Pd nanoparticle loaded TiO2 semiconductor for

J Mater Sci: Mater Electron DOI 10.1007/s10854-017-7585-z

Pd nanoparticle loaded TiO2 semiconductor for photocatalytic degradation of Paraoxon pesticide under visible-light irradiation Amir Homayoun Keihan1 · Hossein Rasoulnezhad2 · Azadeh Mohammadgholi3 · Sharareh Sajjadi4 · Reza Hosseinzadeh5 · Mousa Farhadian6 · Ghader Hosseinzadeh7

Received: 2 June 2017 / Accepted: 20 July 2017 © Springer Science+Business Media, LLC 2017

Abstract Overuse of the organophosphorus pesticides such as Paraoxon in agriculture industry has raised significant threats to the environment by contamination of soils and groundwaters. Therefore, extensive studies have been carried out to develop an effective method for removing of these poisonous pollutants from contaminated resources. In the current study, Pd nanoparticle loaded TiO2 nanocomposites with different weight percentages of Pd were prepared via a facile photoreduction method and for the first time, were used for photocatalytic degradation of Paraoxon under visible-light irradiation. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and photoluminescence spectroscopy techniques. In these nanocomposites, the presence of Pd nanoparticles enhances * Ghader Hosseinzadeh [email protected] 1

Molecular Biology Research Center, Baqiyatallah University of Medical Sciences, Tehran, Iran

2

Department of Electrical & Electronics Engineering, Standard Research Institute (SRI), Karaj, Iran

3

Department of Biology, Central Tehran Branch, Islamic Azad University, Tehran, Iran

4

Department of Biology, Roudehen Branch, Islamic Azad University, Roudehen, Iran

5

Medical Laser Research Group, Medical Laser Research Center, ACECR, Tehran, Iran

6

Young Researchers and Elite Club, Khomeinishahr Branch, Islamic Azad University, Khomeinishahr, Iran

7

Department of Polymer Science and Engineering, University of Bonab, Bonab, Iran

the photocatalytic activity of TiO2 by their surface plasmon resonance effect and also by narrowing the band gap energy of TiO2. The results of photocatalytic activity measurements indicate that the nanocomposite with 0.8 wt% content of Pd (PT0.8) has the best photocatalytic activity. The result of total organic carbon test shows that Paraoxon was completely mineralized by PT0.8 photocatalyst after 120 min, under visible-light irradiation.

1 Introduction Organophosphorus compounds are the most widely used insecticides, accounting for about 38% of total used pesticides worldwide. The irreversible binding of these compound to the acetylcholine esterase (AChE) enzyme inhibits the normal function of this enzyme and thereby the hydrolysis of acetylcholine is stopped and acetylcholine is accumulated in body [1]. In spite of the usefulness of pesticides in agriculture industry, overuse of the pesticide compounds has raised significant threats to the environment by contamination of soils and groundwater. Therefore, in the last decade, extensive studies have been carried out to develop an effective method for water treatment to remove these poisonous pollutants. Biodegradation [2], photolysis [3], heterogeneous and homogeneous hydrolysis [4], and photocatalytic degradation [5] methods are the currently used techniques in this field. Among the several methods for treatment of aqueous environments contaminated with the organic pollutants, photocatalytic water treatment techniques, has some advantages such as low-cost, its environmentally friendly nature, and sustainable treatment technology [6]. For this reason, this technique has attracted considerable attention during the past decades. Among all semiconductor

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MS Code : JMSE-D-17-01976

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J Mater Sci: Mater Electron

photocatalysts, TiO2 semiconductor is one of the most promising photocatalyst in photocatalytic water treatment technology, because of its high oxidizing power, low production cost, high chemical stability, and low toxicity [7]. However, the major drawback of TiO2 is its relatively large intrinsic band gap energy (3.2 eV for the anatase phase and 3.0 eV for the rutile phase [8]), and therefore TiO2-based photocatalysts are only active in the UV region of the electromagnetic spectrum (~5% of solar light) [9]. Therefore, for increasing the photocatalytic activity of TiO2 under the solar light irradiation it is necessary to reduce the band-gap energy of TiO2 [10]. However, by narrowing of the band gap energy, the photogenerated electron–hole recombination rate increases, and with increasing of the electron–hole recombination rate, the photocatalytic activity decreases [11]. In recent years, various methods including: increasing the porosity of TiO2 [12], compositing with carbon nanomaterials such as graphene and carbon nanotube [13–15], doping with metal or nonmetal materials [16–18], surface modification and sensitizing [19] have been developed for narrowing the band gap energy of TiO2 and also for reducing the electron–hole recombination rate in this photocatalyst. One promising method to enhance the photocatalytic efficiency of TiO2 is loading of metal nanoparticles on its surface [20, 21]. According to previous studies, presence of the metal nanoparticles can shift the Fermi level of TiO2, which results in the enhancement of its photocatalytic activity [22]. Moreover, because of the high electric conductance of noble metal nanoparticles, presence of them can increase charge separation on TiO2 surfaces and by minimization of charge-carrier recombination rate can improve the photocatalytic activity of TiO2 [23]. Pd nanoparticles with sizes larger than 10 nm show the surface plasmon resonance (SPR) effects [24], and Leong et al. used this feature of Pd nanoparticles for improvement of the photocatalytic performance of TiO2 [25]. In the present work, Pd nanoparticles were loaded on TiO2 nanoparticles in different weight percentage via photo-reduction of Pd2+ by UV irradiation, in a solution containing TiO2 nanoparticles, palladium chloride, and formic acid as a sacrifice component. The prepared nanocomposites were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray (EDX) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) spectroscopy and diffuse reflectance spectroscopy (DRS) techniques. This nanocomposites were used, for the first time, for photocatalytic degradation of Paraoxon pesticide.

2 Materials and methods 2.1 Materials Titanium dioxide nanoparticles (P25, containing 70% anatase and 30% rutile) were purchased from Degussa, Germany. Paraoxon in analytical grade was obtained from Sigma-Aldrich. Palladium chloride (PdCl2) and formic acid, all in analytical grade, were purchased from Merck. 2.2 Synthesis of Pd nanoparticle loaded TiO2 For preparation of TiO2 suspension, 0.2 g TiO2 nanoparticles were dispersed in 100 mL deionized water at pH 2 (using 1 M nitric acid). A desired amount of palladium chloride and formic acid (the molar ratio of formic acid to palladium was set 20:1) were added into the TiO2 suspension. After 2 h vigorous magnetic stirring in dark condition, in order to reduction of Pd2+ cations to Pd ‎ nanoparticles, the mixture was irradiate with UV light for about 1 h at room temperature. In this photoreduction process, formic acid was used as a sacrifice agent for reduction of Pd2+ cations to Pd ‎ nanoparticles. After the complete photo-reduction of Pd2+ cations, the resulted products were separated from the reaction medium by centrifuging at 12,000 rpm, and finally washed with ethanol three times and dried at 60 °C. In this work for selecting the optimal amounts of Pd ‎ nanoparticles in the structure of the Pd loaded TiO2 nanocomposite, a series of these nanocomposites with various contents of Pd were prepared and by comparing their photocatalytic activity with each other the optimal composition was selected. The nanocomposites with Pd weight percentages of 0, 0.2, 0.4, 0.8, and 1.6 wt% were prepared and labeled as P25, PT0.2, PT0.4, PT0.8, and PT1.6. 2.3 Catalyst characterization X-ray diffraction patterns of the samples were acquired using Philips X’Pert MPD Pro X-ray diffractometer with Cu Kα irradiation (λ = 1.54018 Å). The surface morphology of the products were characterized by JEOL SEM. TEM and HRTEM images of the PT0.8 sample were taken with JEOL HRTEM (JEM-2100F). Diffuse reflectance UV–Vis spectra of the samples in the wavelength range of 200–700 nm were measured in the reflectance ‎ mode using Avaspec-2048-TEC. XPS spectra of the PT0.8 sample were obtained by using a Gammadata-Scienta Esca 200 (Uppsala, Sweden) hemispherical analyzer equipped with an Al Kα (1486.6 eV) X-ray source. All binding energy values were calibrated by using the value of the C 1s peak at 284.6 eV as a reference. PL emission spectra of the prepared samples were recorded in the wavelength range of

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400–550 nm upon excitation at λex = 300 nm using a VARIAN (Cary Eclipse) fluorescence spectrophotometer. 2.4 Photocatalytic activity test In this work, for evaluation of the photocatalytic performance of the P25, PT0.2, PT0.4, PT0.8, and PT1.6 samples in the degradation of Paraoxon pesticide, as a model of organophosphorus compounds, a 570 W Xenon lamp (OSRAM Co.) was used as an irradiation source, and a L41 UV cutoff filter (Kenko Co.) was used to remove UV light (λ < 400 nm). For preparation of the reaction suspensions, 20 mg of the different photocatalyst samples were added into a 100 mL aqueous solution of Paraoxon with an initial concentration of 31 mg/L. Prior to irradiation, the reaction suspension was stirred using a magnetic stirrer in darkness for about 2 h to establish an adsorption–desorption equilibrium between the Paraoxon and the photocatalyst surface. Then the suspension containing Paraoxon and photocatalysts was irradiated under the visible-light with continuous stirring. At given time intervals, 2 mL of the suspension was taken and immediately centrifuged, then the remaining concentration of Paraoxon was determined by UV–Vis spectrophotometer (Varian 100 Bio, Cary). For each photocatalyst, the above mentioned photocatalytic activity test was repeated three times. The mineralization of Paraoxon, was followed by total organic carbon (TOC) measurements using a total organic carbon analyzer (Shimadzu TOCVCSH; Japan). For determination of the reactive species (hvb+ (hole), OH· and O2·− radicals) responsible for the photocatalytic degradation of Paraoxon, the photocatalytic activity tests of the PT0.8 photocatalyst were repeated in the presence of the various kinds of scavengers, including tert-Butanol (tBuOH) as an OH· scavenger, KI as an hvb+ scavenger, and benzoquinone as an O2·− scavenger [26]. In these experiments, the scavengers with initial concentration of 100 mM were added to the reaction mixture (containing photocatalyst and Paraoxon) before the visible irradiation.

3 Results and discussion 3.1 XRD Figure 1 shows the X-ray diffraction patterns for the P25, PT0.2, PT0.4, PT0.8, and PT1.6 samples. In the XRD patterns of the all samples, the peaks located at 2θ = 25.1°, 37.8°, 48°, 54.2°, 55.2°, 62.8°, 68.9° and 70.7° are related to the diffraction from the (101), (004), (200), (105), (211), (204), (116), and (220) planes of the TiO2 anatase phase (JCPDS No. 21-1272), respectively, and the peaks at 2θ = 27.4°, 36.2° and 41.2° are due to

Fig. 1 XRD patterns of the P25, PT0.2, PT0.4, PT0.8, and PT1.6 samples

the diffraction from the (110), (101), and (111) crystal planes of the TiO2 rutile phase (JCPDS No. 21-1276). In the XRD patterns of PT0.4, PT0.8, and PT1.6 samples, two peaks at 2θ = 40.1° and 46.7° could be detected which can be attributed to the diffraction from the (111) and (200) planes of face center cubic (FCC) metallic palladium (JCPDS No. 46-1043), however, because of the low content of Pd in PT0.2 sample, these peaks could not be distinguished in the XRD pattern of this sample. The crystal structure and anatase to rutile ratio of TiO2 were preserved after the formation of Pd loaded TiO2 nanocomposite, therefore presence of Pd doesn’t interfere in the crystal structure of TiO2. Because of the nanocrystalline nature of the samples, the all of these samples have broad diffraction peaks, however in comparison with the XRD patterns of the Pd loaded TiO2 samples, P25 sample have broader diffraction peaks, which can be attributed to


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the growth of the TiO2 nanoparticles during the photoreduction of Pd nanoparticles on their surface. 3.2 SEM and TEM images The surface morphology of the TiO2-P25, and PT0.8 samples were investigated by SEM. As Fig. 2a depicts, TiO2-P25 nanoparticles has nearly spherical morphology with sizes in the range of 30–50 nm. During the photoreduction process and by compositing of TiO2 with Pd, the sizes of TiO2 nanoparticles in PT0.8 increase (Fig. 2b), however, both samples have nearly the same morphology (spherical morphology). As reported in literature, by increasing of the ionic strength of the solution in acidic

condition, the thickness of the electrical double layer (EDL) can be decreased, in such condition, nanoparticles agglomeration and particle size increases [27, 28]. For this reason and because of the Ostwald ripening effect, in comparison with size of P25, size of PT0.8 sample has increased. The co-existence of Pd, Ti, and O elements in the PT0.8 nanocomposite, and successful formation of the Pd nanoparticle loaded TiO2 were confirmed by energy-dispersive X-ray spectroscopy (EDS) elemental analysis (Fig. 2b). Precise particle sizes of the PT0.8 sample was measured using TEM. As TEM image in Fig. 2c indicates the average particle size of TiO2 nanoparticles in PT0.8 sample is about 50 nm and Pd nanoparticles with sizes range of 12–20 nm clearly could be seen at the surface of TiO2 nanoparticles.

Fig. 2 a EDS elemental analysis and SEM image of P25 sample, b EDS elemental analysis and SEM image of PT0.8 sample, c TEM image of PT0.8 sample (arrows show the Pd nanoparticles), and d HRTEM image of PT0.8 sample


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In bright-field TEM image, because of the mass contrast and high atomic number of Pd, Pd nanoparticles are observed darker [29], however these particles were further analyzed by HRTEM. In HRTEM image of PT0.8 sample (Fig. 2d), the lattice fringes with a d-spacing of 0.224 and 0.352 nm are clearly observed which are associated with the lattice planes of Pd(111) and TiO2(101), respectively. Therefore, by using photoreduction method, Pd nanoparticles were successfully loaded on TiO2 nanoparticles. 3.3 DRS UV–Vis absorption spectra of the P25, PT0.2, PT0.4, PT0.8, and PT1.6 samples were demonstrated in Fig. 3. In comparison with the absorption spectra of P25, the Pd nanoparticle loaded TiO2 samples exhibit significant redshift to visible region in their absorption spectra, indicating narrowing of the P25 band gap with loading of Pd nanoparticle on its surface. According to literature, because of the surface plasmon resonance (SPR) effect, Pd nanoparticles with sizes larger than 10 nm show absorption band in visible region, therefore the broad absorption band for all of the Pd nanoparticle loaded TiO2 samples (PT0.2, PT0.4, PT0.8, and PT1.6 samples) is related to the SPR effect of the Pd nanoparticles [24]. With increasing the Pd contents from PT0.2 to PT1.6 sample there is a negligible change

Fig. 3 UV–Vis absorption spectra of the P25, PT0.2, PT0.4, PT0.8, and PT1.6 samples

in the red-shift of the absorption spectra to visible region, however, the intensity of the absorption band related to the SPR of Pd nanoparticles increases from PT0.2 to PT1.6 sample. The values of the band-gap energy (Eg) of samples can be estimated from the x-intercept of a fitted tangent line to the linear part of the Tauc plots by using Tauc’s equation:

n 𝛼hv = A(hv − Eg ) ∕2 where α, A, h, ν, and Eg denote the absorption coefficient, constant value, Planck’s constant, light frequency, and band-gap energy, and the value of n is 1 and 4 for a directand indirect band-gap semiconductors, respectively [30]. The plots of (αhν)1/2 versus hν (Tauc plots) for the prepared samples (indirect-band-gap semiconductor) are shown in inset of Fig. 3. The estimated band-gap energy (Eg) of TiO2 (P25), PT0.2, PT0.4, PT0.8, and PT1.6 samples are 3.29, 2.86, 2.85, 2.81, and 2.79 eV, respectively. From these band gap energy values of the samples it can be concluded that, the first addition of Pd (i.e. PT0.2 sample) has a remarkable effect on the band gap energy of TiO2 however, the effect of the subsequent loading of Pd nanoparticles on the narrowing of the TiO2 band gap energy is negligible. 3.4 XPS The surface composition and chemical state of elements in the PT0.8 sample were further analyzed using XPS. Figure 4a shows the XPS survey spectrum of the PT0.8 sample, the co-presence of O, Ti, and Pd elements in this sample indicates successful loading of Pd nanoparticle on TiO2. For precise analysis of chemical state(s) of Pd element, core level XPS spectrum of Pd element in the PT0.8 sample is demonstrated in Fig. 4b. Based on the literature, owning to the Pd 3d5/2 and Pd 3d3/2 transitions, there are two peaks in the core level XPS spectrum of palladium. As Fig. 4b indicates, the peak related to the Pd 3d5/2 transition can be deconvoluted to two peaks centered at 334.8 and 336.8 eV, corresponding to the Pd and PdO species of palladium, respectively, and the peak related to the Pd 3d3/2 transition can be deconvoluted to two peaks at 340.0 and 342.0 eV, for the Pd and PdO species, respectively [31]. Therefore, some of the palladium atoms are oxidized to palladium oxide. Although, because of the SPR effect of metallic Pd, presence of the metallic Pd can increase the photocatalytic activity of the TiO2 photocatlysts, however, as reported by Lee et al. [32], presence of PdO can provide some oxygen vacancy in TiO2, and thereby PdO can also enhance the photocatalytic activity of TiO2. Therefore, the co-presence of these two species of palladium (i.e. Pd and


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Fig. 4 a, b XPS survey and core-level XPS spectrum of PT0.8 sample, respectively

PdO) in TiO2 can remarkably improve the photocatalytic activity of TiO2 by their synergic effect. 3.5 Photoluminescence spectroscopy The intensity of the PL emission is directly proportional to the electron–hole recombination rate, that is, the greater the PL emission intensity, the faster the recombination rate, and vice versa. Therefore, PL spectroscopy can provide some useful information about the recombination rate of the photo-generated electrons and holes and charge separation [33]. As the results of the PL spectra of the P25, PT0.2, PT0.4, PT0.8, and PT1.6 samples in Fig. 5 indicate, with increasing the Pd contents in the Pd nanoparticle loaded TiO2 samples, the PL emission intensity and, consequently, the electron–hole recombination rates decreases. However, increasing the Pd contents up to the 0.8 wt% (i.e. PT1.6 sample) increases the PL emission intensity and the electron–hole recombination rate. In the case of metal doped TiO2, high loading of metal atoms causes that the metal atoms act as a recombination center and in this way enhance the recombination rate of the photo-generated electron hole pairs [34]. There are many reports in the literature about the role of metal atoms as electron–hole recombination center in the metal doped TiO2 when the concentration of the metal atoms exceed an optimal value [35–37]. Therefore, the metal atoms in the structure of TiO2 can act as trapping and recombination centers. Below the optimal concentration, the trapping role is dominant, however, above the optimal level, some surface enrichment of metal

Fig. 5 Photoluminescence (PL) emission spectra of the P25, PT0.2, PT0.4, PT0.8, and PT1.6 samples at λex = 300 nm

atoms causes that the dominant role shifts to the recombination one [38]. 3.6 Photocatalytic performance measurements The photocatalytic performances of the different photocatalysts for the visible-light photodegradation of Paraoxon are


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shown in Fig. 6a. In the Ct/C0 versus irradiation time plots, C0 is the Paraoxon concentration after the adsorption equilibrium and Ct is the remaining concentration of Paraoxon at the time of t. There is no considerable photodegradation in Paraoxon in the absence of any photocatalysts under visible-light irradiation. In comparison with the Pd nanoparticle loaded TiO2 samples, the bare TiO2-P25 photocatalyst, because of its large band gap energy, has relatively low photocatalytic activity under visible-light irradiation. However, as discussed in previous sections, by loading of Pd nanoparticle on TiO2, the band-gap energy of this semiconductor shifts to visible region (Fig. 3) and its photocatalytic activity is enhanced. In the case of the low initial concentration of the pollutants, the pseudo first order kinetic model could be used for determination of the photocatalytic degradation reaction rate constant (k). Therefore in the current work, the reaction rate constants (k) of the Paraoxon photocatalytic degradation on the different photocatalysts were calculated from this model given by Eq. (1): ( ) Ct ln = − kt (1) C0 Figure 6b shows the ln(Ct/C0) versus irradiation time plots for the photocatalytic degradation of Paraoxon on the different photocatalyst samples. The calculated reaction rate constants of the P25, PT0.2, PT0.4, PT0.8, and PT1.6 samples are 0.0127, 0.0439, 0.0702, 0.0885, and

0.0541 min−1, respectively. As could be seen, among these samples, the PT0.8 photocatalyst has the highest reaction rate constant (k) and photocatalytic activity, and loading of Pd nanoparticles in this samples increases the photocatalytic activity of the TiO2 (P25) about seven times. The loading of Pd nanoparticles on the surface of TiO2 increases the visible-light absorption of TiO2 by their SPR effect, in such condition the visible-light stimulated conduction electrons of Pd nanoparticles are transferred to the conduction band of TiO2 and increase the photocatalytic activity of TiO2 in visible-light region [23, 39]. However, in the case of PT1.6 photocatalyst, high loading of Pd nanoparticles has destructive effect on photocatalytic activity. As reported in literature, in high loading of metal nanoparticles, some of the metal nanoparticles can act as a recombination center for the photo-generated electron hole pairs and in this way reduce the photocatalytic activity [40], therefore the optimum amount of the metal nanoparticles must be loaded. One of the important parameters in powder photocatalysis for practical applications is the stability of the photocatalyst samples under the reaction condition. In this regard, the stability of the PT0.8 photocatalyst was evaluated by a series recycle experiments on the photocatalytic degradation of Paraoxon under the same reaction conditions. The results of this test have been shown in Fig. 7a. As can be seen, even after five times of recycling, the PT0.8 photocatalyst maintains 86.6% of its initial activity for the photocatalytic degradation of Paraoxon, which indicates good stability of this photocatalyst in this condition. Furthermore,

Fig. 6 a Photocatalytic performance, and b corresponding pseudo first order kinetics of different photocatalysts for the visible-light photocatalytic degradation of Paraoxon


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Fig. 7 a Cycling runs for the visible-light photocatalytic degradation of Paraoxon on PT0.8 photocatalyst, and b comparison between the XRD pattern of (1) PT0.8 sample and (2) PT0.8 sample after five cycles of the paraxon photocatalytic degradation reaction

for further evaluation of the stability of PT0.8 sample, its XRD pattern was recorded after five times of recycling. The obtained result is shown in Fig. 7b, and there is no remarkable change in comparison with the related XRD pattern of this sample before the photocatalytic reaction, so the prepared sample has considerable stability. To determine the possible photocatalytic degradation mechanism of Paraoxon on the PT0.8 photocatalyst, the active species (hvb+ (hole), OH· and O2·− radicals), responsible for the photocatalytic activity of the photocatalyst, must be determined. Figure 8 shows the photocatalytic activity of the PT0.8 photocatalyst for degradation of Paraoxon in the presence of the different scavengers. As the results of this figure indicate, among the surveyed scavengers, tBuOH (as a scavenger of OH·) has the largest destructive effect on the photocatalytic activity of PT0.8, and KI (as a scavenger of hvb+) has the smallest destructive effect. From these results it can be concluded that the hydroxyl (OH·) radicals play the main role in the photocatalytic degradation of Paraoxon on the PT0.8 photocatalyst and role of the hvb+ (hole) in this reaction is not significant. 3.7 Mineralization During the photocatalytic degradation of organic compounds, some intermediates with high toxicity may be produced, so it is necessary to evaluate the complete degradation of these compounds and their intermediates. In this regard, in the present work, the TOC test was done for evolution the mineralization of Paraoxon on PT0.8 photocatalyst under visible-light irradiation. As the results of Fig. 9

Fig. 8 photocatalytic activity of PT0.8 sample for the visible-light photocatalytic degradation of Paraoxon in presence of the different scavengers

indicate, by the reaction of the photogenerated O2·− and OH· radicals with Paraoxon and its photocatalytic degradation intermediates on the PT0.8 surface, Paraoxon was completely mineralized after about 120 min and converted to the inorganic compounds such as NO3−, PO43−, and CO2. Therefore, by using the photocatalytic degradation method,


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Fig. 9 TOC removal efficiency (%) for the photocatalytic degradation of Paraoxon on PT0.8 photocatalyst under visible-light irradiation

Paraoxon pesticide can be successfully removed from the contaminated water. 3.8 Discussion Based on the obtained results, the possible photocatalytic degradation mechanism of Paraoxon is illustrated in Scheme 1. Based on the SPR effect, the conduction electrons of Pd nanoparticles are stimulated by visible-light irradiation. Subsequent transferring of these electrons to the conduction band (CB) of TiO2, and their reaction with dissolved oxygen, produce peroxide radicals (O2·−). Peroxide radicals themselves can react with Paraoxon and decompose it, or their reactions with proton cation can

Scheme 1 Schematic illustration of the photocatalytic degradation of Paraoxon on PT0.8 photocatalyst under visible-light irradiation

finely produce hydroxyl (OH·) radicals. On the other hand, visible-light irradiation can excite the VB electrons of TiO2 to the Fermi level (EF) of Pd nanoparticles [22] (formation of the hole (hvb+) in VB of TiO2), which following reaction of the hole with water can also create OH· radicals. These radicals are highly reactive, and reaction of them with Paraoxon finally mineralized this pesticide to inorganic compounds such as NO3−, PO43−, and CO2. Because of the high electric conductivity of Pd nanoparticles, the photogenerated electron hole recombination rate in the Pd nanoparticles loaded TiO2 nanocomposite decreases and thereby, the photocatalytic activity of this nanocomposite increases. Photocatalytic performance of the PT0.8 sample was compared with that of the most important reported photocatalysts in degradation of organophosphorus pesticides, and the results has been summarized in Table 1. As can be seen in this table, the TiO2 sample reported by Senthilnathan et al. [41], has the highest photocatalytic degradation rate of 15.3 × 10−2 min−1, however, it must be noticed that this high Photocatalytic performance was obtained under UV radiation. In this table, among the visible light active photocatalysts, the PT0.8 sample has the highest photocatalytic activity in degradation of the Paraoxon organophosphorus pesticide.

4 Conclusion In the present study, the Pd nanoparticles were successfully loaded on TiO2 nanoparticles via a simple photoreduction method and for the first time, this nanocomposite was used for photocatalytic degradation of Paraoxon (as a model of

Table 1 Comparison of the photocatalytic performance of the PT0.8 sample with other reported photocatalysts in degradation of organophosphorus pesticides Photocatalyst

Pesticide

TiO2

Parathion

Bi-doped TiO2 La-doped TiO2 N-doped TiO2

Parathion Parathion Parathion

ZnO ZnS/TiO2 Ag/TiO2 Ag/TiO2 CoO–TiO2 TiO2 thin film Carbon/Cu2O/epoxy PT0.8

Parathion Parathion Parathion Paraoxon Paraoxon Paraoxon Paraoxon Paraoxon

Radiation UV Visible UV UV UV Visible UV Visible UV Visible Visible UV–Vis Sunlight Visible

K (min−1) −2

15.3 × 10 1.1 × 10−2 5.6 × 10−2 3.81 × 10−2 2.3 × 10−2 5.5 × 10−2 2.00 × 10−2 6.56 × 10−2 3.44 × 10−2 7.57 × 10−2 3.51 × 10−4 6.90 × 10−3 8.41 × 10−3 8.85 × 10−2

Refs. [41] [41] [42] [43] [41] [41] [44] [45] [46] [13] [47] [48] [49]


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organophosphorus compounds) under visible-light irradiation. Anchoring of Pd nanoparticles on the surface of TiO2 nanoparticles, decreases the band gap energy of TiO2, and Pd nanoparticles by their SPR effects and by reduction of the electron–hole recombination rate improve the photocatalytic activity of TiO2 in the visible light region. It was found that the optimal weight percentage of Pd nanoparticles for loading on TiO2 is 0.8 wt%. The prepared PT0.8 nanocomposite photocatalyst shows good stability in the reaction condition, and Paraoxon was completely mineralized to inorganic compounds with less toxicity after 120 min visible light irradiation on this photocatalyst. The results of this work could be useful for researchers in development of efficient photocatalysts for water treatment from organophosphorus pesticides.

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