Facile Synthesis of Ag/TiO2 by Photoreduction Method and Its Degradation Activity of Methylene Blue under UV and Visible Light Irradiation

A series of Ag/TiO2 with various Ag contents were prepared by photoreduction method. Commercial TiO2 from Evonik-Degussa was used as the catalyst. Ag was used as the cocatalyst. This facial synthesis method is cheap and easy. TiO2 was suspended in water with various concentrations of silver nitrate. The solution was illuminated by UV light for 36 h. Ag would deposit on the surface of TiO2. This method can deposit all Ag cation in the starting material on TiO2 after 36 h irradiation by UV light. X-ray diffraction, high resolution-TEM, and X-ray photoelectron spectroscopy were used to characterize the surface, morphology and chemical composition of the catalysts. Photocatalytic degradation of methylene blue in water on these catalysts was carried out under UV and visible light irradiation, respectively. The methylene blue concentration in water was measured by a UV-vis spectrophotometer. The results showed that the bulk structure of TiO2 did not change and some of Ag was incorporated into the surface of TiO2 lattice. The change in the electronic state of Ti on surface is attributed to the replacement of titanium atoms by silver atoms on the TiO2 surface structure which induced visible light response and enhanced the photocatalytic activity. 1 wt% Ag is the optimum loading to have high activity.


Introduction
Titania photocatalyst has been extensively studied in the last two decades [1]- [20]. The TiO 2 from Evonik-Degussa (P25) has been known [1] [2] to have high photo catalytic activity under UV light irradiation, since it contains both anatase and rutile structure, and the interface between these two phases is the active site. Doping silver on TiO 2 has been known to have plasmon effect and results in high activity under visible light irradiation [3] [4]. It was reported that silver particles can serve as electron traps assisting electron-hole separation. Silver particles also accelerate electron excitation by producing a local electrical field, which can improve the photocatalytic activity of TiO 2 [5]- [12]. Many researchers used sol-gel method and other technique to synthesize AgTiO 2 . These methods are tedious and costive. In addition, some silver material was lost during preparation. Some researchers used impregnation to prepare AgTiO 2 , which resulted in big Ag particles.
Generally, surface modification of the semiconductor with noble metal nanoparticles expands the absorption of the semiconductor from UV light to visible light region, due to the surface plasmon resonance absorbance feature, and improves the electron-hole pair separation and consequently the rate of photocatalytic reaction grows [13]- [30]. Noble metal nanoparticles demonstrate strong and wide surface plasmon resonance (SPR) absorption under the visible light owing to the collective oscillations of their conduction band electrons by absorbing visible region. The frequency of the SPR band can be adjusted by modifying the shape and size of the noble metal nanoparticles, which can considerably enlarge the visible-light absorption and can be utilized to exploit efficient plasmonic photocatalysts under visible light. The noble metal nanoparticles are deposited on the surface of semiconductor in the plasmonic photocatalysts, and the metal nanoparticles serve as an element for collecting visible light because of their surface plasmon resonance while the metal-semiconductor interface efficiently departs the photogenerated electrons and holes. As a consequence, several works belonging to the deposition of noble metal nanoparticles such as gold (Au) and silver (Ag) on TiO 2 nanoparticles and their application used as photocatalysts have been fulfilled with great success [27] [28] [29] [30] [31]. The photocatalytic activity of noble metal nanoparticles doped TiO 2 toward the degradation of various environmental pollutants, such as methylene blue, methyl orange, rhodamine blue and 4-cholorophenol, has been well recorded [3] [18]- [33]. However, their photocatalytic efficiency does not arrive the standard demanded for practical applicability. Doping metal on titania can increase active sites and repression of e-/h+ pairs recombination [20]- [45].
The aim of this study was to develop a facile method to prepare Ag/TiO 2 , which is simple, low cost and have high activity under UV light and visible light irradiation. The effects of Ag loading on the activity of the catalyst were studied extensively.

Materials
Titanium dioxide (P25) was obtained from Evonik Degussa Company. Silver ni-

Catalysts Preparation
Ag/TiO 2 catalysts were prepared by photo-reduction method.

High-Resolution Transmission Electron Microscopy (HRTEM)
The observation in morphology and structure of TiO 2 and Ag/TiO 2 particles, including dimension of samples, were investigated by HRTEM on a JEOL JEM-2010 operated at 200 kV. The detail has been described in previous literature [5] [6] [7] [8]. The suspended particles were deposited on the holey carbon-coated copper grid (300#) (Ted Pella). The chemical composition of the samples was determined by Energy-dispersive X-ray spectroscopy (EDS) attached on the HRTEM with accelerating voltage of 200 kV, and using silicon detector.

X-Ray Photoelectron Spectroscopy (XPS)
The XPS spectra of the samples were recorded with a Thermo VG Scientific Sigma Prob spectrometer. The detail has been described in previous literature [5] [6] [7] [8]. The XPS spectra were collected using Al Kα radiation at a voltage and current of 20 kV and 30 mA, respectively. The base pressure in the analyzing chamber was maintained in the order of 10 -7 Pa. The pass energy was 23.5 eV and the binding energy was calibrated by contaminant carbon (C 1S = 284.5 eV).
The peaks of each spectrum were organized using XPSPEAK software; Shirley type background and 30:70 Lorentzian/Gaussian peak shape were used for deconvolution of the spectra.

Photocatalytic Degradation of Methylene Blue
In this study, the decomposition of methylene blue in aqueous solution under both UV light and visible light irradiation was used as the standard testing reaction. The detail has been described in previous literature [5]

XRD
The crystalline structures of TiO 2 and Ag/TiO 2 were characterized by XRD. All

HR-TEM
The morphology of the as-prepared catalyst was investigated by HR-TEM. Figure 2 shows that AgO particles were spherical shape, and the lattice spacing was 0.241 nm, in consistent with (111) plane of silver oxide.

XPS
The XPS spectra in the Ti 2p region of the samples are shown in Figure 4. The           which is the major species for degradation of methylene blue [38]- [45]. Figure 8 and Figure 9 show the XPS spectra of Ag 3d.
The Ag nanoparticles were used as surface traps which can capture the electrons from the conduction of TiO 2 and inhibit the electron/hole pair recombination [38]- [44]. These electrons further move from conduction of TiO 2 to the Ag nanoparticles and react with the dissolve oxygen to generate the superoxide    Figure 10 shows the adsorption amount of methylene blue on the catalyst. The amount of methylene blue adsorption increased after loading silver concentration. Multilayer methylene blue was adsorbed on the surface of catalyst, indication that adsorption of methylene blue is not the rate-determining step.

Photocatalytic Degradation of Methylene Blue Aqueous Solution
The results of photocatalytic activity under UV light irradiation are shown in Figure 11. It can be seen that the 1Ag/TiO 2 had the highest activity among all of the samples under UV light illumination. The photocatalytic activity of Ag/TiO 2 decreased when the silver content was more than 1 wt%. As silver concentration is more than the optimum amount, Ag becomes the recombination center of the photo-induced electrons and holes, which is harmful for photocatalytic reactions.
The photodegradation of organic dyes is suited to the Langmuir-Hinshelwood model. The slope of ln(C 0 /C) plotted versus irradiation time (min or h) demonstrates the rate constant of the reaction of the sample, as shown in Figure 12.
H.-C.       [40]. As shown in Figure 14 and With increment of silver doping, the quantity and size of silver cluster became bigger progressively and the energetic properties of the doped silver may be closed to that of bulk silver making the silver sites turn into recombination center of electrons and holes.
Moreover, higher amount of silver would enshroud more TiO 2 surface and prevent the contact between organics and TiO 2 , which would decrease lots of received photons and increase diffuse distance.

Conclusions
A series of Ag/TiO 2 were prepared by photoreduction method. The samples were The advantages of using photoreduction to prepare Ag/TiO 2 are as following.
1) All of Ag cations in starting material could be deposited and reduced on TiO 2 surface; 2) TiO 2 is commercially available and is cheap; 3) the bulk structure of TiO 2 is fully crystallized, and only surface structure is changed, resulting in high photocatalytic activities; 4) the preparation method is simple.