Abstract
Ti doped Bi2O3 (TDB) and Ag ion modified Ti doped Bi2O3 (Ag@TDB) photocatalysts were prepared by framework replacement synthesis method with different Ag loadings (0.05, 0.3, 0.75, and 1.0 mol/L AgNO3). The structural properties of the prepared catalysts were studied by scanning electron microscope (SEM), X-ray diffraction (XRD), BET surface area, and UV/Vis diffuse reflectance (DRS). The XRD spectra of the Ti doped Bi2O3 calcined at 650°C showed the diffraction peaks of a mixture of Bi12TiO20 and Bi4Ti3O12, with bits of mixed crystallite consisting of TiO2 and B2O3. A high blue shift in the range 650–550 nm was detected in the DRS band. This blue shift increased with the decreasing Ag content. The photocatalytic activities of the catalysts were evaluated for the degradation of crystal violet (CV) under UV light irradiation. The results indicated that the degradation rate of CV by using 1.0 mol/L AgNO3 doped bismuth titanate composite photocatalyst (1.0 Ag@TDB) was 1.9 times higher than that by using the bare Ti doped Bi2O3 photocatalyst. The higher activity of Ag@TDB is due to the enhancement of electron-hole pair separation by the electron trapping of silver particles.
1. Introduction
Photocatalytic degradation of organic pollutants is an alternative and harmless technology for the removal of organic impurities [1]. The crucial step for its application is to develop new, efficient, and clean catalysts. Recently, bismuth-based oxides photocatalysts such as Bi2O3, BiVO4, and Bi2WO6 have attracted much attention due to their attractive photocatalytic properties under visible light irradiation [2–4]. Bi2O3 has been widely used in gas sensors, solid oxide fuel cells, optical coatings, and ceramic glass manufacturing due to its high refractive index, dielectric permittivity, marked photoluminescence properties, and thermal stability [5]. Furthermore, Bi2O3 has also shown higher photocatalytic activities in splitting water and decomposing pollutant. Despite the positive attributes of Bi2O3, low quantum yields hinder its practical application.
To improve the photocatalytic properties of catalysts, transition-metal ions with 3D electron structure, such as Fe(III), Sm(III), and Zn(II), have been widely used to modify the photocatalysts because they can act as shallow charge traps [6–9]. In our previous research, transition-metal ion Ti(IV) doped bismuth-based oxides photocatalysts have been confirmed to show higher photocatalytic activity. The decolorization rates of crystal violet (CV) in aqueous solution with Ti(IV) doped Bi2O3(TDB) have been proved to be double compared with that with bare Bi2O3 [10]. Pentachlorophenol could be also removed efficiently by using TDB, and its reductive dechlorination route under visible light irradiation was firstly revealed [11]. The removal ratios of pentachlorophenol and 2,4-dichlorophenol were 99.7% and 99.2%, respectively, by using TDB photocatalysis combined with laccase catalysis [12]. The results showed that Ti (IV)-doping improved the light absorption capacity and enhanced the photogenerated electron yield of Bi2O3, which was mainly attributed to the narrowed band gap () and red shift of light absorption edge [10].
The photocatalytic efficiency of catalysts is closely related to the rates of recombination of photoexcited electrons and holes [13]. Insertion of noble metal (e.g., Pt, Au, and Ag) could reduce the rate of charge carrier recombination and then improve the photocatalytic activity of catalysts. For example, Sobana et al. [14] reported that the photocatalytic activity of silver doped TiO2 was far much higher than that of the bare TiO2 for the degradation of azo dyes. Additionally, the photonic efficiency of all metal-doped TiO2 (i.e., Pt, Au, and Pd) was found to be higher than bare TiO2 due to the acceleration of hydroxyl radical formation and decreasing recombination [15]. Rayalu et al. [16] used Au/TiO2 as a catalyst in hydrogen generation under visible light illumination for their high photocatalytic activity. Evidence indicated that the photocatalytic activity could be enhanced by the impregnation of the semiconductor surfaces with optimal loading.
The aim of this study was to assess the photocatalytic activity of TDB by additional doping with Ag ion. A series of Ag ion modified Ti doped Bi2O3 (Ag@TDB) photocatalysts were prepared by the framework replacement synthesis method. The influences of the Ag ion contents on the surfacial properties, optical absorption, and crystal shape of the photocatalysts were studied. The photocatalytic degradation of CV was explored by using Ag@TDB, and the optimal Ag ion loading content was obtained. Finally, the promotion mechanisms of Ag@TDB were proposed.
2. Materials and Methods
2.1. Catalyst Preparation
The bare TDB was prepared by a hydrothermal synthesis technique as described in detail elsewhere [11]. The TDB photocatalysts were then dipped in an aqueous solution of silver nitrate (AgNO3) with proper concentration after 2 h of illumination under UV light (at 254 nm) from a G15W/T8 Sylvania tube lamp (in the presence of atmospheric oxygen). After dipping, the catalysts were thoroughly washed with ultrapure water and dried at 150°C for 24 h. The concentrations of AgNO3 were 0.05, 0.3, 0.75, and 1.0 mol/L. The prepared photocatalysts were labeled as 0.05 Ag@TDB, 0.3 Ag@TDB, 0.75 Ag@TDB, and 1.0 Ag@TDB, respectively.
2.2. Catalyst Characterization
Detailed surface images of the photocatalysts were obtained by means of a scanning electron microscope (SEM, JSM-6360LV, Hitachi). The structures of the photocatalysts were characterized by X-ray diffraction (XRD, D/MAX-rb, Rigaia). UV-visible absorbance spectrophotometry (DRS, Varian-CARY 1E UV-visible spectrophotometer, Varian) was used to measure the absorption spectrometry. The surface area and pore sizes were measured using a gas sorption analyzer (ASAP-2010 M, Micromeritics).
2.3. Photocatalytic Degradation of Crystal Violet
The photocatalytic efficiencies of catalysts were evaluated by the degradation of CV in aqueous solution. In all cases, a volume of 100 mL CV solution was used to conduct the photocatalytic experiments with regard to the effects of catalyst dosages (0.2 g/L, 1.0 g/L, 1.6 g/L, and 2.0 g/L), catalyst types (TDB, 0.05 Ag@TDB, 0.3 Ag@TDB, 0.75 Ag@TDB, and 1.0 Ag@TD), and initial CV concentrations (5, 10, 20 mg/L). The dye solution containing the appropriate quantity of the photocatalyst powder was magnetically stirred before and during illumination. Prior to irradiation, the suspension solution was firstly ultrasonicated for 15 min and then transferred to the photocatalytic reactor. The reactor was covered by aluminium foil to prevent UV light leakage. The suspension was irradiated under an 8 W UV lamp (UV354L, 354 nm, Beony S.T.). A suspension solution of 5 mL was withdrawn after periodic interval of irradiation and analyzed after filtration. The photocatalytic degradation ratio of CV was monitored by measuring the absorbance in a UV-Vis spectrophotometer.
3. Results and Discussion
3.1. Characterization
The phase and composition of TDB photocatalysts were characterized by XRD. Figure 1(a) shows the XRD pattern of the bare TDB photocatalyst. The diffraction peaks can be indexed to a mixture of Bi12TiO20 and Bi4Ti3O12, with bits of mixed crystallite consisting of TiO2 and B2O3. The monoclinic phase was mixed with three-phase (, , ). Bismuth-based oxides exist in four stable crystalline phases under atmospheric pressure: , , , and [17]. The crystalline phase significantly affects the structural, textural, and catalytic properties of photocatalyst [10].
(a)
(b)
The prepared TDB catalysts show light yellow colors. Dipping the TDB catalysts in AgNO3 aqueous could darken the particles, revealing the presence of silver compounds [18, 19]. In order to explore the influence of Ag ion doping on the optic absorption of photocatalysts, a UV-Vis spectrophotometer was used to measure their absorption spectra with the wavelength from 240 to 800 nm. As shown in Figure 1(b), all the catalysts have good properties of visible light absorption. The absorption peak of TDB even extends to >500 nm. Comparably, the absorption peaks of all the Ag@TDB catalysts have blue shift phenomenon. In particular, the photocatalyst 1.0 Ag@TDB exhibits an absorption peak at 450 nm. The degree of blue shifting of catalysts decreased with the increase of the Ag ion content in the catalysts. The blue shift in the UV absorption spectra has been also reported in ceria-containing nanomaterials in recent years, and the quantum size effect has been proposed to explain this phenomenon for the pure ceria nanocrystals [20, 21]. Considering the blue shift in the UV spectra for the Ag@TDB system, the energy level lied beneath the valence band formed by the hybrid track of Bi 6s and O 2p for the bare TDB. The impurity levels of Ag@TDB were located between valence band and conduction band of TDB due to Ag ion doping, which potentially led to the blue shift. This phenomenon was not convenient for visible light absorption but for photocatalytic reactions under UV light illumination.
The morphology of the photocatalysts was characterized by SEM. Figure 2 reveals that particle diameters ranging from 80 to 200 nm varied in size and shape. These particles were easy to cluster into aggregates ranging from 5 to 200 m, and their shapes were mostly spherical and ellipsoidal in the low-resolution image. Compared with the TDB, the shape of the Ag@TDB changed from duct-like to scale-like and then into slice with the increasing Ag ion content from 0.05 to 1.0 mol/L. The morphology of the photocatalyst is closely related to their activities [2]. The Brunauer Emmett Teller (BET) surface areas of the photocatalysts were 20~60 m2/g. There was no significant change in the surface area after metallisation of TDB particles. The average pore size estimated by the Barrett Joyner Halenda (BJH) method was 1.7 nm, indicating that these catalysts have mesoporous structure and good photocatalytic performances.
3.2. Evaluation of the Photocatalytic Activity
3.2.1. Effect of Catalyst Concentration
To remove pollutants economically and efficiently, it is important to find the optimal amount of catalyst. The effect of photocatalyst TDB concentration from 0.2 to 2.0 g/L on the photodegradation of CV solution of 5 mg/L was explored under UV irradiation under room temperature. As shown in Figure 3, the degradation rates increased from 0.335 to 0.47 with the increase of the catalyst dosage from 0.2 to 1.0 g/L. The degradation rate was almost constant when the dosage of the catalyst was beyond 1.0 g/L. At the low catalyst concentration, the increase of TDB particle concentrations led to the increase of the number of absorbed photons and the number of the adsorbed dye molecules, enhancing the photocatalytic efficiency. As the concentration of catalyst was higher than the optimal value, the degradation rates decreased due to the increasing turbidity of the suspension and light scattering. Hence, 1.0 g/L was used as the catalyst dosage for the following photocatalytic reactions.
3.2.2. Effect of Initial Crystal Violet Concentration
The effect of initial CV concentration on the degradation efficiency was investigated by changing the initial concentration from 5.0 to 20.0 mg/L at constant catalyst loading (1.0 Ag@TDB, 1.0 g/L). The results are shown in Figure 4. The degradation rate of CV at low concentration was high. The degradation ratio decreased from 92.1% to 45.0% with the increase of initial CV concentration from 5 mg/L to 20 mg/L, suggesting that the increase of the dye concentration decreased the removal ratio. Similar results have been reported for the photocatalytic degradation of other dyes in aqueous solution (e.g., quinolone yellow, acid red, and reactive dyes 4) [22–24]. An explanation to this phenomenon was that the increase of dye concentration led to the increase of the amount of dye adsorbed on the catalyst surface, reducing the active sites for adsorption of hydroxyl ions and the generation of hydroxyl radicals. The increase of dye concentration could also intercept the path of photons reaching the catalyst surface and decrease the adsorption of photons on catalyst particles. Additionally, at high concentration, the dye molecules would compete for UV light absorption with the catalyst, which may also reduce the degradation efficiency [25].
3.2.3. Effect of Catalysts Type
The photocatalytic activities of TDB and Ag@TDB were evaluated and compared by degradation of CV at 5 mg/L in aqueous solution under UV light irradiation. As shown in Figure 5, the removal rate of CV is below 5% in the dark, indicating that the adsorption of catalysts could be negligible. The removal ratio of CV was 33.5% by using TDB photocatalyst in 80 min. Under the same reaction conditions, the removal rations of CV were 36.1%, 62.5%, 86.64%, and 91.5% by using 0.05 Ag@TDB, 0.3 Ag@TDB, 0.75 Ag@TDB, and 1.0 Ag@TDB photocatalysts, respectively. The results indicated that all the Ag modified particles exhibited higher activities than the bare TDB particles. The degradation rate of CV increased with the increase of the Ag ion doping content. The main reason might be that Ag ions on Ag@TDB surface acted as sites where electrons accumulated. Better separation of electrons and holes on Ag@TDB catalyst surface allowed more efficient channeling of the charge carriers into useful reduction and oxidation reactions rather than recombination reactions [26, 27]. The degradation rate with 0.75 Ag@TDB was 1.4 times that with 0.05 Ag@TDB. The degradation rate with 1.0 Ag@TDB was only 0.05 times that with 0.75 Ag@TDB. This change can be due to the fact that Ag particles can act as recombination centers as the Ag content is beyond its optimal value [27, 28]. Furthermore, the active sites of catalyst were fully occupied by Ag ions. The rest of Ag ions played slight role on CV degradation, leading to slight rise of CV degradation rate with higher Ag ion doping. Additionally, it was observed that the color of catalyst became darker during the irradiation. The main reason was that the reduction potential of silver ion is prone to photocatalytic reduction Ag+ + e− → Ag0, producing metallic silver on catalyst surface.
4. Conclusions
The doping of silver alters the structure of TDB catalyst and leads to the blue shift. The degradation rations of CV were obtained by using TDB and Ag@TDB photocatalysts in aqueous solution under UV light irradiation. The results indicated that the degradation efficiency of CV was significantly impacted by its initial concentrations, photocatalyst amounts, and photocatalyst types. The optimal amount of photocatalyst was 1.0 g/L at the CV concentration of 5 mg/L. For photocatalyst type, the photocatalytic activities of Ag@TDB were significantly higher those that of bare TDB. There are two main reasons: (1) Ag ion doped improved the photocatalyst activities due to trapping photoelectrons of Ag ion and (2) the photocatalyst has mesoporous structure. CV could be effectively degraded by Ag@TDB under UV light irradiation, indicating that Ag@TDB is expected to exhibit high practicability and good performance for the degradation of dyes.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This study was financially supported by the National Natural Science Foundation of China (no. 21077010), the Fok Ying-Tong Education Foundation of China (Grant no. 121077), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (no. 20110003110023).