Efficient photodecomposition of herbicide imazapyr over mesoporous Ga2O3-TiO2 nanocomposites

Accepted Manuscript Title: Efficient Photodecomposition of Herbicide Imazapyr over Mesoporous Ga2 O3 -TiO2 Nanocomposites Authors: Adel A. Ismail, Ibrahim Abdelfattah, M. Faisal, Ahmed Helal PII: DOI: Reference:

S0304-3894(17)30638-6 http://dx.doi.org/10.1016/j.jhazmat.2017.08.046 HAZMAT 18811

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

29-5-2017 12-8-2017 16-8-2017

Please cite this article as: Adel A.Ismail, Ibrahim Abdelfattah, M.Faisal, Ahmed Helal, Efficient Photodecomposition of Herbicide Imazapyr over Mesoporous Ga2O3-TiO2 Nanocomposites, Journal of Hazardous Materialshttp://dx.doi.org/10.1016/j.jhazmat.2017.08.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Efficient Photodecomposition of Herbicide Imazapyr over Mesoporous Ga2O3-TiO2 Nanocomposites Adel A. Ismaila,b*, Ibrahim Abdelfattahc, M. Faisald, Ahmed Helalb a

Wastewater Treatment and Reclamation Technologies (WTRT), Water Research Center, Kuwait Institute for Scientific Research, P.O. Box 24885 Safat 13109 Kuwait. Tel: (+965) 24989820/6079, Fax: (+965) 24989820. E. mail: [email protected] b Advanced Materials Department, Central Metallurgical R& D Institute, CMRDI, P.O. Box 87, Helwan 11421, Cairo, Egypt. c Water Pollution Research Dept., National Research Centre, 33 EL Bohouth St. (Former EL Tahrir St.), P.O. 12622, Dokki, Giza, Egypt. d Advanced Materials and Nano-Research Centre, Najran University, P.O. Box: 1988, Najran 11001, Saudi Arabia. Graphical abstract

RESEARCH HIGHLIGHTS 

One pot synthesis of mesoporous Ga2O3-TiO2 nanocomposites has been demonstrated.



Heterogeneous photocatalytic process was employed for degrading of imazapyr.



XRD and TEM exhibited crystalline anatase TiO2 (10±2 nm with mesoporous structure.



0.1% Ga2O3-TiO2 is able to degrade 98% of imazapyr herbicide along 180 min.



Photodegradation rate of 0.1%Ga2O3-TiO2 is 3-fold higher than that UV-100.

Abstract: The unabated release of herbicide imazapyr into the soil and groundwater led to crop destruction and several pollution-related concerns. In this contribution, heterogeneous photocatalytic technique was employed utilizing mesoporous Ga2O3-TiO2 nanocomposites for degrading imazapyr herbicide as a model pollutant molecule. Mesoporous Ga2O3-TiO2 nanocomposites with varied Ga2O3 contents (0 - 5 wt%) 1

were synthesized through sol-gel process. XRD and Raman spectra exhibited extremely crystalline anatase TiO2 phase at low Ga2O3 content which gradually reduced with the increase of Ga2O3 content. TEM images display uniform TiO2 particles (10±2 nm) with mesoporous structure. The mesoporous TiO2 exhibits large surface areas of 167 m2g-1, diminished to 108 m2g-1 upon 5% Ga2O3 incorporation, with tunable mesopore diameter in the range of 3 - 9 nm. The photocatalytic efficiency of synthesized Ga2O3TiO2 nanocomposites was assessed by degrading imazapyr herbicide and comparing with commercial photocatalyst UV-100 and mesoporous Ga2O3 under UV illumination. 0.1% Ga2O3-TiO2 nanocomposite is considered the optimum photocatalyst, which degrades 98% of imazapyr herbicide within 180 min. Also, the photodegradation rate of imazapyr using 0.1%Ga2O3-TiO2 nanocomposite is nearly 10 and 3fold higher than that of mesoporous Ga2O3 and UV-100, respectively. The high photonic efficiency and long-term stability of the mesoporous Ga2O3-TiO2 nanocomposites are ascribed to its stronger oxidative capability in comparison with either mesoporous TiO2, Ga2O3 or commercial UV-100.

Keywords: Mesoporous; Ga2O3-TiO2; Nanocomposites; Photodegradation; Imazapyr herbicide.

I.

Introduction

Many metal oxides containing d0 electron configurations like Ti, Zr, Nb, and Ta oxides and d10 electron configurations like Ga, Ge, In, Sn, and Sb oxides exhibiting outstanding photocatalytic performances[1]. Semiconductors with a d10 electron configuration have attracted considerable attention for superior photocatalytic activities, mainly due to their conduction bands being formed by hybridized sp orbits with a large dispersion, which makes them able to generate photoexcited electrons with large mobility.[2-7] As a representative of such d10 materials, gallium oxide (Ga2O3) exhibits high activity in water-splitting and in the degradation of organic pollutants. [2-7] Among the five polymorphs of Ga2O3, the β phase with a monoclinic structure was reported to be commonly formed under ordinary conditions [8]. β-Ga2O3 is the most stable polymorph over the entire temperature range until the melting point, whereas all polymorphs are metastable and convert into the β-Ga2O3 at temperatures > 750-900 °C [9, 10]. Ga2O3 has drawn much attention for its potential application in optoelectronic devices, gas sensors, spintronic devices, due to their wide bandgap energy (Eg = 4.2 to 4.7 eV) and good luminescence properties.[11-13] In addition, Ga2O3 is not classified as hazardous according to Worksafe Australia criteria. This nontoxic Ga2O3 has a significant potential for photocatalytic air purification, particularly for the elimination of toxic aromatic 2

compounds[4]; Ga2O3 was modified to explore its photocatalytic activity[14-17]. It is therefore reasonable to believe that the Ga2O3 photocatalyst is stable and does not suffer from the problem of photocorrosion[4]. Since TiO2 photocatalyst came into existence for H2O splitting through the electrochemical photolysis in 1972[18], a lot of efforts have been made to find out unprecedented photocatalyst materials. Mixed oxides have paid attention for their potential application in photovoltaic, photo/-catalytic, electronic, and energy storage.[19, 20] Doping of different transition metals ions (Fe3+, W6+, Ru3+, and Cu2+) into TiO2 have been documented to enhance its photocatalytic activity and generate of an optical absorption.[21-24] Ga2O3-TiO2 composite has been also reported to be an interesting material possessing huge potential purposes in photocatalysis, electrocatalysis, and sensors[25-27]. Now a days mesoporous materials proven to be the potential applicants for separation technology, batteries, catalysis, gas sensors, molecular sieves and electronics applications.[28-31]. Crystallinity and long term thermal stability of mesoporous TiO2 makes it an interesting material for photocatalytic applications which may be advantageous compared to discrete nanoparticles.[32-34] Imazapyr [((RS)-2-(4-methyl-5-oxo-4-propan-2-yl-1H-imidazol-2-yl) pyridine-3-carboxylic acid] herbicides are considered to be nonselective for grasses control and it displays no aquatic utilization; however, it is observed in the water surface through spray drift and it can deeply permeate into the groundwater causing potential unfavorable environmental impacts.[35,36] Imazapyr preserves in soil without degradation within 6 -12 months and it is highly soluble in water, and it damages plants even at its minimal concentration in soil [0.05 mg/kg].[35,36] The maximum permission of herbicide or pesticide in drinking water is 0.1 μg/L by The European Union.[37] The removal of imazapyr has been investigated by different techniques such as electrochemical oxidation, adsorption and chemical oxidation[36,37]. Chemical oxidations such as chlorine dioxide, ozone and chlorine have disadvantage when employed for degradation of organic compounds, owing to the secondary products of oxidation are poisonous [38]. Advanced oxidation processes (AOPs) are effective new approaches characterized by •OH radical generation. Among AOPs, heterogeneous photocatalysis employing TiO2 photocatalyst is achieving maximum productivity with minimum lost effort for treatment technology.[39] Recently, photocatalytic degradation of herbicide imazapyr was widely evaluated by commercial TiO2 and mesoporous TiO2, Al2O3 and WO3 doped TiO2 [22, 32, 34, 40, 41]. To the best of our knowledge, there are no reports of photodegradation of herbicide imazapyr using mesoporous Ga2O3-TiO2 nanocomposites. Thus, a fundamental study that can verify the correlation between Ga2O3-TiO2 nanocomposites at Ga2O3 contents and their photodegradation is of great importance to be addressed. In this contribution, our strategy for synthesizing mesoporous Ga2O3-TiO2 nanocomposites depend upon one step process through 3

hydrolysis homogeneous mixture of tetrabutyl orthotitanate with Ga(NO3)3 in the presence of triblock copolymer. Since each Ga2O3 exhibited some photocatalytic activity, we expected that the mesoporous Ga2O3-TiO2 photocatalysts could demonstrate promising photocatalytic performances for herbicide imazapyr degradation.

II. Experimental II.1. Materials: Tetrabutyl orthotitanate Ti[OC(CH3)3]4 (TBOT), Gallium(III) nitrate hydrate, (Ga(NO3)3·xH2O, 99.9%), Imazapyr (C13H15N3O3> 99%), HCl, C2H5OH, CH3OH, H3PO4 and CH3COOH, the triblock copolymer surfactant EO106-PO70EO106 (F-127), MW 12600 g/mol), were purchased from Sigma-Aldrich. Commercial TiO2 (Hombikat UV-100, 100% anatase and 230 m2g-1 surface area) was kindly provided by Sachtleben Chemie GmbH. II.2. Preparation of mesoporous G2O3-TiO2 nanocomposites: Mesoporous G2O3-TiO2 nanocomposites were prepared via sol-gel process using F127 triblock copolymer. Typically, 1.6 g of F127 was added to 30 mL of ethanol with stirring for 60 min, and then 0.74 mL of HCl , 3.5 mL of TBOT and 2.3 mL of CH3COOH were added dropwise to F127 solution under magnetic stirring for 30 min [22,23]. The calculated amount of Ga(NO3)3·xH2O was added to the mixture mesophase (F127-TBOT- CH3COOH) with vigorously stirring for 60 min to obtain 0.5, 1, 2, 3 and 5 wt% Ga2O3-TiO2 nanocomposites. The prepared mesophase was put into 40% humidity chamber at 40 °C for 12 h to evaporate C2H5OH and form gel and dried at 65 °C for 24 h. Afterward, it was calcined at 500 °C in air at a heating rate of 1 °C/min to reach 500 °C for 4 h to remove the template and to produce mesoporous G2O3-TiO2 nanocomposites at different G2O3 content. II.3. Characterization X-ray diffraction (XRD) was carried out on a Bruker AXS D4 Endeavour X diffractometer using Cu Kα1/2, λα1=154.060 pm, λα2 = 154.439 pm radiation. JEOL JEM-2100F electron microscope (Japan) was employed to determine transmission electron microscopy (TEM) images operated at 200 kV. Spectrofluorophotometer, (RF-5301 PC, Japan, SHIMADZU, 400 W, 50/60 Hz) was used to record photoluminescence (PL) at room temperature employing xenon lamp excitation source (150 W) at excitation λ ~365 nm. The nitrogen adsorption and desorption isotherms at 77 K were performed employing a Quantachrome Autosorb equipment and the obtained samples were degassed at 200 °C overnight. The sorption data were analyzed using the Barrett-Joyner-Halenda (BJH) model with Halsey equation[42]. Fourier transforms infrared spectrometer (FT-IR) spectrum was measured in KBr dispersion 4

in the range of 400 to 4000 cm-1 using Perkin Elmer. Raman-scattering was recorded at room temperature with the Ar+ laser line (ʎ; 513.4 nm) as an excitation source. II.4. Photocatalytic activity tests 0.05 g photocatalyst and 10 mM KNO3 was added in 50 mL of water and then it was sonicated in an ultrasonic cleaning bath for 15 min to disperse the photocatalyst. KNO3 was added to keep the ionic strength of the solution to equivalent the excess of HCl (pH = 4). The imazapyr concentration [0.08 mmol L-1] was maintained to carry out the experimental work. To reach adsorption equilibrium, the imazapyr and photocatalyst were kept under continuous magnetically stirring at 300 rpm for 4 h at 25± 1 oC. Illumination experiments were conducted under top irradiation of a borosilicate glass beaker and the photonic flow was ρ = 2 mWcm-2. The samples were taken at regular time intervals for analysis. The analysis of imazapyr concentrations was measured through high performance liquid chromatography (HPLC) system from Agilent Technologies 1260 Infinity composed of a G1311C-1260 Quat pump and a G1365D-1260 MWD UV Detector adjusted to 254 nm. An Agilent Eclipse plus C18 column employing at room temperature was performed as stationary phase, and a mixture of water and methanol (70:30 %v/v) employing as mobile phase at pH value ~ 3 by adding H3PO4 . The flow rate was fixed at 0.8 mL.min-1 and the retention time at 4.60 min. A calibration curve (R2 =0.9996) was determined from the patrons of 6 different concentration analysis at range 0-0.08 mmol L-1. The HPLC analysis was measured 2-3 replicates, allowing initial reaction rates to be considered with a mean experimental error ± 5 %. This error is determined to be the sum of the HPLC instrument error and the error intrinsic to mathematical calculations from the experimental concentration as a function of time plots. The initial rate for imazapyr photodegradation was calculated through the first 30 minutes of UV irradiation. The photonic efficiency was calculated as given in the following equation[43].



r  100 I

where ξ is the photonic efficiency (%), r the photodegradation rate of imazapyr (mol L-1s-1), and I the incident photon flux (7.03 x 10-6 Ein L-1s-1). The UV-A incident photon flow was measured by ferrioxalate actinometry [44].

Results and Discussion Ga2O3-TiO2 nanocomposites synthesized from TBOT and Ga(III) as the inorganic precursors with employing triblock copolymer as the template were achieved by a simple one-step. The mesophase 5

containing nanoparticles were grown slowly and polymerize to form transparent gel. The produced amorphous inorganic nanoparticles were heat treated at 500 °C to nucleate and grow the nanocrystalline matrix and obtain Ga2O3-TiO2 nanocomposites at different Ga2O3 contents. XRD patterns of mesoporous TiO2 and Ga2O3-TiO2 nanocomposites at different Ga2O3 contents are shown in Figure 1. XRD patterns were compared with the JCPDS-ICDD standards for anatase (21-1272). The patterns for TiO2 are essentially identical, displaying peaks at 25.4°, 36.4° , 48.1° , 54.2° and 62.8° that are compatible with the (101), (004), (200) ), (211) and (213) planes related tetragonal anatase. It is clearly seen that the crystallinity of anatase slightly decreases with the increase of Ga2O3 contents. On the other hand, no evident diffraction Ga2O3 peaks assigned owing to its amorphous phase and it cannot be transformed to crystalline β- Ga2O3 at 500 °C [45]. The anatase intensity peaks decrease with the increase of Ga2O3 contents and no significant shifts of the anatase peaks were observed, suggesting that Ga2O3 incorporated into TiO2 lattice. This finding could prove the Ga3+ substitutes Ti4+ as ionic radius of Ti4+ (0.75 Å) is very adjacent to that of Ga3+ (0.62 Å)[46]. The crystallinity of the mesoporous TiO2 and Ga2O3-TiO2 nanocomposites at different Ga2O3 contents obtained upon calcination at 500 °C was conducted by Raman spectroscopy (Figure 2a). The Raman spectra of all prepared samples evidence that these samples entirely consists of anatase phase at Eg [144.4, 199, 641 cm-1], B1g [399, 518 cm-1], A1g [518 cm-1][47]. While with increasing Ga2O3 contents, the peak intensity of TiO2 anatase phase was gradually reduced upon 3%Ga2O3. However, in case of 5%Ga2O3, the intensity peak of anatase phase was declined. It can be deduced that a certain extent of Ga3+ can be integrated in anatase due to peak positions are not shifting any more. The results obtained from Raman spectra are compatible with the XRD analysis (Figure 1).

FT-IR spectra demonstrate the absorbance of TiO2 and Ga2O3-TiO2 nanocomposites at different Ga2O3 contents as given in Figure 2b. For mesoporous TiO2, two main peaks assigned at 3400 cm-1 and 1626 cm1.

The weak absorption at 1626 cm-1 is attributed to the deformation vibration for H-O-H bonds of the

surface-adsorbed Ti-OH bonds and H2O molecules.[48] The band at 3400 cm-1 recognizes the OH¯ groups of Ti-OH at weak surface active sites[49]. The isolated OH¯ group vibrations absorption bands at 3665 and 3715 cm-1 were not detected, which suggesting a high degree anatase surfaces hydration [50]. It is believed that pure Ga2O3 exhibits two main absorption bands at 680-720 and 460-500 cm-1 and pure anatase has absorption bands at 550-670 cm-1. The FTIR spectra exhibit no distinct absorption peaks that can be explained by C-H stretching and bending modes, indicating that the templates were removed by annealing at 500 °C. 6

Nitrogen adsorption-desorption isotherms of the mesoporous TiO2, 0.1%Ga2O3-TiO2 and 5% Ga2O3-TiO2 nanocomposites were depicted in Figure 3. The pore size distribution at a mean value was calculated from the adsorption branch on the basis of the BJH model.[42] The surface areas and the pore parameters of mesoporous TiO2, Ga2O3, Ga2O3-TiO2 nanocomposites are summarized in Table 1. The pore size distribution showed a narrow pore diameter range at 3.2-8.9 nm. The samples show similar type-IV isotherms for mesoporous solids. Also, the hysteresis loop indicates the loss in long-range ordering of the mesopores and it can explained by the voids between non-ordered particles.[51] The main pore sizes increased to 8.9 nm upon addition of 5%Ga2O3. Specific surface area of mesoporous TiO2 ~167 m2.g-1 was slightly reduced to 108 m2g-1 after adding 5%Ga2O3, which suggested that at high Ga2O3 content it blocked the pores of TiO2. The addition of Ga2O3 into mesoporous TiO2 network at low contents does not appear to collapse the TiO2 mesoporous structure.

TEM images of mesoporous TiO2 and 0.1%Ga2O3-TiO2 and 3%Ga2O3-TiO2 nanocomposites are presented in Fig. 4. Mesoporous TiO2 and 0.1%Ga2O3-TiO2 are consisted of spherical and rhombohedra nanoparticles, as demonstrated by the TEM image in Fig. 4a,b. TiO2 nanoparticles are not agglomerated and quite uniform in shape and size with an average diameter of about 10 nm (Fig. 4a,b ). It exhibits the randomly mesoporous channels and TiO2 nanoparticles have an average size of 5-10 nm (Fig. 4a,b). At loadings of 3%Ga2O3 into mesoporous TiO2 network, agglomerates form showing amorphous Ga2O3 into TiO2 networks (Fig. 4c). Fig. 4d shows the HRTEM image of the resulting anatase TiO2 nanoparticles, one can clearly observe a 0.35 nm aligned anatase phase grown along [101] direction. The selected area electron diffraction (SAED) patterns (Fig 4d, inset) further confirm that anatase are composed.

Photocatalytic activity evaluation It is well known that TiO2 is the most efficient candidate for photocatalysis reactions and the addition of Ga2O3 at very low concentration has led to increase the separation efficiency of charge carriers pairs. The photocatalytic performances of mesoporous TiO2, Ga2O3 and Ga2O3-TiO2 nanocomposites at different Ga2O3 contents were assessed by degrading herbicide imazapyr and comparing with either commercial mesoporous Ga2O3 or Hombikat UV-100 under UV illumination. The adsorption of imazapyr has been conducted in dark for 6 h and the findings reveals that only 5-10% imazapyr was adsorbed for all prepared photocatalysts. Figure 5a shows the relation between imazapyr concentration and illumination time within 180 min. The initial photodegradation rates for all prepared samples are summarized in Table 1. The 7

results indicated that the imazapyr degradation rate of mesoporous Ga2O3 was 2.51×10-8 molL-1s-1. Mesoporous TiO2 was increased from 1.75×10-7 to 2.41×10-7 molL-1s-1 with the increase Ga2O3 content from 0 - 0.1% and then decreased to 3.63×10-8 molL-1s-1 at 5%Ga2O3 content whereas the imazapyr photodegradation rate using commercial Hombikat UV-100 was 9.28×10-8 molL-1s-1. These results indicates that the photodegradation rate of imazapyr using 0.1%Ga2O3-TiO2 nanocomposite is nearly 3 and 10-fold higher than that mesoporous Ga2O3 and UV-100, respectively. Additionally, the photocatalytic efficiency of imazapyr using mesoporous TiO2 and 0.1%Ga2O3-TiO2 nanocomposite was 76% and 98%, respectively within 180 min, however, the photocatalytic efficiency of mesoporous Ga2O3 and 5% Ga2O3-TiO2 nanocomposite was 19 and 33%, respectively. (Figure 5b & Table 1). The photonic efficiency of Ga2O3-TiO2 nanocomposites, pure Ga2O3 and UV-100 for imazapyr photodegradation was calculated and summarized in Table 1. The results revealed that the photonic efficiency of mesoporous TiO2 is 2.49%, which increases with the increase Ga2O3 content up to 0.1%Ga2O3-TiO2 nanocomposite with the maximum photonic efficiency being 3.43%. Subsequently, the photonic efficiency gradually decreases with increasing Ga2O3 content to reach 0.51 % at 5% Ga2O3-TiO2 nanocomposite, however, the photonic efficiency of pure Ga2O3 and UV-100 photocatalyst is 0.36 and 1.42%, respectively. Such high photonic efficiency of the mesoporous TiO2 as compared with commercial UV-100 can be attributed to facile diffusion of imazapyr to reach the active sites and and low light scattering of photons[22, 33, 51]. On the other hand, the addition of Ga2O3 more than 0.1% leads to decrease the photodegradation rates and hence reduce the photonic efficiency (Table 1). One can be explained the reasons beyond the high photocatalytic performance of the mesoporous 0.1% Ga2O3-TiO2 nanocomposite as compared with either pure Ga2O3 or commercial UV-100 : i) the larger specific surface area of 0.1% Ga2O3-TiO2 nanocomposite is advantageous for liquid-solid photocatalysis due to the enlarged irradiated area. But this alone cannot explain the superior photocatalytic activity of the 0.1% Ga2O3-TiO2 nanocomposite, as compared with that of commercial UV-100, ii) 3D mesoporous TiO2 and 0.1%Ga2O3-TiO2 networks provide more accessible sites for both adsorption and photocatalysis potential, more imazapyr molecules are adsorbed and then photocatalytically degraded as a result of fast transport of imazapyr moiety to the active sites, while it is hindered by commercial UV-100 photocatalyst, iii) the irradiated mesoporous 0.1% Ga2O3-TiO2 photocatalyst possesses the light utilization more efficiently as a result of the lower light scattering of the mesoporous photocatalysts.[29, 33, 52] In general, at very low Ga2O3 content, 0.1% Ga2O3-TiO2 nanocomposite exhibits superior photocatalytic performance. However at high Ga2O3 content, its wide band gap and low crystallinity might be one obstacle. This has been 8

attributed to Ga2O3 acts as a charge recombination center and slow reduction of molecular O2 by the trapped photoelectrons reaction has occurred, whereby e-/h+ recombination is favoured. On the other hand, the decrease of photocatalytic activity of pure Ga2O3 can be explained by its amorphous phase materials with large band gap energy and it is needed UVC illumination (emission wavelength 254 nm) to supply excitation light; however, in the present work UVA illumination (emission wavelength 365 nm) was employed. From above results, the proposed mechanism could be suggested: If surface defect states exist, it may be able to trap the electron or hole, the recombination is suppressed and the rate of oxidation-reduction reactions may be increased (scheme 1). The large populations of Ga2O3 defects have been found in samples prepared at low Ga2O3 content. The large numbers of defects consist of robust acceptor states in the band gap, trapping the holes and preventing recombination.[52] Various defect bands promote the electron-hole pair separation rate.[53] The enhanced photocatalytic performance is mainly derived from the large numbers of acceptor states accompany with Ga2O3 defects especially in amorphous phase (scheme 1). The acceptor states not only expand the light absorption edge of UV but also retard the rate of electron-hole pair recombination. It believe that both large numbers of defects and acceptor states are responsible for enhancing photocatalytic performance.[53] However, 5% Ga2O3-TiO2 nanocomposite is lower photocatalytic performance than all samples due to the higher recombination rate of the electron-hole pairs. To confirm the suggested mechanism, the PL was employed to explore the influence of Ga2O3 on Ga2O3-TiO2 conjunction. The PL spectra were performed by exciting mesoporous TiO2 and Ga2O3-TiO2 nanocomposites at 365 nm and the observed transitions represent the electronic behavior of the samples are shown in Fig. 6. The results reveal that mesoporous TiO2 and 0.1%Ga2O3-TiO2 nanocomposite display one luminescence emission at 468 nm due to the recombination of charge carriers excited on Ga2O3 surface.[54] With increasing the Ga2O3 content, luminescence emission peak was assigned at 445 nm. The emission at 445 nm can be explained by surface defects,[55,56] Also, the intensity of PL emission spectra of Ga2O3-TiO2 nanocomposites at 5wt% Ga2O3 content is higher than that mesoporous TiO2 and 0.1%Ga2O3-TiO2 (Fig. 6). These results further emphasizes that the recombination rate of electron-hole of Ga2O3-TiO2 nanocomposites at high content is faster than that the mesoporous TiO2 and 0.1%Ga2O3TiO2 nanocomposite. The decrease of PL intensity of the 0.1%Ga2O3-TiO2 nanocomposite point out a high photonic efficiency [22]. Thus, at 0.1% Ga2O3-TiO2 nanocomposite, the Ga2O3 impurity level represented as a charge carriers separation center. However, at 5wt% Ga2O3 content, the Ga2O3 impurity energy level is considered to be as a charge carriers recombination center.[22] Therefore, with the optimal 9

Ga2O3 content, the PL spectrum has a minimal intensity at 0.1 wt% Ga2O3-TiO2 nanocomposite, which matching with the photocatalytic reactions under UV illumination as shown above. The reproducibility and stability of 0.1% Ga2O3-TiO2 nanocomposite is significant criteria for feasible applications. The photocatalytic stability and recyclability of 0.1% Ga2O3-TiO2 nanocomposite was evaluated for the photodegradation of herbicide imazapyr up to five cycling runs as shown in Fig. 7. For each reaction cycle, the photocatalyst was separated from the reaction mixture by centrifugation. The findings demonstrate the effect of cycle times of photocatalyst on the photodegradation efficiency of herbicide imazapyr. After recycling five times, the photodegradation efficiency of herbicide imazapyr continues to maintain over 95%. This slight decrease in photocatalytic performance after five cycling runs might be due to the loss of photocatalysts during centrifugation and washing processes. However, only little reduce in degradation performance of 0.1% Ga2O3-TiO2 nanocomposite after 5 recycling runs showing the highly stable nature of designed 0.1% Ga2O3-TiO2 nanocomposite. IV.

Conclusion

In summary, mesoporous Ga2O3-TiO2 nanocomposites at different Ga2O3 (0-5 wt %) have been synthesized by sol-gel method. XRD revealed that crystalline anatase phase at low Ga2O3 contents was proved; the crystallinity was reduced at high Ga2O3 contents. On the other hand, no evident diffraction Ga2O3 peaks assigned owing to its amorphous phase and cannot be transformed to crystalline β- Ga2O3 through calcination at 500 °C. TEM images indicated that the prepared materials have mesopores structure with 10±2 nm particle size. Mesoporous 0.1% Ga2O3-TiO2 nanocomposite was found to be highly photoactive and stable toward mineralizing imazapyr under UV illumination. The overall photodegradation rate of 0.1% Ga2O3-TiO2 nanocomposite is significantly 10 and 3-fold higher than that mesoporous Ga2O3 and commercial UV-100 Hombikat, respectively. The superiority of 0.1% Ga2O3-TiO2 nanocomposite is explained by the large surface area, high crystallinity and mesoporous structure. With these characteristics of 0.1% Ga2O3-TiO2 nanocomposite, the separation of photoinduced electron hole pairs is suppressed. It believe that both large numbers of defects and acceptor states are responsible for enhancing photocatalytic performance.

10

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Figure captions Figure 1. XRD patterns of of mesoporous TiO2(a), Ga2O3-TiO2 nanocomposites at different Ga2O3 contents 0.5% Ga2O3 (b), 1% Ga2O3 (c), 3% Ga2O3 (d), and 5% Ga2O3 (e) calcined at 500 °C for 4 hours. Shifted for sake of clarity. Figure 2. (A) Raman spectra of mesoporous TiO2(a), Ga2O3-TiO2 nanocomposites at different Ga2O3 contents; 0.5% Ga2O3 (b), 1% Ga2O3 (c), 3% Ga2O3 (d), and 5% Ga2O3 (e). (B) FT-IR spectra of mesoporous TiO2(a), Ga2O3-TiO2 nanocomposites at different Ga2O3 contents; 0.5% Ga2O3 (b), 1% Ga2O3 (c), 3% Ga2O3 (d), and 5% Ga2O3 (e) Shifted for sake of clarity. Shifted for sake of clarity. Figure 3. N2 sorption isotherms and pore size distributions (inset) of the mesoporous of Nitrogen adsorption-desorption isotherms of the mesoporous TiO2, 0.1%Ga2O3-TiO2 and 5% Ga2O3-TiO2 nanocomposites. Figure 4. TEM images of mesoporous TiO2(a) and 0.1%Ga2O3-TiO2(b) and 3%Ga2O3-TiO2(c) nanocomposites. HRTEM image of TiO2 anatase phase using (101) (d), The insets show the SAED patterns for the anatase phase(d). Figure 5. (a) Change in imazapyr concentration as a function of illumination time in the presence of mesoporous TiO2 and Ga2O3-TiO2 nanocomposites at different Ga2O3 contents compared with Hombikat UV-100 as a reference.(b) Comparison of photodegradation efficiency of mesoporous TiO2 and Ga2O3TiO2 nanocomposites at different Ga2O3 contents compared with Hombikat UV-100 as a reference for the decomposition of imazapyr. Photocatalyst loading, 1 g/L; 0.08 mmol l-1 aqueous solution of imazapyr (O2saturated, pH= 4; T = 25± 1 oC); reaction volume, 50 mL; Io = 2 mW/cm2 (ca. > 320 nm). Figure 6. PL spectra of mesoporous TiO2 and Ga2O3-TiO2 nanocomposites at different Ga2O3 contents (0.1% Ga2O3, 0.5% Ga2O3, 1% Ga2O3, 3% Ga2O3, and 5% Ga2O3). Figure 7. Recyclability up to 5 times of Imazapyr photodegradation over 0.1% Ga2O3-TiO2 nanocomposite.

14

Figure 1

Intensity (a.u.)

(e)

(d) (c) (b)

(a) 20

30

40

50

60

2/ o

15

70

80

Figure 2

(A)

(e)

Intensity (a.u.)

(d) (c) (b)

(a) 1000

800

600

400

200

Wavelength/ cm-1

(B)

Intensity (a.u.)

(e) (d) (c) (b) (a) 4000

3000

2000

1000

Wavelength/ cm-1 16

Figure 3

160

120 100 80

Pore volume/ (cc/g)

Volume adsorbed/ (cc/g)

140

0.6 0.5 0.4 0.3 0.2 0.1 0.0

60

2

4

6

8

10 12 14 16

Pore diameter/ (nm)

40 Meso-TiO2

20

0.1% Ga2O3-TiO2 5% Ga2O3-TiO2

0 0.2

0.4

0.6

Relative pressure P/Po

17

0.8

Figure 4

(a)

(b)

(d)

(c)

18

Figure 5

Meso-TiO2 Ga2O3 0.1% Ga2O3-TiO2 0.5% Ga2O3-TiO2 1% Ga2O3-TiO2 3% Ga2O3-TiO2 5% Ga2O3-TiO2 UV-100

1.0 0.9 0.8 0.7

0.5 0.4 0.3 0.2 0.1 0.0 0

20

40

60

80

100

120

140

160

180

Illumination time/ min

(b)

100

% Photondegradation effieincy

Co-C

0.6

(a)

80

60

40

20

0

0

0 -1 V U

O2

i -T O3 a2

O2

i -T O3 a2

G

G

5%

3%

iO 2 -T O3 a2

G

G

iO 2 -T O3 a2 G iO 2 1% -T O3 a2

1%

5% 0.

0.

O2

Ti

O3 a2

o-

es

G

M

19

Figure 6

60

PL intensity ( a.u.)

Meso-TiO2 0.1% Ga2O3-TiO2

0.5% Ga2O3-TiO2 40

1%Ga2O3-TiO2 3%Ga2O3-TiO2 5% Ga2O3-TiO2

20

0 400

450

500

Wavelength/ nm

20

550

600

Figure 7

% Photodegradation efficiency

100

80

60

40 1

2

3

4

Recycle Times

21

5

Scheme 1. Schematic explanation of the suggested mechanism to illustrate the enhancement of photocatalytic performance of Ga2O3-TiO2 nanocomposites for Imazapyr photodegradation under UV illumination.

22

Table 1. Textural properties of mesoporous Ga2O3-TiO2 nanocomposites at different Ga2O3 contents calcined at 500 °C and commercial UV100 Hombikat and their photocatalytic properties.

Meso-TiO2

SBET/ m2g-1 167

Dp (nm) 6.53

Vp (cm3/g) 0.267

r x 107 ( molL-1s-1) 1.75

Photonic efficiency, % 2.49

Removal efficiency,% 76.0

Pure Ga2O3

25

-

-

0.251

0.36

19.4

0.1%Ga2O3-TiO2 142

8.86

0.265

2.41

3.43

98.1

0.5%Ga2O3-TiO2 135

8.91

0.276

1.35

1.93

79.1

1%Ga2O3-TiO2

122

8.82

0.279

1.09

1.56

74.7

3%Ga2O3-TiO2

116

4.60

0.310

0.663

0.934

63.7

5%Ga2O3-TiO2

108

3.20

0.380

0.363

0.517

33.0

UV-100

232

-

-

0.982

1.42

63.0

Samples

SBET Surface area, Vp pore volume, Dp pore diameter.

23