Taking cetyltrimethylammonium bromide (CTAB) as the template and using TiO2 as the substrate, coral-globular-like composite Ag/TiO2-SnO2 (CTAB) was successfully synthesized by the sol–gel combined with a temperature-programmed treatment method. X-ray diffraction, scanning electron microscopy (SEM), UV–vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, SEM combined with X-ray energy dispersive spectroscopy, and N2 adsorption–desorption tests were employed to characterize samples' crystalline phase, chemical composition, morphology and surface physicochemical properties. Results showed that composites not only had TiO2 anatase structure, but also had some generated SnTiO4, and the silver species was metallic Ag0. Ag/TiO2-SnO2 (CTAB) possessed a coral-globular-like structure with nanosheets in large quantities. The photocatalytic activity of Ag/TiO2-SnO2 (CTAB) had studied by degrading organic dyes under multi-modes, mainly using rhodamine B as the model molecule. Results showed that the coral-globular-like Ag/TiO2-SnO2 (CTAB) was higher photocatalytic activity than that of commercial TiO2, Ag/TiO2-SnO2, TiO2-SnO2 (CTAB), and TiO2-SnO2 under ultraviolet light irradiation. Moreover, Ag/TiO2-SnO2 (CTAB) composite can significantly affect the photocatalytic degradation under multi-modes including UV light, visible light, simulated solar light and microwave-assisted irradiation. Meanwhile, the photocatalytic activity of Ag/TiO2-SnO2 (CTAB) was maintained even after three cycles, indicating that the catalyst had good usability.
In recent years, with environmental pollution increasing, photocatalysis as a kind of advanced technology has shown huge potential for pollutant degradation, energy conversion, selective oxidation, and organic synthesis. In all kinds of methods of water pollution treatment, the semiconductor photocatalysis technology has been widely studied due to its environment-friendliness, and lower cost (Liao et al. 2008; Chen et al. 2013). The photocatalytic technology usually refers to the photocatalytic reaction occurring light irradiation if the light energy is the same as or higher than the band gap of the semiconductor. That is, the semiconductor absorbs the light energy and is excited, resulting in photogenerated electrons gathering on the conduction band and photogenerated holes on the valence band. Photogenerated electrons and photogenerated holes react with molecules (oxygen adsorption, surface hydroxyl group or water) absorbed on the surface of the semiconductor, respectively, generating strong oxidizing substances such as the superoxide radical (·O2−) and the hydroxyl free radical (·OH) that can react with organic pollution molecules, so that organic molecules are degraded and mineralized by active groups (Fan et al. 2009; Yuan et al. 2011; Dimitrijevic et al. 2013). However, according to relatively recent research, band gaps of some pure semiconductor materials are wide and the semiconductors have low efficiencies for light (Liang et al. 2012; Li et al. 2014a), of which the practical application is limited to some extent. Researchers have carried out a lot of work, including increasing the light response range, promoting the transfer of the interface charge, and decreasing the recombination of photogenerated electrons and photogenerated holes to improve the photocatalytic activity of TiO2, SnO2, and other semiconductor materials. Examples include the following.
(a) Noble metal doped. Fermi levels of Ag, Au, Pt, Pd, etc. are lower, with noble metal depositing on the surface of the semiconductor, and the Schottky barrier can be formed to capture photogenerated electrons, thus effectively reducing the recombination of electrons and holes (Chen et al. 2014; Li et al. 2014b).
(b) Semiconductor composited. Semiconductors (WO3, SnO2, ZnO, etc.) are compounded with different band width gap to improve the utilization rate of the solar spectrum (Li et al. 2014c).
(c) The template used. The template is conducive to the formation of a special surface, thus changing the specific surface area and the pore size of the semiconductor material, which further improves the photocatalytic activity owing to charge carriers being effectively transferred (Merrill et al. 2013; Jiang et al. 2015).
On the basis of research, we take cetyltrimethyl ammonium bromide (CTAB) as the template and use TiO2 as the substrate to synthesize Ag/TiO2-SnO2 (CTAB) by the sol–gel combined with temperature-programmed treatment method in this paper. The main ideas of the synthesized design are as follows. (1) TiO2 (3.2 eV) and SnO2 (3.8 eV) are relatively common semiconductor photocatalytic materials, whose band gaps are different; through the coupling of TiO2 and SnO2, using the synergistic effect to increase the light response range, the photocatalytic activity is improved (Beltran et al. 2008; Katoch et al. 2015). (2) During the synthesis process, noble metal Ag is doped to take advantage of the electron capture ability of Ag to decrease the recombination of photogenerated electrons and photogenerated holes. At the same time, surface plasmon resonance (SPR)) effect of Ag makes the composite absorption in the visible region, thus enhancing the photocatalytic activity (Li et al. 2008; Shan et al. 2013). (3) In the synthesis reaction, the template CTAB is added. Utilizing the electrostatic and steric effect of CTAB to influence the growth direction of the composite in the process of synthesis, a better morphology of the composite is obtained (Pan et al. 2011; Kumar et al. 2014).
Under the above guidance ideas, we expect that Ag/SnO2-TiO2 (CTAB) composite can form a special structure and has an excellent morphology under the action of the template CTAB and Ag. In addition, we select rhodamine B (RhB) as the model molecule to study the photocatalytic activity of the as-synthesized Ag/SnO2-TiO2 (CTAB) under multi-mode photocatalysis.
Tetra isopropyl titanate (TTIP, 98%) was purchased from Shenzhen Chunhe Merrill Chemical Technology Limited Company. Methanol (CH3OH, 99.5%) and isopropanol (CH3CHOHCH3, 99.7%) were purchased from Tianjin Tianli Chemical Reagent Factory. Tert-butanol was purchased from Meryer (Shanghai) Chemical Technology Co., Ltd. Ethylenediamine tetraacetic acid disodium salt, silver nitrate (AgNO3) and CTAB were purchased from Tianjin Kaitong Chemical Reagent Limited Company. Crystalline stannic chloride (SnCl4·5H2O, 99%) was purchased from Tianjin Guangfu Institute of Fine Chemicals. RhB, methylene blue (MB), methyl orange (MO), salicylic acid (SA) and neutral red (NR) were purchased from Beijing chemical plant. All reagents are analysis pure. Deionized and doubly distilled water was used in all the experiments.
Synthesis of Ag/SnO2-TiO2 (CTAB)
In the typical process, according to the molar ratio of Ag:Ti: CTAB = 0.1:1:0.05, AgNO3, TTIP, and CTAB were added to 10 mL isopropanol solution under stirring at room temperature to form a water gel. Then the water gel was transferred into a Teflon-lined autoclave and heated at 160 °C (2 °C/min) for 24 h and cooled down to the room temperature naturally. SnCl4 solution (0.75 mmol) was added to the above Teflon-lined autoclave under stirring, ultrasonicated for 30 min, kept at 160 °C (2 °C/min) for 24 h then cooled down to room temperature. The as-prepared product (molar ratio of Ti:Sn = 13.33:1) was washed three times with the deionized water and ethanol, dried at 80 °C for 10 h, and finally calcined at 600 °C for 7 h. Ag/TiO2-SnO2, TiO2-SnO2 (CTAB), and TiO2-SnO2 were synthesized by the same method without adding CTAB or silver nitrate solution in the process.
X-ray diffraction (XRD) pattern was used to analyze the crystalline structure by the XRD analysis (German Bruker-AXS (D8) with Cu Kα as X-ray radiation under 60 kV and 80 mA and with the 2θ ranging from 20° to 80°). Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was undertaken using an S-4300 SEM, Hitachi, working voltage 5 kV. The X-ray photoelectron spectroscopy (XPS) spectra of the samples were measured with a VG-ADES400X, using a MgK-ADES source; the residual gas pressure was less than 10−8 Pa. UV–vis diffuse reflectance spectra (UV–vis/DRS) were recorded with a UV–vis spectrophotometer (TU-1901, Beijing General Analytical General Company, China) using BaSO4 as reflectance standard. The absorbance of sample solution was determined by the TU-1901 UV–vis double-beam spectrophotometer. The surface area and pore size of sample were obtained using Quan-chrome NoveWin2 USA Contador physisorption apparatus, with a measuring temperature of 77 K.
Multiple mode photocatalytic degradation tests
The photocatalytic activities of the as-synthesized composite material Ag/TiO2-SnO2 (CTAB) were evaluated by photocatalytic degradation of RhB under multi-modes including UV, visible light, microwave-assisted and simulated solar light. The experimental device is composed of a cylindrical quartz outer tube and a quartz glass sleeve in which a built-in visible light source is a 400 W Xe lamp (the main emission line is greater than 410 nm, the inner sleeve is made of No. 11 glass to filter out the ultraviolet light emitted). The ultraviolet light source is a 125 W high-pressure mercury lamp (the wavelength of the main emission line is about 313.2 nm). The microwave-assisted photocatalytic device is made of a quartz tube, filled with metal mercury and inert gas Ar; the emission wavelength is about 280 nm and the power is 15 W. Simulated sunlight photocatalytic reaction is formed by a BL-GHX-V photocatalytic reactor: the light source is a 1,000 W external Xe lamp (external type, Shanghai Bison Instruments Co. Ltd, the emission spectrum was close to the full spectrum), distance 8.5 cm between the lamp and the reaction liquid, constant temperature by ethanol recycling equipment for keeping the temperature in the reaction system. RhB was used as the model dye molecule, but MB, MO, SA and NR were also involved in those photocatalytic experiments under UV light irradiation.
UV light mode: 0.15 g photocatalysts was suspended in 90 mL of RhB solution (50 mL/L) under ultrasonication for 10 min, then stirred for 30 min in the dark to ensure the adsorption/desorption equilibrium between RhB and photocatalyst powders. The high-pressure mercury lamp was placed into a jacketed quartz tube that was soaked in the solution, magnetically stirred continuously, and suspensions were kept at constant temperature by circulating water through the jacket during the entire process. Three millilitres of suspension was sampled and centrifuged to remove the photocatalyst particles. Then, the absorption spectrum of the centrifuged solution was recorded using a TU-1901 UV–vis spectrophotometer. The change in RhB concentration was determined by monitoring the optical intensity of the absorption spectra at 499 nm.
Visible light mode: 0.30 g photocatalyst was suspended in 220 mL of RhB solution (50 mL/L). Other steps of the degradation were the same as the UV mode, except the lamp source was a 400 W Xe lamp.
Microwave-assisted mode: 500 mL of RhB (50 mg/L) solution and 0.50 g photocatalysts were placed in a microwave reactor and stirred for 30 min in the dark to ensure the adsorption/desorption equilibrium between RhB and photocatalyst powders. The microwave reactor employed for the degradation of RhB was purchased from Yuhua Instrument Limited Company of China. It consisted of a cylindrical glass reactor (capacity 600 mL) with a 600 mm long water reflux condenser, connected through a communication pipe. The microwave discharge electrode lamp (MDEL) was placed into the reaction solution, with about two-thirds of the MDEL being in the reaction solution. Three silicone tubes were connected to the equipment, which could let water in and out, and let air out through the hole in the microwave reactor. Air was bubbled into the solution through a sintered glass filter, fixed at the bottom of the reactor for passing oxygen, as well as for mixing the catalyst and the solution. At given time intervals, 3 mL of suspension was sampled and centrifuged to remove the photocatalyst particles. Then, the absorption spectrum of the centrifuged solution was recorded using a TU-1901 UV–vis spectrophotometer.
Simulated solar light mode. D: during the process of the simulated solar light irradiation, 90 mL of RhB (50 mg/L) solution and 0.15 g photocatalysts were placed in a quartz photoreactor and stirred for 30 min in the dark. The subsequent steps of the experiment were the same as for the UV light mode above, except the lamp source was a 1,000 W external Xe lamp.
To determine the photocatalytic oxidation pathways of the as-synthesized composite Ag/TiO2-SnO2 (CTAB), tert-butanol (1 mM), ethylenediamine tetraacetic acid disodium salt (1 mM, EDTA), and methanol (1 mM) were selected as scavenger agents for oxygen radical anions, holes and hydroxyl radicals, respectively, for the photocatalytic degradation of RhB over Ag/TiO2-SnO2 (CTAB) under UV light irradiation. Other steps of the degradation were the same as the UV mode.
RESULTS AND DISCUSSION
At the same time, in order to further investigate the effect of the CTAB template action and Ag deposition on diffraction peaks of as-synthesized composites, four kinds of as-synthesized composites, Ag/TiO2-SnO2 (CTAB), TiO2-SnO2 (CTAB), Ag/TiO2-SnO2, and TiO2-SnO2, were analyzed by XRD analysis, shown in Figure 1(b). According to the Scherrer formula, crystallite sizes of as-composites were calculated and are shown in Table 1. From Table 1, the crystallite size of Ag/TiO2-SnO2 (CTAB) is significantly increased; in addition, the crystal structure of composites is not obviously changed through the CTAB action.
|Sample .||Ag/TiO2-SnO2 (CTAB) .||Ag/TiO2-SnO2 .||TiO2-SnO2 (CTAB) .||TiO2-SnO2 .|
|Sample .||Ag/TiO2-SnO2 (CTAB) .||Ag/TiO2-SnO2 .||TiO2-SnO2 (CTAB) .||TiO2-SnO2 .|
D* is the average crystallite size of sample calculated using the Scherrer equation.
According to the Kubelka–Munk energy curves made by above results, band gap energy (Eg) values of Ag/TiO2-SnO2 (CTAB), Ag/TiO2-SnO2, TiO2-SnO2 (CTAB), and TiO2-SnO2 were calculated and are shown in Table 1, respectively. Seen from Table 1, band gaps of Ag/TiO2-SnO2 (CTAB) and Ag/TiO2-SnO2 composite materials are lower than that of TiO2-SnO2 (CTAB) and TiO2-SnO2, indicating that the former two composites will show good photocatalytic capability.
N2 adsorption–desorption analysis
The Brunauer–Emmett–Teller (BET) surface area (SBET), average pore size and pore volume of the four kinds of composite materials are shown in Table 1. Results show that the specific surface area of the sample Ag/TiO2-SnO2 (CTAB) is better than that of Ag/TiO2-SnO2, but the BET value of the sample with doping of Ag decreases, attributed to its morphology changes. According to SEM images, the composite material formed by the Ag deposition is composed of the spherical coral and irregular quartet structure, respectively, and these particles are bigger, which can make the specific surface area of composites be smaller. In addition, pore size distribution curves of these composite materials show that the pore structure and the distribution of composite materials are more uniform. The pore size distribution curve of Ag/TiO2-SnO2 (CTAB) forms two relatively concentrated areas at 2 nm and 11.3 nm, mainly due to the structure of the coral globule.
The photocatalytic activity of Ag/TiO2-SnO2 (CTAB) is better than those of Ag/TiO2-SnO2 (without using the template CTAB), TiO2-SnO2 (CTAB), and TiO2-SnO2 under the same condition and the reasons may be as follows. (1) Noble metal Ag can effectively capture photogenerated electrons and promote the transfer of photogenerated electrons; photogenerated electrons reacting with the oxygen adsorbed on the surface of composites to form superoxide radical. (2) Ag/TiO2-SnO2 (CTAB) has certain visible light absorption for the SPR effect of Ag, which can improve the utilization rate of light. The addition of silver can reduce the band gap energy of composites and increase the range of light response in the visible region. According to the photocatalytic degradation activity of the four kinds of composites, it can be found that the photocatalytic activity of composites increases with the addition of silver. (3) SnO2 is reacted with TiO2, generating SnTiO4 in the composite process, so that the photocatalytic activity of the composite is enhanced due to the existence of SnTiO4. (4) The influence of Ag and CTAB on the morphology and structure of as-synthesized samples and the special morphology can increase scattering of light and effective transfer of carriers. Ag can affect the structure of the product, and CTAB can affect the growth process of the composite through the electrostatic and steric effects such that a double layer forms on the surface of particles. In addition, the product presents a coral-globule structure with the action of CTAB and Ag, and this special structure plays a key role in the photocatalytic degradation activity of Ag/TiO2-SnO2 (CTAB) composite material.
In summary, the photocatalytic degradation activity of composites can be attributed to the different specific surface area, the photogenerated electrons captured by Ag, and the effective transfer of the photogenerated electron–hole pairs caused by the special structure of the composite.
Capture experiment and possible photocatalytic mechanism
Taking CTAB as the template, using TiO2 as the substrate, coral-globular-like composite Ag/TiO2-SnO2 (CTAB) was successfully synthesized by the sol–gel combined with temperature-programmed treatment method. The composite material has a good crystalline structure, and the morphology is uniform coral-globular. Ag/TiO2-SnO2 (CTAB) shows the highest photocatalytic activity under multi-mode photocatalytic experiments, due to the SPR effect of noble metal Ag, the template action of CTAB, and the specific surface area. Meanwhile, the special morphology provides a convenient way of charge carrier transfer, which can effectively promote the separation of electrons and holes. The SPR effect of Ag can form the Schottky barrier on the metal semiconductor surface to achieve the purpose of improving photocatalytic activity. Electrostatic and steric effects of CTAB can affect the crystal growth of Ag/TiO2-SnO2 (CTAB) to change the morphology and the specific surface area.
This study was supported by the National Natural Science Foundation of China (21376126), Natural Science Foundation of Heilongjiang Province, China (B201106), Scientific Research of Heilongjiang Province Education Department (12511592), Government of Heilongjiang Province Postdoctoral Grants, China (LBH-Z11108), Open Project of Green Chemical Technology Key Laboratory of Heilongjiang Province College, China (2013), Postdoctoral Researchers in Heilongjiang Province of China Research Initiation Grant Project (LBH-Q13172), Innovation Project of Qiqihar University Graduate Education (YJSCX2015-ZD03), College Students' Innovative Entrepreneurial Training Program Funded Projects of Qiqihar University (201610221112), and Qiqihar University in 2016 College Students Academic Innovation Team Funded Projects.