate and 0.03 mol glucose were dissolved in 10 ml ultrapure water under magnetic constant stirring in 90°C thermostat water bath. The brown thick colloid formed after 2h which was transferred to crucible and allowed to dry at 90°C overnight. The dried solid were further calcined in muffle from room temperature to 600°C with a heating rate of 5°C/min in air atmosphere and kept for 2h at 600°C in order to form ZnO.

For silver-modified ZnO, 0.0005mol, 0.001mol and 0.002mol silver nitrate were added into the above solution, respectively. The three gray colloids were dried and calcined as above ZnO to form different concentration of Ag/ZnO which were denoted as 2.5%, 5% g, 10% Ag/ZnO. 2.5%, 5% and 10% refer to the molar ratio of Ag/ZnO calculated on the basis of the initial amount of AgNO3 and Zn (Ac)2·2H2O.

2.2

Catalyst Characterizations

The X-ray powder diffraction (XRD) patterns were performed on a Dmax-3 β diffractometer with nickel-filtered Cu K α radiation. The X-ray photoelectron spectra (XPS) were recorded on a VG Multilab 2000 spectrometer with Cu K α radiation (1486.60eV). The SEM and TEM images were obtained using a JSM-35CF scanning electron microscope and a Tecnai G20 (FEI Corporation of Holland) Transmission Electron Microscope, respectively. The UV-vis diffuse-reflectance spectra of the as-synthesized samples were measured on a UV-vis spectrometer (UV-2550).

3.1

The Structure and Optical Properties of Nanozied Ag/ZnO Heterostructure

The XRD patterns of the as-synthesized samples with different Ag content are shown in Figure 1(a). Only hexagonal wurtzite ZnO (JCPDS 36-1451) was detected for the sample without Ag, while for other samples doping with Ag, face-centered-cubic (fcc) metallic Ag (JCPDS 04-0783) marked with “*” were also identified except for hexagonal ZnO, however, there is no remarkable shift of all diffraction peaks, implying that no Zn1-xAgxO solid solution is formed and the change of the lattice parameters of ZnO nanocrystals should be negligible. Furthermore, a consistent decrease in the intensity of relative ZnO peaks but an increase of silver peaks can be noted with the increase in concentration of Ag from 2.5 to 10 %.

Figure. 1

(a) The XRD patterns of the nanosized ZnO, Ag/ZnO heterostructure with different content Ag and (b) the predecessor. These (*) and (▼) marks indicated the peaks corresponding to Ag and C4H6O4Zn.

In order to make sure the chemical reaction for the formation of ZnO and Ag/ZnO heterostructure, the XRD patterns of the four precursors were detected as Figure 1(b). There is only a zinc acetate phase (marked with ▼) with the PDF no. of 01-0089 in the XRD pattern of ZnO precursor as a comparison sample. However, two main phases according to the XRD results of Ag/ZnO heterostructure precursor samples were detected. The intensity of zinc acetate (no. 01-0089) decreases but which of Ag (no. 87-0718) increases with the content of Ag from 2.5% to 10%. So we can infer that the existence of ZnO in the final samples come from the thermal decomposition of zinc acetate in the precursor but the metallic Ag generate in the first step of the chemical method.

In order to find the relationship between the two phases of ZnO and Ag, the surface structure of the representative sample with Ag content of 5.0 % was investigated by XPS analysis, and the corresponding experiment results are shown in Figure 2. The survey scan spectrum in Figure 2(a) revealed that the various signature of the electron orbital of Zn 2p, O 1s and Ag 3d were presented corresponding to the binding energies. The positions of Zn 2p3/2 and Zn 2p1/2 peaks are at the value of 1021.3 and 1044.5 eV, respectively. These two symmetric single peaks confirm that Zn element exists mainly in the form of Zn2+ chemical state on the sample surfaces as shown in Figure 2 (b). The energy peak located at 530.4 eV (O 1s) shown in Figure 2 (c) corresponds to O2- in O–Zn bonding.28 Moreover, there is a shoulder peak around 531.4 which could be ascribed to OH- caused by the contaminated surface in the air.29 Compared with pure metallic Ag, The binding energy of Ag 3d3/2 and Ag 3d5/2 for 5% Ag/ZnO shows a remarkably shift to the lower binding energy from 368.2 and 374.2 to 367.2 and 373.2 eV in Figure 2 (d), implying a strong interaction between Ag and ZnO.22 Similar result was found in the literatures about Ag/ZnO.30,31 When ZnO and Ag interact, the new Femi energy level is formed in order to keep stable state. Because the Femi energy level of Ag is higher than that of ZnO, the free electrons above the new Femi energy level of Ag could flow into ZnO causing the electron deficiency of Ag, partially generating monovalent Ag in which possesses a lower binding energy of Ag 3d as compared with Ag0.32,33

Figure.2

The representative XPS spectrum of the 5% nanosized Ag/ZnO heterostructure. (a) The survey spectrum, (b) Zn 2p3/2 and Zn 2p1/2 spectra, (c) O 1s spectrum, (d) Ag 3d5/2 and Ag 3d5/2 spectra.

Figure 3 shows the TEM images of the representative 5.0 % Ag/ZnO heterostructure. A low-magnified TEM image of this sample shows a high yield of nanosized Ag/ZnO heterostructure with diameter in the range of 30-50nm, as presented in Figure 3 (a). There is almost no aggregation and the shapes are without unification among the different nanoparticles. From the higher magnification image (Figure 3b) we can see the hexagon shape crystal with the side length of about 25 nm at (0001) plane which may be the ZnO nanocrystal according with the previous study of our groups.34 The darker particle in the left of Figure 3 (b) different from the hexagon ZnO nanocrystal may be the doping Ag nanoparticles. The similar result was already found in the literature.35 Figure 3 (c) is a high resolution TEM image of ZnO nanocrystal with 0.26nm as its fringe distance of (0002) plane, one angle of the hexagon can be clearly seen below the image. The FFT pattern taken from the square region of Figure 3 (c) reveals that this nanocrystal is ZnO exactly. Figure 3 (d) shows the interface between the ZnO and Ag nanocrystals, the two insets at the corner of left are the FFT patterns taken from the square region of the ZnO and Ag HRTEM images, respectively. The plane fringe with 0.26nm is assigned to the (0002) plane of wurtzite ZnO and the fringes interplanar distance of 0.23 is in good agreement with (111) plane of Ag. Figure 3(d) confirmed that the Ag growth along the (0002) plane of ZnO.

Figure.3

The low (a) and high resolution (b), (c) TEM images of the 5% Ag/ZnO heterostructure. The inset of (c) shows the FFT pattern of the selected region with the rectangle

The UV-Vis diffuse-reflectance spectra of unmodified and silver-modified ZnO are presented in Figure 4. It can be seen that there is almost no shift in the band edge absorption of the four samples which implies that the silver addition does not make any significant change in its band gap.36 There is only one absorption band in ZnO UV-Vis diffuse-reflectance spectrum which can be assigned to the absorption of the ZnO semiconductor. However, two prominent absorption bands are found in the Ag-coated ZnO UV-Vis diffuse-reflectance spectra. The former higher absorption bands in the UV region are similar to the above of ZnO, the latter lower one can be attributed to the characteristic absorption of surface plasmon resulting from metallic Ag in Ag/ZnO heterostructure.37 Moreover, the intensity of the latter absorption bands increase with the increasing concentration of Ag. The appearance of two kinds of characteristic absorption bands confirms that the as-synthesized samples are composed of Ag and ZnO phases. From the literature, we can see that the low absorption in the visible light range should be ascribed to oxygen defects from Ag doping.38 So Ag doping could create the crystal defects which improve the optical absorption in the visible region, this may be one reason of the promotion photocatalytic activity.

Figure.4

The transmittance spectra of the nanosized ZnO and Ag/ZnO heterostructure with different content of Ag

3.2

The Photocatalytic Performance

The photocatalytic activities of the as-prepared samples with pure ZnO and different Ag content are shown in Figure 5, where C0 is the original concentration of the methyl orange and C is the concentration after photocatalytic reaction at different time. As seen in Figure 5, all the Ag/ZnO heterostructure samples exhibit higher photocatalytic activities compared to the pure ZnO and the 5% Ag/ZnO sample shows the highest photocatalytic activity. The photocatalytic activities enhanced with the increase of Ag content until 5%. However, the photocatalytic activity of the Ag/ZnO heterstructure decreases with the increase of Ag content when the value exceeds 5%. Similar results were found by in the literature.22,30 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.24 Based on the phenomenon of our paper, we can infer that higher surface loading of metal deposits decrease the catalytic efficiency may because the reductive availability surface for absorption of light and pollutant. This viewpoint can also be supported by the intensity of the violet absorption band from ZnO in figure 4, we can see that the intensity of 2.5 % Ag is higher than the pure ZnO, but which decrease when the content of Ag exceeding than 2.5%. Because of the couple function of absorption from both ZnO and Ag nanoparticles, the photocatalytic activity of the nanosized Ag/ZnO heterostructure is not increasing with the adding of Ag content.

Figure.5

The photocatalytic activity of the pure nanosized ZnO and the Ag/ZnO heterostructure with different content Ag.