C@Ag/TiO₂: A Highly Efficient and Stable Photocatalyst Active under Visible Light

Bei Jin, Xiaosong Zhou, Xuyao Xu, Lin Ma, Zhijun Wu, Yingshan Huang

School of Chemistry Science & Technology, Development Center for New Materials Engineering & Technology in Universities of Guangdong, Zhanjiang Normal University, Zhanjiang, China.
School of Chemistry Science & Technology, Development Center for New Materials Engineering & Technology in Universities of Guangdong, Zhanjiang Normal University, Zhanjiang, China.

DOI: 10.4236/wjnse.2013.31001                

Abstract

In this paper, preparation and characterization of C@Ag/TiO₂ nanospheres compound photocatalysts was reported. C@Ag nanosphere was firstly synthesized via hydrothermal reaction, and followed by a sol-gel process to obtain the functionalized C@Ag/TiO₂ nanosphere which has highly efficient visible light catalytic ability towards methyl orange (MO). The morphology of the obtained compound was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) technologies. From which we can see that the as-prepared samples show a spherical structure with a diameter of approximately 200 nm, and the silver particle in core was about 10 nm. The catalytic ability of the synthesized photocatalysts under visible light irradiation shows that C@Ag/TiO₂ possesses higher photocatalytic activity towards MO degradation than that of N-P25 (TiO₂). Furthermore, the C@Ag/TiO₂ photocatalysts exhibited excellent reusability with almost no change after five runs. Finally, the possible photocatalytic mechanism of catalyst under visible light was discussion and proposed.

Keywords

Nanoparticles; Sol-Gel Preparation; Visible Light Photocatalyst; Surface Plasma Resonance

1. Introduction

The photocatalysis technique has a promising application prospect in pollutant decomposition and hydrogen evolution via the generation of •OH radicals and other oxidative species [1-3]. The semiconductor TiO₂ is considered as one of the best photocatalysts. However, the light response range and the photo-efficiency of TiO₂ are limited because of its wide band gap (3.2 eV) [4,5]. Therefore, the creation of simple, efficient, and sustainable photocatalysts that work well with visible light is a major challenge in this research field [6-8]. Until now, several methods have been reported to improve the photo-catalyzed efficiency of TiO₂. Among them, surface modification via the addition of metals which can enhance the photocatalytic activities is widely studied, and many metals, such as Pd, Pt, Rh, Ru, Ag, have been investigated to extent the absorption wavelength of noblemetal/TiO₂ compounds into the visible region [9-14]. The reason is that deposition of metals on the surface of TiO₂ would produce traps to capture the photo-induced electrons or holes, leading to the reduction of electronhole recombination and thus improving the photocatalytic efficiency [15]. Because of non-toxic, relatively inexpensive and obvious modification effect, loading Ag to improve the TiO₂ catalytic activity has been raised extensive attention. However, a problem is that Ag nanoparticles, which are chemically very reactive, would be easily oxidized when directly contact with TiO₂. Romanyuk et al. confirmed that Ag could have been oxidized at the Ag-TiO₂ interface to form eventually a 10 nm thick layer of silver oxide (AgO) at room temperature by employing time-of-flight secondary ion mass spectroscopy [16]. To prevent this oxidation, a passive material, such as SiO₂, must be coated on the surface of Ag nanoparticles so as to separate them from TiO₂. Zhang et al. reported core/shell nanofibers of TiO₂@carbon embedded by Ag nanoparticles with well-dispersed distribution of small Ag NPs in the carbon layer [17]. However, as the near-field amplitude decays in a rough estimation exponentially with the distance from the nanoparticles surface, the protection layer has to be kept sufficiently thin [18].

In this paper, we prepared C@Ag nanospheres by a simple hydrothermal method, and then using the sol-gel, C@Ag/TiO₂ composite visible light catalyst was synthesized. MO was used simulation pollutant, and the degradation experiments of the photocatalysts were carried out under visible light. The results showed the photocatalytic activity of the C@Ag/TiO₂ composite is 3 times higher than that of N-P25. Moreover, stability test showed that the composite photocatalyst is almost inactivating even after five cycles and the good stability is due to the protective effect of the coated carbon layers.

2. Experimental Section

2.1. Preparation of the Catalysts

A solution of AgNO₃ (1 mL, 0.02 M) was added to a glucose solution (40 mL, 0.5 M) with stirring to form clear solutions, which was placed in a 50 mL Teflonsealed autoclave and maintained at 180˚C for 12 h [19]. The precipitate was collected and washed with distilled water and absolute alcohol three times, respectively, and oven-dried at 80˚C for 24 h.

Ti[OCH(CH₃)₂]₄ (3 mL, Aldrich, 97%) and isopropyl alcohol (40 ml) mixed solution added dropwise to a solution of HNO₃ (50 ml, pH = 1.0) containing an amount of C@Ag, after aged for 10 h at room temperature, and then  dried at 110˚C for 20 h. The resultant gel was calcined at 500˚C for 2 h under a nitrogen atmosphere, which was denoted as CAT-X (X denotes percentage content of C@Ag).

2.2. Characterization of the Catalysts

The surface morphology was examined by a scanning electron microscopy (LEO1530VP, LEO), and a transmission electron microscopy (JEOL, JEM2010). UV-Vis diffuse reflection absorption spectra (UV-Vis/DRS) of the samples were recorded by an UV-Vis spectrometer (U3010, Hitachi) equipped with an integrating sphere accessory in the diffuse reflectance mode (R) and BaSO₄ as a reference material. The chemical nature of C, Ag, Ti, O have been studied by X-ray photoelectron spectroscopy (XPS) in Krato Axis Ultra DLD spectrometer with Al Ka X-ray (hv = 1486.6 eV) at 15 kV and 150 W. The binding energy was referenced to C 1s line at 284.6 eV for calibration.

2.3. Photocatalyst Reaction

The photocatalytic reaction was conducted in the XPA-II photochemical reactor (Nanjing Xujiang Machine-electronic Plant). A 500 W Xe lamp was used as the simulated solar light source, and a house-made filter was mounted on the lamp to eliminate infrared irradiation. MO (20 mg/L) was used as contamination [12]. 20 mg photocatalyst powder was dispersed in 200 mL reaction solutions by ultrasonicating for 15 min, then the suspension was magnetically stirred in dark for 1 h. Air was blown into the reaction medium at a flow rate of 200 mL/min during the photocatalytic reaction. One 8 mL of the suspension was sampled and filtered. The concentration of the remaining MO was measured by a Hitachi UV-3010 spectrophotometer. The degradation ratio was calculated by X = (A₀ − A)/A₀ × 100%.

3. Results and Discussion

3.1. Morphology and Structures of the Samples

The morphology of the C@Ag/TiO₂ sample is examined by SEM and TEM, and the images are shown in Figure 1. From SEM image, it is clear that C@Ag/TiO₂ is composed of many monodispersed spherical particles with a diameter of about 200 nm (shown in Figure 1(a)). The spheres are relatively uniform with very smooth surface. Figure 1(b) is a TEM image of C@Ag/TiO₂ sample and the samples shows spherical particles morphology. The diameters are approximately 200 nm, which is in agreement with the result of SEM. It is interesting that the middle of Ag nucleus can clearly be seen after magnified (shown in Figure 1(b) inset). A diameter is approximately 10 nm. The energy dispersive spectroscopic (EDS) analysis (Figure 1(c)) of the C@Ag/TiO₂ sample reveals the existence of C, Ag, O, and Ti elements. The element content of C, Ag, O, and Ti in the compounds investigated with the results of EDX is 5.35, 0.20, 31.55, 62.90 at%, respectively. It reveals that the samples were consists of C, Ag and TiO₂.

Figure 2 displays the XRD patterns of the prepared C@Ag and C@Ag/TiO₂ samples. Figures 2(a) exhibits the characteristic diffraction peaks of Ag (JCPDS file No.

Conflicts of Interest

The authors declare no conflicts of interest.

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