Imazalil sulphate pesticide degradation using silver loaded hollow anatase TiO 2 under UV light irradiation

Silver loaded hollow anatase TiO2 particles with the location of silver are inside (Ag@TiO2) and outside (Ag/TiO2) the hollow TiO2 structure have been successfully synthesized by a depositionprecipitation and template methods. The effects of silver nanoparticles location on Ag@TiO2 and Ag/TiO2 has been evaluated in the photodegradation efficiency of imazalil sulphate pesticide in aqueous suspension under ultraviolet irradiation. The Ag/TiO2 showed better photocatalytic performance for the degradation of imazalil sulphate, compared to Ag@TiO2 and hollow TiO2 microspheres. A higher photocatalytic activity of Ag/TiO2 compared to Ag@TiO2 and hollow TiO2 can be considered as an evidence of enhanced charge separation of Ag/TiO2 photocatalyst as confirmed by photoluminescence spectroscopy.


INTRODUCTION
Hollow TiO 2 microspheres are associated with a high surface area, low density, easy recovery, ability to deliver of drugs, high surface permeability (Yu et al., 2007b) and have multiple light reflection and diffraction (Kondo et al., 2007).It is also believed that the structural features also improved its light harvesting ability by allowing more light to penetrate into its interior (Kondo et al., 2007;Li et al., 2007b).This light-harvesting ability makes hollow TiO 2 as promising photocatalyst (Kondo et al., 2007).The photocatalytic activity of TiO 2 is due to the formation of a photo-induced electron and a positive hole which occurs as a result of ultraviolet light absorption which corresponds to the energy gap (Herrmann, 1999).These species are believed to be mobile and are capable of initiating many photocatalytic reactions.However, the fast recombination of photogenerated electrons and holes limits both the photocatalytic efficiency and activity of TiO 2 .Therefore, the photocatalytic activity of TiO 2 can be improved by controlling the steps involved during the photocatalysis by TiO 2 .These steps include e -and h + generation, followed by their separation, migration and the reaction on the surface with adsorbed species.The photoinduced charge separation in bare TiO 2 particles has a very short lifetime, which is due to the recombination of charges.So, it is vital to prevent electron-hole recombination before a designated chemical reaction occurs on the surface of TiO 2 .High recombination rate of the photogenerated electron-hole pairs limit the industrial application of TiO 2 .Since charge separation is found to be a major problem, many attempts were made to improve the photocatalytic activity of TiO 2 by modifying the surface or bulk properties.This includes coupling of two semiconductors (Tada et al., 1998), metal deposition (Sclafani and Herrmann, 1998;Seery et al., 2007;Cozzoli et al., 2004;Xu et al., 2005), surface chelation and doping (Xu et al., 2005;Chatterjee and Mahata, 2002;Tada et al., 1998).
Although this type of catalyst's structure is effective, metals on the surface of the semiconductor are easily corroded and dissolved (Hirakawa and Kamat, 2005).To overcome these drawbacks, the noble metals are incorporated into the core and the semiconductor, such as TiO 2 , acts as the shell (Tom et al., 2003;Ung et al., 1998;Pastoriza-Santos et al., 2000;Chan and Barteau, 2005).Among all of these types of particles, TiO 2 @Ag particles have been intensively investigated because of their potential applications in photocatalysis and fabrication of electronic devices (Pastoriza-Santos et al., 2000;Tom et al., 2003;Hirakawa and Kamat, 2004b;Jang et al., 2006).
In the previous paper (Baharvand et al., 2014), we reported the synthesis of hollow anatase TiO 2 microspheres by employing carbon particles as a template.In the present paper, we investigated the functionalization of hollow anatase TiO 2 with Ag nanoparticles inside and outside, with the aim of improving the photocatalytic efficiency.First, two types of noble metal modified hollow anatase TiO 2 , such as silver core anatase TiO 2 shell (TiO 2 @Ag) and surface modified hollow anatase TiO 2 with silver (Ag/TiO 2 ) were prepared (see Fig. 1).Second, the effect of modification of hollow TiO 2 with Ag in photocatalytic decomposition of imazalil sulphate pesticide in aqueous medium was investigated.This is the first report in which photocatalytic activities of Ag nanoparticles modified hollow anatase TiO 2 microspheres are examined in the degradation of imazalil sulphate pesticide.

Synthesis of Ag/TiO2 microspheres
The Ag/TiO 2 microspheres, where the location of Ag nanoparticles are attached on the external surface of hollow TiO 2 , were prepared by deposition-precipitation method using urea and previously synthesized hollow anatase TiO 2 .The synthesis of hollow TiO 2 spheres is given in details in reference (Baharvand et al., 2014).TiO 2 (0.1 g) was added to a 10 mL aqueous solution containing AgNO 3 (Ag 1 wt/wt %) and 0.25 g urea.The suspension was then refluxed at 80 °C and kept at this temperature for 4 h under continuous stirring.The sample was then collected by centrifugation, washed repeatedly with deionized water, dried for 12 h at 80 °C and then calcined at 300 °C for 4 h.

Synthesis of Ag@TiO2 microspheres
The Ag@TiO 2 particles, silver loaded hollow anatase TiO 2 particles with the location of Ag are inside the hollow TiO 2 structure is by template method.2.4 mL of aqueous solution AgNO 3 was added dropwise into 40 mL of aqueous fructose solution (0.5 M) under vigorous stirring.After stirring for 20 min, the solution was transferred and sealed in a Teflon-sealed autoclave.The autoclave was placed in an oven at 160 °C for 5 h, yielding Ag@C microspheres.2.6 mL of TTIP was added to a colloidal suspension of Ag@C under vigorous stirring and then the mixture was aged under ambient conditions for one day.The product was collected by centrifugation and washed with ethanol, then dried at 60 °C in an oven.Ag@TiO 2 were finally obtained after the sample was calcined at 600 °C for 3 h in a furnace.

Characterization techniques
The prepared photocatalysts were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDX), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction spectroscopy (XRD), diffuse reflectance ultraviolet-visible spectroscopy (DR UV-Vis), Brunauer-Emmett-Teller surface area analysis, X-ray Fluorescence Spectroscopy (XRF) and photoluminescence spectroscopy (PL).The morphology of the samples was examined using FESEM-EDX (JEOL JED-2300) and TEM (JEOL JEM-2100, 200 kV).The FTIR spectra of the samples were obtained using a Perkin-Elmer spectrometer at a range of 400-4000 cm -1 .XRD experiments were carried out at room temperature using a Bruker AXS D8 Automatic Powder Diffractometer with Cu K α radiation (λ = 1.5406Å, 40 kV, 30 mA) to identify the crystal phase of the samples.The DR UV-Vis spectra were recorded on a UV-Vis-NIR spectrophotometer (Perkin Elmer Lambda 35) ranging from 200 to 800 nm at room temperature.PL spectra of samples were obtained on a Perkin-Elmer LS 55 luminescence spectrometer with 150 W Xe lamp, excitation at 355 nm.The surface area of the samples was determined by single point BET method.The BET surface area was measured on a Micromeritics ASAP 2010 with the gas composition of 30% N 2 and 70% He.XRF measurements were performed by a Rigaku EDXRF (Japan) spectrometer.

Photocatalytic testing
Photocatalytic testing of the catalysts prepared in the photodegradation of imazalil sulphate pesticide were performed at room temperature for 5 h under UV irradiation by using a 6 W UV lamp (Spectroline® ENF-260 C/FE) with the wavelength peak at 365 nm.The reaction mixture containing imazalil sulphate (100 mL, 15 mg L -1 ) and catalyst (50 mg) was put into a simple degradation box and magnetically stirred throughout the degradation process.For the first 1 h, the suspension was left to reach complete adsorptiondesorption equilibrium in dark condition.After 1 h, 10 mL of the solution was taken out and labeled as the initial time (t 0 ) of the degradation process.The UV lamp was switched on to irradiate the sample over a 5 h period.Samples were taken out using a 10 mL plastic syringe at the different time interval (1, 2, 3, 4 and 5 h) after irradiation and then filtered using a milipore membrane syringe filter of 0.45 µm in order to remove the photocatalyst particles.The degraded samples were measured using a UV-Visible spectrophotometer (Perkin Elmer Lambda 25) and the percentage of photodegradation was calculated.

RESULTS AND DISCUSSION
Physicochemical properties of Ag@TiO2 and Ag/TiO2 microspheres FESEM images of Ag@TiO 2 and Ag/TiO 2 microspheres are shown in Fig. 2. It can be seen from the Fig. 2(a) that the synthesized particles are spherical in shape with an average diameter of about 536 nm.The white arrow in Fig. 2(a) shows partly broken sphere, thus confirming that the spheres are hollow inside.Fig. 2(b) shows Ag/TiO 2 spheres of about 670 nm in average diameter with a rough surface.
TEM image shown in Fig. 3(a) displays the core-shell structure of Ag@TiO 2 particles.It can be seen that the TiO 2 layer thickness is around 63 nm.The Ag nanoparticles can be clearly observed from the TEM image, as labeled by the white circles.It can be concluded that Ag@TiO 2 core-shell particles have been successfully fabricated.The TEM image of hollow anatase TiO 2 after Ag loading is given in Fig. 3(b).The presence of Ag nanoparticles deposited on hollow anatase TiO 2 can be clearly observed in the TEM image in Fig. 3(b).It can be observed that the Ag@TiO 2 spheres have larger Ag nanoparticles than Ag/TiO 2 because the high temperature during the calcination process provides sufficient energy for the growth of Ag particles.In addition, HRTEM images of the Ag@ TiO 2 and Ag/TiO 2 particles are shown in Figs.3(c) and (d), in which the crystal lattice of TiO 2 and Ag can be identified.Lattice fringes with a lattice spacing of 0.35 and 0.24 nm correspond to the (1 0 1) plane of anatase TiO 2 and Ag (1 1 1) plane, respectively.Further information on the elemental composition of the samples can be provided by EDX analysis.Fig. 4 shows the EDX spectrum of synthesized Ag@TiO 2 while the elemental composition of Ag@TiO 2 sample is summarized in Table 1.The EDX results confirmed the presence of Ti, O and Ag elements in the microspheres.The spectrum (Fig. 5) shows that Ag/TiO 2 microspheres consist of Ti, O and Ag elements with the corresponding percentages given in Table 2.The distribution of Ag in heterostructure was confirmed by the EDX elemental mapping as shown in Fig. 6.The EDX elemental mapping clearly shows the presence of Ag particles on TiO 2 , which should correspond to the darker small spots in HRTEM image (Fig. 3(d)).
The FTIR spectrum of the Ag@C microspheres is shown in Fig. 7(a).The bands at 3119, 1707, 1605, 1190 and 797 cm -1 are attributed to the vibrations of ─OH, C═O, C═C, C─O and aromatic C─H outof-plane bending, respectively.The presence of water is supported by the OH stretching mode peak appearance at 3119 cm -1 .The FTIR spectrum of Ag@TiO 2 anatase TiO 2 microspheres (Fig. 7(b)) shows a peak at about 3334 cm -1 and an absorption band around 1643 cm -1 , which are assigned to the vibrations of the surface adsorbed H 2 O and Ti─OH bonds (Li et al., 2007a).The broad band at 507-873 cm -1 originated from the TiO 2 .The peak at 784 cm -1 is assigned to the symmetric O─Ti─O stretching while the peak at 595 cm −1 is due to the vibration of Ti─O bond (Nur, 2006).This confirms the formation of the inorganic shell and the removal of the organic components.The FTIR spectrum of Ag/TiO 2 particles is shown in Fig. 7(c) and shows similar peaks with Ag@TiO 2 microspheres.
Where D is the crystallite size of the catalyst, λ is the X-ray Fig. 3 TEM images of (a) TiO2@Ag, (b) Ag/TiO2 and (c, d) HRTEM images of Ag@TiO2 and Ag/TiO2, respectively.
wavelength (1.54 Å), β is the full width at half maximum (FWHM) of the diffraction peak (radian), K is a coefficient (0.89) and θ is the diffraction angle at the maximum peak.The crystallite size of the TiO 2 in the Ag@TiO 2 and Ag/TiO 2 estimated from their diffraction peaks width is 23.82 and 13.83 nm, respectively.The observed result is in good agreement with the obtained result from the TEM analysis and confirmed that the particles were composed of silver and anatase TiO 2 .
Fig. 4 EDX spectrum of Ag@TiO2 sample.Fig. 9 shows the DR UV-Vis spectra of the hollow anatase TiO 2 , Ag@TiO 2 , and Ag/TiO 2 microspheres.The onset of the absorption edge for hollow anatase TiO 2 sample is at λ ≈ 400 nm.It is observed that the absorption spectra of Ag@TiO 2 and Ag/TiO 2 photocatalysts are obviously different from that of hollow anatase TiO 2 .The weak peak observed at around λ = 404-650 nm is due to the surface plasmon absorption of electrons in Ag nanoparticles (Linic et al., 2011;Scirè et al., 2009;Cozzoli et al., 2004).If the typical plasmon peak of Ag colloidal nanoparticles (Lee et al., 2008b), which appears at λ ≈ 400-420 nm, is compared with those of Ag@TiO 2 and Ag/TiO 2 microspheres, a large red shift can be clearly observed.This behaviour can be related to a change in the metal particle size, where decreasing the size will result in a shift to the UV region (Albiter et al., 2013).However, it has also been explained in terms of a strong interaction between metallic silver and TiO 2 (Jiang et al., 2012;Zhang et al., 2012;Ismail, 2012;Zhang et al., 2011d;Liga et al., 2011;Wodka et al., 2010;Ashkarran et al., 2011;Liang et al., 2011;Xie et al., 2010;Awazu et al., 2008;Kubo and Tatsuma, 2006).The energy band gap of crystalline semiconductors can be calculated using the following equation: Where α is the absorption coefficient, h is Planck constant, v is the incident light frequency, A is a constant, E g is the band gap and n depends on the nature of the transitions.The transitions may have values of 1/2, 2, 3/2 and 3, which correspond to allowed direct, allowed indirect, forbidden direct and forbidden indirect transitions, respectively.In TiO 2 , n = 2 is usually considered.Experimental diffuse reflectance data cannot be used directly to measure the absorption coefficients (α) because of scattering contributions to the reflectance spectra.Scattering coefficient depends weakly on the energy and Kubelka-Munk function (F(R ∞ )), (R ∞ is the diffuse reflectance) can be considered to be proportional to the absorption coefficient within the narrow range of energy containing the absorption edge features.In such way, the determination of the absorption edge energy can be estimated from the (F(R ∞ )•hv) 1/n versus hv plot.In particular, the experimental band gap value can be obtained from the x-intercept of the straight tangent line to this plot.The indirect band gap energies (E g ) of the hollow anatase TiO 2 , Ag@TiO 2 , and Ag/TiO 2 are estimated to be approximately 3.16, 3.07 and 3.01 eV, respectively, (Fig. 10).These results indicate that the band gap energy of Ag@TiO 2 and Ag/TiO 2 was lower than that of hollow anatase TiO 2 .
Table 1 Chemical composition of the Ag@TiO2 from EDX analysis.Fig. 11 shows the photoluminescence spectra of hollow anatase TiO 2 , Ag@TiO 2 , and Ag/TiO 2 particles.It can be seen that the PL spectrum of the hollow TiO 2 is more intense than the spectra of Ag@TiO 2 and Ag/TiO 2 samples.The decrease in the intensity may be due to the capturing of the photoinduced electron by Ag, thus inhibiting recombination of electron-hole pairs.The lower PL intensity of the Ag/TiO 2 sample clearly implies lower recombination rate (Liqiang et al., 2006;Hirakawa and Kamat, 2004a;Chen et al., 2007).
From the single point BET measurement, the specific surface areas are 23.26 m 2 g -1 for hollow anatase TiO 2 , 15.22 for Ag@TiO 2 and 22.29 m 2 g -1 for Ag (out) /hollow anatase TiO 2 .When Ag nanoparticles were loaded onto the TiO 2 , aiming at improving the photocatalytic performance, the surface area value of the hollow anatase TiO 2 remained unchanged even after Ag loading, probably due to the small amount of Ag deposited on the surface of TiO 2 .The surface area of Ag@TiO 2 sample was less than that of the hollow anatase TiO 2 , which might be due to the fact that the particles aggregated and formed bigger particles.
The amount of Ag in Ag@TiO 2 and Ag/TiO 2 samples was determined by XRF measurement.Table 3 and 4 illustrate Ag content in the prepared samples.Experimental Ag mass % measured by XRF technique is in good agreement with the calculated one, revealing the effectiveness of the current preparation methods.

Evaluation of photocatalytic activity
The photocatalytic activity of Ag modified hollow anatase TiO 2 microspheres in the environment was tested out in the photodegradation of imazalil sulphate and its imazalil sulphate photodegradation efficiency was compared with that of commercial TiO 2 .The photodegradation efficiency was calculated using the Eq. (3).
Photodegradation efficiency (%) = (1-C t /C 0 ) × 100 (3) Where C 0 is the concentration of the compound before illumination and C t is the concentration of solution after time t.Fig. 12 shows the photodegradation efficiency of imazalil sulphate.The photodegradation efficiency was in the following order: Ag/TiO 2 > hollow anatase TiO 2 > Ag@TiO 2 > commercial anatase TiO 2 .The highest efficiency was shown by Ag/TiO 2 photocatalyst.5 h of irradiation resulted in degradation of 27.9% for imazalil sulphate.From Fig. 12, it can be seen that the presence of Ag nanoparticles on the surface of hollow TiO 2 gave a more efficient photodegradation than the other photocatalysts, which demonstrates the efficiency of Ag nanoparticles in reducing the recombination rate of the electron-hole pair.Even though the prepared photocatalyst contains Ag in the metallic form, it can act as an electron sink (Wang et al., 2013).When the reaction mixture is irradiated by UV, the photoexcited electrons from the valence band of TiO 2 gets captured by the Ag, allowing the generated hole to decompose the pesticides (Chiarello et al., 2008;Wang et al., 2013).Electron migration from the conduction band of TiO 2 to the metallic Ag particles is feasible since the Fermi level of TiO 2 is higher than that of Ag metal (Wang et al., 2013).When the quantity of Ag is little and the photogenerated electrons effectively migrate to Ag, better separation of electrons and holes would be accomplished.The role of Ag is to increase separation of the electronhole pair, decreasing recombination rate of the electron-hole pair and in turn, enhanced the photocatalytic efficiency of the Ag/TiO 2 photocatalyst.
Additionally, the Ag nanoparticles can impact photoluminescence properties of TiO 2 .photoluminescence phenomenon is a kind of photophysical process, and the photocatalytic reaction is a kind of photochemical process (Liqiang et al., 2006).The photoluminescence and photocatalytic processes are nearly associated with dynamic behaviours of photoinduced charge carriers of TiO 2 .Consequently, there are definite relationship between photoluminescence and photocatalysis, and the inherent relationship between photoluminescence intensity and photocatalytic performance can be shown on the basis of photoluminescence characteristics (Liqiang et al., 2006).Photoluminescence may be effectively utilized to screen recombination and generally, low-intensity photoluminescence signals indicate lower recombination rates (Liqiang et al., 2006).
According to Fig. 13, it can be seen that the lowest photoluminescence intensity of the sample corresponds to the highest photocatalytic efficiency.This is because the lower the photoluminescence intensity, the stronger the capacity of the Ag to capture photoinduced electrons, the higher the separation rate of photoinduced electrons and holes, and the higher the photocatalytic efficiency.
In other words, the Ag nanoparticles have a great effect on the separation and recombination processes of photoinduced charge carriers of Ag/TiO 2 sample and can further influence the photoluminescence performance.Therefore, it can be suggested that the photocatalytic efficiency of samples can be evaluated by means of photoluminescence measurements.
It is also proposed that Ag is more efficient when it comes to photodegradation of molecules with smaller particles (9-20 nm particle size of Ag on the TiO 2 's surface compared with 45 nm the average size of Ag particles inside the hollow TiO 2 sample).Smaller particles, implying a larger contact area between Ag and TiO 2 , were found to result in better charge separation and gave better photodegradation performance.This is in agreement with the results shown in Fig. 12.

CONCLUSION
Hollow anatase TiO 2 containing Ag, in the different location, has been successfully synthesized.This was proven by the images obtained using TEM.Apart from that, the existence of Ag was also confirmed by XRF and EDX.DR UV-Vis spectra showed the existence of absorbance peak for Ag at around 404-650 nm.The Ag's particles size was measured by TEM.The results showed that the size of Ag particles inside the hollow anatase TiO 2 sample was larger (45 nm) than its particles size when it was located outside (9-20 nm).The photocatalytic efficiency of the hollow anatase TiO 2 and hollow anatase TiO 2 containing Ag was evaluated by the degradation of imazalil sulphate pesticide under UV light irradiation.The Ag/TiO 2 particles exhibited the highest performance in the photodegradation of the aqueous phase of imazalil sulphate pesticide (35.9%) compared with the other photocatalysts used in this work.The higher photocatalytic efficiency of Ag/TiO 2 particles is related to the role of Ag on the surface of hollow anatase TiO 2 particles.The deposition of Ag onto the surface of hollow anatase TiO 2 is shown to be beneficial in increasing the efficiency of pesticide photocatalytic degradation.Ag nanoparticles can act as an electron acceptor centre, allowing electron and hole pair separation and hence leading to the overall improved photocatalytic efficiency of hollow anatase TiO 2 .

Table 2
Weight percentage of Ti, O and Ag of Ag/TiO2 microspheres.

Table 3
Elemental analysis by XRF for Ag@TiO2 microspheres.

Table 4
Elemental analysis by XRF for Ag/TiO2 microspheres.