Ag2CrO4 nanocomposites: Efficient

Journal of Photochemistry and Photobiology A: Chemistry 341 (2017) 57–68

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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem

Invited feature article

Novel TiO2/Ag2CrO4 nanocomposites: Efficient visible-light-driven photocatalysts with n–n heterojunctions Solmaz Feizpoora , Aziz Habibi-Yangjeha,* , S. Vadivelb a b

Department of Chemistry, Faculty of Science, University of Mohaghegh Ardabili, P.O. Box 179, Ardabil, Iran Department of Chemistry, PSG College of Technology, Coimbatore, Tamil Nadu, India

A R T I C L E I N F O

Article history: Received 31 December 2016 Received in revised form 15 March 2017 Accepted 18 March 2017 Available online 21 March 2017 Keywords: TiO2/Ag2CrO4 n-n heterojunction Visible-light-driven Photocatalyst

A B S T R A C T

In this study, novel TiO2/Ag2CrO4 nanocomposites were fabricated by a facile refluxing method using commercial TiO2, silver nitrate, and sodium chromate at low temperature of 96 C. The as-prepared photocatalysts were characterized by XRD, EDX, SEM, TEM, UV–vis DRS, FT–IR, BET, and PL techniques. Compared with the TiO2 sample, TiO2/Ag2CrO4 nanocomposites exhibited highly enhanced photocatalytic activity in degradations of rhodamine B (RhB), methylene blue (MB), and fuchsine dyes under visible-light irradiation. Among the prepared photocatalysts, the TiO2/Ag2CrO4 (50%) nanocomposite showed the superior activity of 8.8, 5.4, and 44-folds relative to the TiO2 sample in degradations of RhB, MB, and fuchsine, respectively. The trapping experiments demonstrated that holes and superoxide anion radicals were the main reactive species in the degradation reaction. Based on the obtained results, the photocatalytic activity enhancement was attributed to strong visible-light absorption, effective separation of photogenerated charge carriers by internal electrostatic field produced through n–n heterojunction formed between counterparts of the nanocomposite, and enhanced surface area. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In recent decades, almost entire of the earth is facing drinking water scarcity, due to contamination of water and discharge of untreated wastewaters to the environment. Accessing to the fresh water has considerable effect on human’s health, because of major influence of the contaminated water to the living systems [1]. As a “green” technology, semiconductor-based photocatalysis, as one of the effective advanced oxidation technology, has attracted much more interests for degradation of environmental pollutants, photoreduction of carbon dioxide, generation of hydrogen gas, disinfection of microorganisms, and synthesis of many organic compounds [2–6]. In the past decades, titanium dioxide (TiO2) has been the most used photocatalyst in the heterogeneous photocatalytic processes, because of its considerable stability, nontoxicity, and abundant resources [7,8]. However, the worst drawback of this semiconductor is its poor activity under visible-light irradiation. This poor photocatalytic activity is related to the wide band gap of about 3.20 eV and high recombination rate of the photogenerated electron-hole pairs [7]. Hence, some strategies such as metal and nonmetal doping, preparation of

* Corresponding author. E-mail address: [email protected] (A. Habibi-Yangjeh). http://dx.doi.org/10.1016/j.jphotochem.2017.03.028 1010-6030/© 2017 Elsevier B.V. All rights reserved.

black titania, and heterojunction formation with narrow band gap semiconductors have been developed to overcome the above mentioned limitations of pure TiO2 [9–12]. It has been confirmed that formation of heterojunction with narrow band gap semiconductors could increase not only utilization of visible-light irradiation, but also separation of electron-hole pairs, leading to generation of more charge carriers and prolonging their life time [9]. Hence, this strategy is more effective and non-expensive method for enhancing photocatalytic activity of TiO2 under visiblelight illumination. Inspired by this strategy, different TiO2-based visible-light-driven photocatalysts such as TiO2/AgBr, TiO2/MoS2, TiO2/Ag2S, TiO2/Bi2WO6, TiO2/BiOI, TiO2/Ag3PO4, TiO2/BiOBr, TiO2/ AgI, TiO2/Ag3VO4, and TiO2/g-C3N4 have been prepared and their activities in different photocatalytic processes were explored [13– 22]. Silver chromate (Ag2CrO4) is a narrow band gap semiconductor with band gap of 1.80 eV [23,24]. Hence, in recent years, some Ag2CrO4-based photocatalysts have been fabricated [25–28]. It is noteworthy that TiO2 and Ag2CrO4 are n-type semiconductors. This means than in the TiO2/Ag2CrO4 nanocomposites, n-n heterojunction could be formed in the junction areas of these semiconductors [28–32]. As a result, it is anticipated that highly enhanced photocatalytic activity will observed for these nanocomposites compared with the pure TiO2. In these regards, we prepared TiO2/Ag2CrO4 nanocomposites, as novel visible-light-driven photocatalysts, using a facile refluxing

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S. Feizpoor et al. / Journal of Photochemistry and Photobiology A: Chemistry 341 (2017) 57–68

Scheme 1. The schematic diagram for preparation of the TiO2/Ag2CrO4 nanocomposites.

method at 96 C. The prepared samples were characterized by means of X-ray diffraction (XRD), energy dispersive analysis of X-rays (EDX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS), Fourier transform–infrared spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) surface analysis, and photoluminescence spectroscopy (PL) instruments. The nanocomposites exhibited enhanced photocatalytic activity in degradations of rhodamine B (RhB), methylene blue (MB), and fuchsine under visible-light illumination. Furthermore, the highly enhanced activity of the TiO2/Ag2CrO4 (50%) nanocomposite was explored and a plausible mechanism was proposed based on n–n heterojunctions. 2. Experimental 2.1. Materials Commercial TiO2 (P25) was obtained from Degussa and employed as received. Silver nitrate and benzoquinone were purchased from Loba Chemie Company. Other chemicals, such as RhB, MB, fuchsine, potassium chromate, 2-propanol, ammonium oxalate, and absolute ethanol were supplied with analytical grade by Merck and employed without further purification. All experiments were carried out using deionized water. 2.2. Instruments The XRD patterns were recorded by a Philips Xpert X-ray diffractometer with Cu Ka radiation (l = 0.15406 nm), employing scanning rate of 0.04 /s in the 2u range from 20 to 80 . Surface

morphology and distribution of particles were studied by LEO 1430VP SEM, using an accelerating voltage of 15 kV. The purity and elemental analysis of the products were obtained by EDX on the same SEM instrument. The TEM investigations were performed by a Zeiss-EM10C instrument with an acceleration voltage of 80 kV. The UV–vis DRS was recorded by a Scinco 4100 apparatus. The FTIR spectra were obtained by a Perkin Elmer Spectrum RX I apparatus. The PL spectra of the samples were studied using a Perkin Elmer (LS 55) fluorescence spectrophotometer with an excitation wavelength of 300 nm. The conditions were fixed in order to compare the PL intensities. Specific surface area and pore properties of the samples were calculated using BET and Barret– Joyner–Halenda (BJH) models with nitrogen adsorption–desorption isotherms collected by Belsorp apparatus at 196 C. Prior to the experiments, the samples were degassed at 120 C for 15 h. The UV–vis spectra during the degradation reactions were studied using a Cecile 9000 spectrophotometer. The ultrasound radiation was performed using a Bandelin ultrasound processor HD 3100 (12 mm diameter Ti horn, 75 W, 20 kHz). 2.3. Preparation of the photocatalysts In a typical procedure for preparation of the TiO2/Ag2CrO4 (50%) nanocomposite, where 50% is weight percent of Ag2CrO4, 0.25 g of the commercial TiO2 was dispersed into 150 mL of water by ultrasonic irradiation for 10 min in a cylindrical glassy reactor provided with water circulation arrangement to maintain its temperature at 25 C with accuracy of one degree. Then, 0.256 g of silver nitrate was added to the suspension and stirred for 60 min at room temperature. Afterwards, an aqueous solution of potassium

Fig. 1. XRD patterns for the TiO2, and TiO2/Ag2CrO4 nanocomposites with different weight percents of Ag2CrO4.


chromate (0.146 g in 50 mL of water) was dropwise added to the suspension and refluxed at 96 C for 60 min. The formed violet suspension was then centrifuged and washed two times with

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water and ethanol and dried in an oven at 60 C for 24 h. The schematic diagram for preparation of the nanocomposites is illustrated in Scheme 1.

Fig. 2. (a) EDX spectra for the TiO2 and TiO2/Ag2CrO4 (50%) nanocomposite. (b)–(f) EDX mapping of the TiO2/Ag2CrO4 (50%) nanocomposite. (g)–(k) EDX mapping of the TiO2/ Ag2CrO4 (40%) nanocomposite.

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2.4. Photocatalysis experiments Photocatalysis experiments were performed at 25 C under a LED lamp with the power of 50 W, as visible-light source. Concentrations of RhB, MB, and fuchsine were 1 10 5, 1.3 10 5, and 0.77 10 5 M, respectively. Other conditions were described in detail in our previous work [28,32]. 3. Results and discussion The phase structure of the samples was studied by powder XRD analysis. Fig. 1 represents the XRD patterns of the TiO2 and TiO2/ Ag2CrO4 nanocomposites with different weight percents of Ag2CrO 4. For the TiO2 sample, the diffraction peaks are corresponding to (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of anatase TiO2, and (110), (101), and (111) crystal planes of rutile TiO2, respectively [33]. For the TiO2/Ag2CrO4 nanocomposites, the diffraction peaks are simply indexed to crystalline TiO2 and orthorhombic phase of Ag2CrO4 (JCPDS No. 26-0952) [26,27]. These XRD patterns did not show any extra peaks of possible impurities, confirming preparation of the TiO2/Ag2CrO4 nanocomposites. In addition, changes of intensity of the XRD peaks for Ag2CrO4 counterpart of the nanocomposite is attributed to changes of size and crystallinity by increasing its weight percent. The elemental compositions of TiO2 and TiO2/Ag2CrO4 (50%) nanocomposite were explored by EDX technique and the results are shown in Fig. 2. The peaks of TiO2 sample are clearly ascribed to

Ti and O elements. In addition, for the TiO2/Ag2CrO4 (50%) nanocomposite, peaks of Ti, O, Ag, and Cr elements are observed. Quantitative analysis of the TiO2/Ag2CrO4 (50%) nanocomposite showed that weight percents of Ti, O, Ag, and Cr elements are 29.8, 29.7, 32.5, and 8.00%, respectively, which they are close to the theoretical percents of 30.0, 29.7, 32.5, and 7.83%, respectively. To further investigate elemental composition and distribution uniformity, the elemental maps for the TiO2/Ag2CrO4 (50%) nanocomposite are displayed in Fig. 2b–f, indicating homogeneous distributions of Ti, Ag, Cr, and O constituting elements in the nanocomposite. In addition, to confirm uniform distribution of the elements in the TiO2/Ag2CrO4 (40%) nanocomposite, the results of EDX mapping are shown in Fig. 2g–k. These analyses demonstrate that Ag2CrO4 tends to integrate with TiO2 nanoparticles firmly and then form heterojunction structures. The detailed morphologies of the TiO2 and as-prepared TiO2/ Ag2CrO4 (50%) nanocomposite were investigated by SEM and TEM techniques and their images are shown in Fig. 3. It can be seen that the commercial TiO2 nearly has spherical morphology (Fig. 3a). As can be observed in Fig. 3b, the TiO2/Ag2CrO4 (50%) nanocomposite has spherical morphology too and aggregation of its particles is lower than that of the pure TiO2. In consistent with the results obtained from the elemental mapping, it is clear that particles of Ag2CrO4 stacked and dispersed on the TiO2 particles (Fig. 3c). Optical absorption spectra of the prepared samples were provided by UV–vis DRS and the results in the range of 270– 800 nm are shown in Fig. 4a. For the TiO2 sample, only a strong

Fig. 3. SEM images for the (a) TiO2, (b) the TiO2/Ag2CrO4 (50%). (c) and (d) TEM images of the TiO2/Ag2CrO4 (50%) nanocomposite.


absorption in the UV region with absorption edge of 400 nm could be observed, due to its wide band gap [8,9]. In contrast to the TiO2 sample, the TiO2/Ag2CrO4 nanocomposites displayed a marked absorption enhancement in the visible-light region. It is well known that during absorption of the light with energy higher than band gap of a semiconductor, electron-hole pairs are produced in the semiconductor by absorption of the irradiated light [2–5]. Hence, the TiO2/Ag2CrO4 nanocomposites produce a large number of electron-hole pairs under visible-light irradiation. It is evident that

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for the nanocomposites, wavelength of the absorption peak for TiO2 counterpart of the nanocomposites decreases relative to the pure TiO2. This decrease can be ascribed to increase of TiO2 band gap through interaction between counterparts of the nanocomposite. Band gaps (Eg) of the prepared sample were estimated using Tauc’s equation ahn = B(hv Eg)n/2. In this equation, a, n, and B are absorption coefficient, the light frequency, and proportionality constant, respectively [34]. The value of n depends on the characteristics of the transition in the semiconductor. The Eg values

Fig. 4. (a) UV–vis DRS spectra for the TiO2 and TiO2/Ag2CrO4 nanocomposites with different weight percents of Ag2CrO4. (b) Plots of (ahn)2 versus hv for the prepared samples.

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Fig. 5. FT-IR spectra for the TiO2 and TiO2/Ag2CrO4 (50%) samples.

were estimated by extrapolation of the linear part of the curves obtained by plotting (ahn)2 versus hv. As can be seen, the Eg values of the prepared nanocomposites are lower than that of the pure TiO2. To obtain more structural information, FT-IR spectra of the TiO2 and TiO2/Ag2CrO4 (50%) samples were provided and the spectra are shown in Fig. 5. For these samples, the broad absorption peak at 3486 cm 1 corresponds to the stretching vibration of O H groups and a weak band at 1627 cm 1 could be attributed to bending vibrations of adsorbed molecules of water over the samples [26]. For the TiO2 sample, there are characteristic wide peaks in the region from 500 to 1000 cm 1, which are related to the Ti O Ti bending vibration [7,8]. Moreover, for the TiO2/Ag2CrO4 (50%) nanocomposite, a strong absorption peak at 892 cm 1 and a shorter one at 853 cm 1 could be assigned to the stretching vibration of Cr O bonds in the CrO42 group [35]. Photocatalytic activity of the samples was investigated by degradation of RhB under visible-light irradiation. The experimental results are shown in Fig. 6. In this figure, along with the photodegradation data, photolysis (without photocatalyst) and adsorption data (without the light irradiation) are also shown. As can be seen from Fig. 6a, RhB can only be slightly degraded under

Fig. 6. (a) Photodegradation of RhB over the TiO2 and TiO2/Ag2CrO4 nanocomposites with different weight percents of Ag2CrO4. UV–vis spectra for degradation of RhB under visible-light irradiation over the (b) TiO2 and (c) TiO2/Ag2CrO4 (50%) nanocomposite.


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Fig. 7. (a) The degradation rate constants of RhB over different samples. (b) PL spectra for the TiO2, TiO2/Ag2CrO4 (50%), and TiO2/Ag2CrO4 (60%) samples. (c) Nitrogen adsorption–desorption data for the TiO2 and TiO2/Ag2CrO4 (50%) samples.

Table 1 Textural properties of the TiO2 and TiO2/Ag2CrO4 (50%) samples. Sample

Surface area (m2 g 1)

Mean pore diameter (nm)

Total pore volume (cm3 g 1)

TiO2 TiO2/Ag2CrO4 (50%)

45.8 84.8

10.3 14.4

0.23 0.46

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Fig. 8. Proposed mechanism for the enhanced photocatalytic activity of TiO2/Ag2CrO4 nanocomposites.

visible-light irradiation without any photocatalyst, indicating that RhB is a stable dye pollutant under the light irradiation. Before the light irradiation, the solutions of RhB and photocatalysts were magnetically stirred in dark for 60 min to establish an adsorption– desorption equilibrium. As can be seen, the commercial TiO2 could degrade less than 45% of the dye after 360 min of the light irradiation. Interestingly, photocatalytic activity of the nanocomposites showed pronounced increase in comparison with the pristine TiO2. Concretely, with increasing Ag2CrO4 content in the nanocomposites until 50%, the photodegradation activity monotonically increases. Notably, it clearly can be seen that the TiO2/Ag2CrO4 (50%) nanocomposite exhibited the highest photocatalytic activity. The spectral changes in the range of 200–700 nm for RhB during the degradation reaction over the TiO2 and TiO2/ Ag2CrO4 (50%) nanocomposite were also investigated and the results summarized in Figs. 6b and c. It was found that the absorption peak of the solution continuously decreases with the increase of the irradiation time. After the light irradiation for 270 min, molecules of RhB were almost completely degraded over the TiO2/Ag2CrO4 (50%) nanocomposite, while as 38% of RhB was degraded over the TiO2 sample. The degradation rate constant of RhB over the samples was determined from the pseudo-first-order rate equation of ln(C/ Co) = kobst, where C is concentration of RhB at time t, Co is the initial concentration of RhB solution, and kobs is the observed firstorder rate constant. The degradation rate constants for RhB over the TiO2 and TiO2/Ag2CrO4 samples are reported in Fig. 7a. It is evident that the rate constant increases continuously with weight percent of Ag2CrO4 and the TiO2/Ag2CrO4 (50%) nanocomposite displays the superior activity. The degradation rate constants of RhB over the TiO2 and TiO2/Ag2CrO4 (50%) samples are 13.2 10 4 and 116 10 4 min, respectively. Hence, activity of this nanocomposite is nearly 8.8-folds higher than that of the TiO2 in the degradation of RhB under visible-light illumination, which indicates that the heterojunction formed between TiO2 and

Ag2CrO4 dramatically enhanced the photocatalytic performance under visible-light irradiation. To explore separation efficiency of the photogenerated electron-hole pairs, PL spectra for the TiO2, TiO2/Ag2CrO4 (50%), and TiO2/Ag2CrO4 (60%) samples were provided in the range of 350–550 nm and the results are displayed in Fig. 7b. It is generally accepted that the PL spectrum with low intensity indicates efficiently separation of the charge carriers,

Fig. 9. The degradation rate constants of RhB over the TiO2/Ag2CrO4 (50%) nanocomposite in presence of various scavengers.


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Fig. 10. (a) The degradation rate constant of RhB over the TiO2/Ag2CrO4 (50%) nanocomposite prepared at different refluxing times. (b) PL spectra of the TiO2/Ag2CrO4 (50%) nanocomposite prepared by refluxing for 60 and 180 min.

leading to participation of more electrons and holes in the oxidation and reduction reactions. It can be observed that all of the samples have similar spectra in this range. However, there is a considerable decrease in the intensity of the PL spectrum for the TiO2/Ag2CrO4 (50%) nanocomposite compared to that of the pure TiO2. This decrease in the PL intensity suggests that recombination of the electron–hole pairs in the nanocomposite was strongly decreased compared with the TiO2, due to formation of heterojunction between TiO2 and Ag2CrO4, resulting in highly enhanced photocatalytic activity for the nanocomposite. Interestingly, intensity of the peaks for the TiO2/Ag2CrO4 (60%) nanocomposite is higher than that of the TiO2/Ag2CrO4 (50%) nanocomposite. This implies that separation of the charge carriers in the TiO2/Ag2CrO4 (60%) nanocomposite is lower than that of the TiO2/Ag2CrO4 (50%) nanocomposite. For this reason, the TiO2/Ag2CrO4 (50%) nanocomposite shows the highest photocatalytic activity. This may be attributed to agglomeration of the excess Ag2CrO4 particles in the nanocomposite, leading to decrease of the heterojunctions between counterparts of the nanocomposite. As a result, similar to many multi-component photocatalysts further increase in weight percent of Ag2CrO4 leads to a decrease in the photocatalytic activity of the nanocomposite [36–38]. Textural properties of the TiO2 and TiO2/Ag2CrO4 (50%) samples were characterized using nitrogen adsorption–desorption experiments and the results are displayed in Fig. 7c. It is evident that the samples have typical IV isotherms with H1 hysteresis, representing that the samples are mesoporous. Specific surface area and pore properties of the samples were calculated by BET and BJH models, respectively and the results are shown in Table 1. It is clear that specific surface area and pore volume of the TiO2/Ag2CrO4 (50%) nanocomposite is remarkably higher than that of the TiO2 sample. As a result, the enhanced photocatalytic activity of the TiO2/Ag2CrO4 nanocomposites relative to the TiO2 sample is related to more harvesting of visible-light irradiation, great suppression of the charge carriers, and enhanced surface area.

On the basis of the band edge positions of TiO2 and Ag2CrO4 semiconductors [39], a schematic diagram for the formation of n-n heterojunction between the counterparts of the TiO2/Ag2CrO4 nanocomposite are proposed. As shown in Fig. 8, due to n-type characteristics of TiO2 and Ag2CrO4 semiconductors, their Fermi levels are located close to the conduction band (CB) [8]. Furthermore, the Fermi level of TiO2 is more negative than that of Ag2CrO4. Consequently, after contacting of these semiconductors to form

Fig. 11. Reusability of the TiO2/Ag2CrO4 (50%) nanocomposite in degradation of RhB in five successive runs.

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heterojunction, electrons transfer from TiO2 to Ag2CrO4, leaving positive and negative charges in the junction surfaces over TiO2 and Ag2CrO4, respectively [30,31]. On the other hand, band gap of Ag2CrO 4 is very lower than that of TiO2. Thus, under visible-light illumination, electron-hole pairs are produced only in Ag2CrO4 rather than TiO2. After photogeneration of the charge carriers over Ag2CrO4, the photogenerated electrons are injected from the CB of Ag2CrO4 into the CB of TiO2, meanwhile the photogenerated holes are effectively collected in the VB of Ag2CrO4. In such a way, the photogenerated charge carriers are effectively separated. Therefore, the probability of electron–hole recombinations is reduced, leading to greatly enhanced photocatalytic activity. In order to explore the role of reactive species produced during the degradation reaction of RhB over the TiO2/Ag2CrO4 (50%) nanocomposite, 2-prOH, ammonium oxalate (AO), and benzoquinone (BQ) were used as the hydroxyl radical (OH), hole (h+), and superoxide anion radical (O2 ) scavengers, respectively. Fig. 9 displays the influence of these scavengers on the visible-light photocatalytic activity of this nanocomposite towards the degradation of RhB. Compared with scavenger-free solution, when AO was added to the reaction system, the degradation rate constant of RhB was strongly inhibited, indicating that h+ was the main active

species generated in the reaction system. In addition, after the addition of BQ, the degradation rate constant was decelerated drastically. Therefore superoxide anion radical was another active species that was responsible for the degradation reaction. However, in the presence of 2-prOH, the degradation rate constant was inhibited a little, representing that OH does not have considerable effect on the degradation reaction. The influence of refluxing time on the photocatalytic activity of the TiO2/Ag2CrO4 (50%) nanocomposite was studied and the results are shown in Fig.10a. It can be seen that the degradation rate constant increases with the refluxing time up to 60 min and then decreases. Fig. 10b shows PL spectra of the nanocomposite prepared by refluxing for 60 and 180 min. Compared with the nanocomposite prepared by refluxing for 180 min, it is evident that there is a decrease in the intensity of the PL spectrum for the nanocomposite prepared by refluxing for 60 min. As a result, it was concluded that separation of the photogenerated electron-hole pairs in the nanocomposite prepared by refluxing for 60 min is better than the other sample, leading to an enhanced photocatalytic activity. In fact, it seems that by changing the preparation time, the heterojunction formed between TiO2 and Ag2CrO4 was slightly destructed [40]. Hence,

Fig. 12. The degradation rate constants of RhB, MB, and fuchsine dyes over the TiO2 and TiO2/Ag2CrO4 (50%) samples under visible-light irradiation.


separation of the charge carriers could not effectively take place for the nanocomposite prepared by refluxing at 180 nm. Besides the excellent photocatalytic activity, the recyclability of the photocatalyst is crucially important for the practical application. Fig. 11 shows the results obtained from the cyclic stability tests for photocatalytic degradation of RhB over the TiO2/Ag2CrO4 (50%) nanocomposite. It is clear that after five cycles, small decrease in its photocatalytic activity was observed, suggesting that the nanocomposite maintains its stability and durability during the degradation reaction. In order to evaluate ability of the TiO2/Ag2CrO4 (50%) nanocomposite for degradation of other dye pollutants, photocatalytic degradation of MB and fuchsine over the nanocomposite and TiO2 under the visible-light irradiation was studied. As can be seen in Fig. 12, photocatalytic activity of the nanocomposite is nearly 5.5 and 44-folds greater than that of the TiO2 in degradations of MB and fuchsine, respectively. Therefore, the TiO2/Ag2CrO4 nanocomposites could have promising applications in heterogeneous photocatalytic processes. 4. Conclusions In summary, novel TiO2/Ag2CrO4 nanocomposites as efficient visible-light-driven photocatalysts were fabricated using a facile refluxing method. The TiO2/Ag2CrO4 (50%) nanocomposite showed enhanced activity of nearly 8.8, 5.4, and 44-folds relative to the TiO2 sample in photocatalytic degradations of RhB, MB, and fuchsine pollutants, respectively. The trapping experiments suggested that h+ and O2 were the main reactive species for the degradation of RhB under visible-light irradiation. The greatly enhanced activity of the nanocomposite was related to increased surface area, efficiently separation of the charge carriers through n-n heterojunctions, and more harvesting of visible light due to the presence of Ag2CrO4, as a narrow band gap semiconductor. Finally, it was observed that the nanocomposite has remarkable stability during the successive five photodegradation runs. Acknowledgment The authors wish to acknowledge University of Mohaghegh Ardabili-Iran, for financial support of this work. References [1] W. Xiao Ping, S. Dian Chao, Y. Tan Dong, Climate change and global cycling of persistent organic pollutants: a critical review, Sci. China 59 (2016) 1899–1911. [2] P.A. Kumar Reddy, P.V. Laxma Reddy, E. Kwon, K.-H. Kim, T. Akter, S. Kalagara, Recent advances in photocatalytic treatment of pollutants in aqueous media, Environ. Int. 91 (2016) 94–103. [3] G. Colón, Towards the hydrogen production by photocatalysis, Appl. Catal. A: Gen. 518 (2016) 48–59. [4] A. Nikokavoura, C. Trapalis, Alternative photocatalysts to TiO2 for the photocatalytic reduction of CO2, Appl. Surf. Sci. 391 (2017) 149–174. [5] C. Yu, W. Zhou, H. Liu, Y. Liu, D.D. Dionysiou, Design and fabrication of microsphere photocatalysts for environmental purification and energy conversion, Chem. Eng. J. 287 (2016) 117–129. [6] J. Chen, J. Cen, X. Xu, X. Li, The application of heterogeneous visible light photocatalysts in organic synthesis, Catal. Sci. Technol. 6 (2016) 349–362. [7] X. Zhang, Y. Wang, B. Liu, Y. Sang, H. Liu, Heterostructures construction on TiO2 nanobelts: a powerful tool for building high-performance photocatalysts, Appl. Catal. B: Environ. 202 (2017) 620–641. [8] L. Gomathi Devi, R. Kavitha, A review on plasmonic metal–TiO2 composite for generation, trapping, storing and dynamic vectorial transfer of photogenerated electrons across the Schottky junction in a photocatalytic system, Appl. Surf. Sci. 360 (2016) 601–622. [9] R. Fagan, D.E. McCormack, D.D. Dionysiou, S.C. Pillai, A. review of solar and visible light active TiO2 photocatalysis for treating bacteria, cyanotoxins and contaminants of emerging concern, Mater. Sci. Semicond. Process. 42 (2016) 2–14. [10] X. Wang, R. Yu, K. Wang, G. Yang, H. Yu, Facile template-induced synthesis of Ag-modified TiO2 hollow octahedra with high photocatalytic activity, Chin. J. Catal. 36 (2015) 2211–2218.

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