The formation of heterojunction structures of semiconductors is one of the most important techniques to increase the photocatalytic efficiency of a photocatalyst. In this paper, Ag/Ag3VO4/TiO2 as a visible light response photocatalyst was prepared easily by a three step process including hydrothermal, precipitation and photoreduction. The Ag/Ag3VO4/TiO2 nanocomposites demonstrated clearly increased visible light absorption and photocatalytic efficiency in degradation of Rhodamine B. The degradation yield of Rhodamine B was detected 97.3% in 45 min under visible light. Compared with Ag3VO4, TiO2 and Ag3VO4/TiO2, Ag/Ag3VO4/TiO2 exhibited the highest efficiency owing to synergetic effect between Ag3VO4 and TiO2 and surface plasmon resonance effect of Ag nanoparticles. So, the Ag/Ag3VO4/TiO2 can be effectively used as an active photocatalyst under visible light and it depicts an ideal potential in elimination organic pollutants.
Ag/Ag3VO4/TiO2 as a visible light response photocatalyst was prepared.
Ag/Ag3VO4/TiO2 plasmonic photocatalyst showed superior photocatalytic activity to degrade RhB.
The heterostructure of Ag3VO4 and TiO2 and the SPR effect of Ag contribute to improved performance.
Considering that heavy metals and organic pollutants are frequently found in industrial wastewater, they can be difficult to eliminate and have significant impacts on environmental protection (Li et al. 2017; Zhao et al. 2019). Until now, different techniques such as reverse biodegradation, chemical oxidation, adsorption, osmosis, coagulation, and flocculation have been utilized to remove these pollutants (Mei et al. 2019). However, the results of these methods cause a secondary pollution, and in this respect, their application is not particularly preferred (Mei et al. 2019). Semiconductor photocatalysis has attracted much attention as a good technique for its unique potential in solving these problems in abundant visible/solar light (Lv et al. 2016). Among the most studied photocatalysts, TiO2 and ZnO are the most cost-effective, nontoxic nature, and high stable materials for utilization in these processes (Jing et al. 2016; Lv et al. 2016; Amini et al. 2019). But, the photocatalytic efficiency of TiO2 is restricted by large band gap energy and rapid recombination of photogenerated carriers (Xu et al. 2016). Thus, various visible-light photocatalysts have been developed to resolve these limitations by methods such as the coupling of TiO2 with narrow-band semiconductors (Xu et al. 2016; Li et al. 2019).
Recently, Ag3VO4 as a silver-based semiconductor has been commonly used in the photocatalytic processes owing to narrow band gap energy (2–2.2 eV) and great efficiency (Sun et al. 2019). The hybridization of Ag 3d and O 2p orbitals greatly disturbs the valence band (VB) of Ag3VO4, so enhancing the mobility of photogenerated holes and the oxidation of organic contaminants (Shekofteh-Gohari & Habibi-Yangjeh 2017; Ramasamy Raja et al. 2018; Li et al. 2019). To reduce the rate of recombination of charge carriers due to its narrow band gap energy, coupling Ag3VO4 with another semiconductor may be a convenient strategy to achieve such enhanced efficiency (Ramasamy Raja et al. 2018). Furthermore, noble metal nanoparticles (NPs) can promote photocatalytic efficiency because of the interaction between plasmon effect and electromagnetic radiation (Sun et al. 2019). Consequently, surface plasmon resonance (SPR) effect of Ag NPs can increase the photocatalytic performance of semiconductors under visible light (Ramasamy Raja et al. 2018; Sun et al. 2019). For instance, a novel Ag/AgBr/BiOBr photocatalyst for degradation of pollutants was prepared by Cheng et al. (2011). Sun et al. prepared Ag/Ag3VO4/g-C3N4 Z-scheme photocatalysts that exhibited high photocatalytic degradation performance on tetracycline (Sun et al. 2019). CoFe2O4/Ag/Ag3VO4 composites by Jing et al. demonstrate excellent photocatalytic and antibacterial activity (Jing et al. 2016). In summary, for effective degradation of organic pollutants, it is clear that Ag/Ag3VO4-based photocatalysts not only accelerate the charge transfer rate, but also improve the visible light absorption range.
The aim of this study is to increase the photocatalytic activity of TiO2 by increasing visible light absorption by Ag and Ag3VO4 doping and facilitating the separation of charge carriers. There are studies in the literature using Ag/Ag3VO4-based photocatalysts, but no study has been reported on photocatalysts prepared by combining TiO2 with Ag/Ag3VO4 and its photocatalysis mechanism. Inspired by the aforementioned studies, Ag/Ag3VO4/TiO2 nanocomposites were synthesized by microwave method and subsequent precipitation and photoreduction method. The efficiency of photocatalysts was examined by degrading Rhodamine B (RhB). The synergistic effect caused by the combination of Ag3VO4 and TiO2 and the strong SPR effect of Ag nanoparticles prolonged the life of the charge carriers and delayed recombination. This results in increased photocatalytic activity. Besides, the results of trapping experiments indicated the hole (h+) and superoxide radicals () are the dominant active species in the process of photocatalysis of RhB. As a result, the possible mechanism for the great photocatalytic efficiency of Ag/Ag3VO4/TiO2 nanocomposite and the stabilization effect of Ag/Ag3VO4/TiO2 was further investigated. This work sheds light on the development of visible active Ag/Ag3VO4 photocatalysts for effective photocatalytic removal of organic pollutants in a short time. Furthermore, our current work may provide a new idea for designing novel TiO2 photocatalysts with effective visible light absorption and surface carriers separation.
MATERIALS AND METHODS
Reagents and apparatus
All the used chemical reagents with analytical grade, such as silver nitrate (AgNO3), ammonium metavanadate (NH4VO3), titanium (IV) isopropoxide (Ti[OCH(CH3)2]4), sodium hydroxide (NaOH) were offered by Merck (Germany). All aqueous solutions were prepared with deionized water (with resistance >18 MΩ/cm, Millipore Milli-Q Plus).
X-ray diffraction (XRD) analyses of all photocatalysts were performed with PANalytical Empyrean diffractometer using Cu Kαirradiation (l = 0.1542 nm) at 60 kV. The surface morphology of the products was identified by using a ZEISS SIGMA 300 scanning electron microscope (SEM). Optical properties were analyzed by UV-vis diffuse reflectance spectroscopy (DRS) using a Shimadzu UV-2600 UV-Vis spectrophotometer. The UV-Vis absorption spectra of the RhB dye solution and photocatalysts were investigated by a Shimadzu UV-2600 UV-Vis spectrophotometer. Fluorescence measurements were performed by Hitachi S-7000 fluorescence spectrophotometer.
Synthesis of photocatalysts
TiO2 nanoparticles were synthesized by microwave method according to the procedure of Ünlü et al. (2018). Ag3VO4 nanoparticles were prepared by co-precipitation method. In this method, 3 mmol AgNO3 was dissolved with 30 mL of deionized water. Then, 30 mL of 1 mmol NH4VO3 was added dropwise the AgNO3 solution and the pH value of the whole system to 9 was adjusted 2 M NaOH. After stirring at 800 rpm for 2 h, the obtained samples were centrifuged, washed, and then dried at 70 °C for 12 h to obtain Ag3VO4. For the preparation of Ag3VO4/TiO2, the Ag3VO4 synthesis procedure was utilized by using 0.05 g TiO2. The content of TiO2 in the Ag3VO4/TiO2 nanocomposite is 22.2% by weight. In the literature, there is a study in which a similar ratio study was conducted and it was observed that this ratio of composite was active (Wang et al. 2013). In the preparation of Ag/Ag3VO4/TiO2, some of the Ag+ ions were reduced on Ag3VO4/TiO2 using a photo-reduction method. The Ag3VO4/TiO2 nanocomposites were photoinduced by 128 W UV light (λ ≥ 365 nm) for 5 min. The synthesis of the Ag/Ag3VO4/TiO2 photocatalyst is represented in Figure 1.
Degradation of Rhodamine-B using the prepared photocatalysts
RESULTS AND DISCUSSION
Characterizations of photocatalysts
XRD patterns supplying knowledge the crystal structure and phase of the photocatalysts are displayed in Figure 2(a). As can be seen in Figure 2(a), the XRD peaks of TiO2 correspond to anatase crystalline phase TiO2 (JCPDS 21-1272) (Wang et al. 2013). The visible peaks in XRD spectra of Ag3VO4 match with a monoclinic structure; this shows that the Ag3VO4 has no second phase (JCPDS No. 43-0542) (Li et al. 2019; Mei et al. 2019). For the Ag3VO4/TiO2 nanocomposites, the main peaks of monoclinic Ag3VO4 and anatase TiO2 are clearly observed, confirming that nanocomposite is formed. The XRD pattern of Ag/Ag3VO4/TiO2 nanocomposites is similar to that of Ag3VO4/TiO2. The diffraction peak stated with ‘#’ corresponds the (111) crystalline plane of metallic Ag (Güy & Özacar 2019). In addition, XPS analysis of Ag 3d was performed to verify the oxidation state of Ag (Figure 2(b)). As can be seen in Figure 2(b), the peaks at 367.5 eV, 368.7 eV, 373.7 eV and 375.04 eV correspond to the binding energies of Ag 3d5/2 and Ag 3d3/2. The peaks at 367.5 eV and 373.7 eV are ascribed to Ag+ in Ag3VO4 (Zou et al. 2016; Zhang & Ma 2017).The peaks at 368.7 eV and 375.04 eV correspond to the Ag0 that confirms the existence of metallic Ag (Wen et al. 2018; Li et al. 2019). According to the results of XRD, the nanocomposites have been successfully synthesized in three steps.
The morphologies of the products was investigated by SEM and the images are displayed in Figure 3. Figure 3(a) exhibits that Ag3VO4 is composed of large irregular structures. TiO2 has the agglomerated small nanoparticles (Figure 3(b)). For Ag3VO4/TiO2 nanocomposite, it is observed that TiO2 consists of small agglomerated nanoparticles and Ag3VO4 structures consist of large irregular nanoparticles (Figure 3(c)). As can be seen in Figure 3(d), silver nanoparticles were doped on binary nanocomposites. Figure 3(e) exhibits EDS analysis for the Ag/Ag3VO4/TiO2 to verify purity and its four- element constitution. Furthermore, Table 1 depicts the atomic percentages for all the photocatalysts. The atomic percentages for the Ag3VO4 represents Ag, V and O elements while it does Ti and O elements for TiO2. Ag3VO4/TiO2 are formed of Ag, V, O, Ti elements which indicate presence of Ag3VO4 and TiO2. When atomic percentages are compared, it is seen that Ag ratio in Ag/Ag3VO4/TiO2 composite is higher than Ag3VO4/TiO2. This confirms that Ag was added to Ag3VO4/TiO2.
|Elements/(at.%) (EDS) .||Ti .||Ag .||V .||O .|
|Elements/(at.%) (EDS) .||Ti .||Ag .||V .||O .|
In order to analyze light absorption features of the photocatalysts the diffuse reflectance measurements were performed and the data were exhibited in Figure 4(a). As demonstrated in Figure 4(a), Ag3VO4, Ag3VO4/TiO2 and Ag/Ag3VO4/TiO2 indicate a reflectance in the visible region. As the Ag3VO4 combined with TiO2, the diffuse reflectance of TiO2 shifts to the right. The diffuse reflectance is described the Kulbelka-Munk plots by the equation F(R) = (1R)2/2R, where R is the diffuse reflectance of the product (Dong et al. 2016; Jing et al. 2017). Kulbelka-Munk plots provide the determination of the band gap of the products. In Figure 4(b), it is seen that only the band gap energy of TiO2 due to its large band gap, corresponds to the UV region. Unlike TiO2, the band gap energy of Ag3VO4/TiO2 and Ag/Ag3VO4/TiO2 shifts to the visible region, owing to the narrow band gap of Ag3VO4. Compared to the band gap energy of Ag3VO4, TiO2 containing nanocomposites have a higher band gap than Ag3VO4. Band gap energies of TiO2, Ag3VO4, Ag3VO4/TiO2 and Ag/Ag3VO4/TiO2 determined by Kubelka-Munk function are 3.20, 2.05, 2.33, and 2.23, respectively (Dong et al. 2016; Jing et al. 2017).
Evaluations of photocatalytic activity
The photodegradation efficiencies of photocatalysts were investigated by degradation of RhB. The RhB degradation performances of the photocatalysts are demonstrated in Figure 5(a). As displayed in Figure 5(a), Ag/Ag3VO4/TiO2 nanocomposite has the greatest performance according to TiO2, Ag3VO4, Ag3VO4/TiO2. The photodegradation efficiencies of TiO2, Ag3VO4, Ag3VO4/TiO2 and Ag/Ag3VO4/TiO2 are 17.1%, 94.5%, 95.0% and 97.3% within 45 min, respectively. From Figure 5(a), it can be observed that the photocatalysis of RhB can be inconsequential in the absence of the photocatalyst, showing that the RhB is steady under visible light. Coupling with TiO2 and the SPR effect of Ag nanoparticles increase the photocatalytic performance of Ag3VO4. In addition, the deposition of Ag3VO4 on TiO2 surface with narrow band gap increases the efficiency of TiO2, which is not very active in the visible region. As can be seen in Figure 5(b), the degradation kinetics RhB via photocatalyst were determined by pseudo-first-order kinetics by linear transform ln(C0/Ct) = kt (Wang et al. 2013; Jing et al. 2016; Atacan et al. 2019; Karimi et al. 2019). The apparent rate constants (k) of TiO2, Ag3VO4, Ag3VO4/TiO2 and Ag/Ag3VO4/TiO2 are 0.0024, 0.0506, 0.0540 and 0.0618 min−1, respectively. Among the photocatalyst, Ag/Ag3VO4/TiO2 has the highest the apparent rate constant. This situation verifies the large spectral response and the influential usage of visible light for Ag/Ag3VO4/TiO2 nanocomposite (Wang et al. 2013; Zhao et al. 2019).
In order to detect the separation capability of electron–hole pairs in the photocatalysts, the photoluminescence (PL) spectroscopy is utilized and the PL spectra of TiO2, Ag3VO4, Ag3VO4/TiO2 and Ag/Ag3VO4/TiO2 are demonstrated in Figure 6(a). Ag/Ag3VO4/TiO2 is clearly decreased, while Ag3VO4 exhibits the strong PL intensity which represents that the charge carriers recombine quickly (Zou et al. 2016). It states that TiO2 and Ag nanoparticles can restrain the recombination of photogenerated electron and holes and increase the photocatalytic efficiency of Ag/Ag3VO4/TiO2 (Wang et al. 2013; Zou et al. 2016; Sun et al. 2019). The PL measurements verify the photocatalytic degradation results.
The radical trapping experiments with various scavengers were carried to analyze the effect of reactive oxygen species (ROS) which are important in the photocatalytic degradation process (Güy & Özacar 2018; Wang et al. 2018; Zhou et al. 2018) and the results are shown in Figure 6(b). In this work, 1 mM benzoquinone BQ, as a scavenger of superoxide radicals (), 1 mM disodium ethylenediaminetetraacetate (EDTA-Na2) as a hole (h+) scavenger and 1 mM tert-butanol (t-BuOH) as a scavenger of hydroxyl radicals (•OH) were utilized. By the addition of t-BuOH into the photocatalytic system, the photodegradation activity of RhB changed very a little in 60 min compared to no scavenger. According to this result, •OH radicals are not the effective ROS in the presence of Ag/Ag3VO4/TiO2. After addition of BQ and EDTA-Na2, the photodegradation performance was significantly decreased, which confirmed that h+ and radicals were dominant ROS in the photocatalytic degradation of RhB.
In the photocatalysis process, the stability and reusability of the photocatalysts are very important. So the cycle experiments were carried out and the results demonstrated in Figure 7. Clearly, the photocatalytic efficiency of Ag/Ag3VO4/TiO2 nanocomposite is still 90% after five successive cycles. This is due to the SPR effect of Ag nanoparticles and the formation of heterostructure between Ag3VO4 and TiO2, which expedites the separation of charge carriers and hinders the recombination (Wang et al. 2013; Mei et al. 2019).
Considering the degradation results, the possible mechanism of the photodegradation of RhB for Ag/Ag3VO4/TiO2 nanocomposite has been proposed and displayed in Figure 8. The determined band gaps of TiO2 and Ag3VO4 are about 3.20 eV and 2.05 eV, respectively. While the valence band potentials of TiO2 and Ag3VO4 are 2.90 eV (Dong et al. 2016) and 1.62 eV (Jing et al. 2017), the conduction band (CB) potentials of TiO2 and Ag3VO4 are −0.30 eV and −0.43 eV, respectively. Under visible light, Ag3VO4 and Ag nanoparticles are simultaneously excited. Meanwhile, the holes (h+) are retained in the VB of Ag3VO4. When Ag nanoparticles are excited with visible light, hot electrons are generated due to its SPR effect. As shown in Figure 8, part of the hot electrons on the Ag nanoparticles migrate to the CB of Ag3VO4. The photogenerated electrons in the CB of Ag3VO4 and hot electrons from Ag nanoparticles transfer into the CB of TiO2. These electrons react with O2 to generate radicals due to CB potential of TiO2 is more negative than the required potential for formation (O2/ = −0.046 eV) (Jing et al. 2016). At the same time, the electrons of Ag nanoparticles are induced by SPR effect can be captured by O2 molecules and converted into radicals. The VB potential of Ag3VO4 nanocomposite (1.62 eV) visible light is not more positive than the redox potential of H2O/•OH (1.99 eV vs NHE) (Jing et al. 2017; Sun et al. 2019). This demonstrates that H2O cannot be oxidized by oxidation of holes in Ag3VO4 to form •OH radicals. So, the RhB can directly be decomposed by holes in the VB of Ag3VO4. Based on the results, it can be stated that both superoxide radicals and holes are dominant species. Trapping experiments also verify this situation.
In summary, Ag/Ag3VO4/TiO2 nanocomposite was successfully synthesized by a simple microwave assisted hydrothermal, precipitation–photoreduction method, which was utilized as a greatly efficient system for the degradation of RhB. Ag/Ag3VO4/TiO2 nanocomposite exhibited the highest efficiency in the removal of the RhB under visible light. This excellent performance was chiefly ascribed to the heterojunction structure, the good separation of charge carriers, more absorption of visible light and SPR of Ag nanoparticles. On the basis of the PL measurements and ROS trapping experiments the mechanism of enhanced photocatalytic performance for Ag/Ag3VO4/TiO2 was well proposed. Furthermore, the Ag/Ag3VO4/TiO2 can exhibit good stability. Consequently, our study supplies an effective strategy for solving wastewater treatment in the future.
M.O. thanks Turkish Academy of Sciences (TUBA) for partial support.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.