La2Sn2O7/g-C3N4 nanocomposites: Rapid and green sonochemical fabrication and photo-degradation performance for removal of dye contaminations

Graphical abstract


Introduction
Nowadays, the deficiency of drinking water sources has become a serious crisis for the future of the world due to the existence of numerous artificial dyes and poisonous organic impurities in aqueous environments. There are many methods to remove artificial dyes from drinking water such as nanofiltration [1], adsorbent [2], biosorption [3] and the photocatalytic process [4,5]. Among them, the photocatalytic activity received more attention because of its privileges of low energy consumption, high stability, environmental and economical friendly [6][7][8][9]. The mechanism of photocatalytic process is described in the following path: (1) the absorption of photons with energy ≥ the bandgap of nanophotocatalyst, (2) photoexcited electrons create at conduction band and the same amount of positive holes at valence band, (3) The various oxidants of OH • , O 2 • and H 2 O 2 could proficiently oxidize dye compounds into harmless combinations of CO 2 and water [10][11][12].
Numerous efforts have been prepared in the field of produce various forms of photocatalyst such as graphene-based nanocomposite [13,14], binary oxides [10,15] and ternary nano-photocatalyst [16]. La 2 Sn 2 O 7 Pyrochlore-type oxide as a semiconductor nano-oxide has an appropriate performance in diverse applications such as photocatalysis [17], energy storage [18,19] and catalysis [20,21]. Owning to the intense efficacy of fabrication approaches on the form and dimension of nanoproducts, the preparation path of pyrochlore La 2 Sn 2 O 7 nano samples is significant. La 2 Sn 2 O 7 nano-sized structures were created via several chemical techniques [17,22,23]. Despite the extensive researches carried out on nano-photocatalyst synthesis and performance, there is still an essential requirement to suggest a beneficial nano-scale sample through a low-cost, fast and eco-friendly way. Recently, green chemistry-based methods have been noticed to the creation of diverse nanostructures due to their non-hazardous and safe features to the environment [24][25][26]. The avail of Broccoli extract to synthesize the nanosized La 2 Sn 2 O 7 structures has not yet been considered. This work offers a rapid eco-friendly sonochemical method to fabrication of La 2 Sn 2 O 7 nanostructures as Uv-light-sensitive photocatalyst with the aid of Broccoli extract, for the first time. Ultrasonic irradiations by increasing reaction rate owning to create of high temperature and pressure in a liquid medium are led to chemical reaction accomplishment in a short time [27]. The sonochemical technique is more favorable in terms of low cost, simply control the form and particle dimension through changing of ultrasonic time and power, low processing temperature, simplicity and potential for large-scale production [28,29]. Compared with other chemical approaches, green sonochemistry has the benefits of quicker reaction rate, lower particle size, more uniformity and purity for the synthesis of nano-scaled powder [30]. Broccoli is rich in phytochemical components including glucosinolates and polyphenols. These agents can conjugate to created nuclei, control their growth and finally create favorable nano-sized samples [31,32]. Therefore, broccoli extract was utilized as both green surfactant and alkaline agent [33]. Despite many research about La 2 Sn 2 O 7 structures, the effect of sonochemical reaction condition using the broccoli extract as a green surfactant on their photocatalytic properties of remain unclear. In the present work, it is aimed at considering nano-sized La 2 Sn 2 O 7 structures synthesized by ultrasonic irradiation in presence of green broccoli surfactant. Graphitic carbon nitride (g-C 3 N 4 ) polymeric component was selected to the formation of La 2 Sn 2 O 7 /CN nanocomposite because of suitable energy gap (2.7 eV), high surface area and cheap synthesis route as well as finally, advanced photocatalytic activity [34]. Numerous researches illustrate that using g-C 3 N 4 has a favorable impact on photocatalytic efficiency [35,36]. In this work, La 2 Sn 2 O 7 / g-C 3 N 4 nanocomposites were fabricated via green and rapid ultrasonic technique in presence of broccoli extract natural surfactant. After characterization of obtained nano-products in various experimental circumstances, photo-degradation efficiency of them was evaluated in several experimental tests such as particle size, weight ratio of LSO:CN, type of dye, scavenger kind, dye and catalyst loading. Moreover, the photo-driven degradation mechanism of erythrosine dye by LSO/CN nanocomposite was studied.

Materials and physical measurements
All the chemical reagents for the synthesis of La 2 Sn 2 O 7 / g-C 3 N 4 nanocomposites such as La(NO 3 ) 3 ⋅6H 2 O (99.99%), SnCl 4 ⋅5H 2 O (98%) and melamine were commercially available and employed without further purification. A multiwave ultrasonic generator (MPI Ultrasonics; welding, 1000 W, 20 KHz, Switzerland), immersed directly in the reaction solution. X-ray diffraction (XRD) patterns were recorded by a Philips-X'pertpro, X-ray diffractometer using Ni-filtered Cu Ka radiation. Fourier transform infrared (FT-IR) spectra were recorded on Nicolet Magna-550 spectrometer in KBr pellets. The electronic spectrum of the sample was taken on Perkin-Elmer LS-55 luminescence spectrometer. Scanning electron microscopy (SEM) images were obtained on LEO-1455VP equipped with an energy dispersive X-ray spectroscopy. The EDX analysis with 20 kV accelerated voltage was done. Transmission electron microscopy (TEM) image was obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV.

Preparation of broccoli extract
An appropriate amounts of fresh broccoli leaves were carefully washed via distilled water and air-dried. Then, the clean leaves were crushed in the food processor and filtered via filter paper. The obtained broccoli extract was kept in a refrigerator for further use.

Sonochemical fabrication of La 2 Sn 2 O 7 nano products
La(NO 3 ) 3 ⋅6H 2 O (99.99%) and SnCl 4 ⋅5H 2 O (98%) salts were purchased from Sigma-Aldrich Company and weighed based on stoichiometric ratios (1:1) and dissolved in deionized water, separately. Then, two solutions were added to each other. Broccoli extract was utilized as a natural surfactant. Meanwhile, NH 3 was added to the cationic solution dropwise till set the pH to 11. Afterwards, A multiwave ultrasonic generator (MPI Ultrasonics; welding, 1000 W, 20 kHz, Switzerland), immersed directly in the reaction solution for 15 min. The attained products were centrifuged, dried and calcined at 900 • C for 5 h. In order to reach favorable dimension and morphology of nano-scaled structures for enhanced photocatalytic performance, the effect of calcination conditions, ultrasonic time and type of alkaline agent was evaluated that has been exemplified in Table 1. Also, a blank test without Broccoli surfactant was carried out. The obtained nanocomponents were considered through various physical analyses. The schematic plan of La 2 Sn 2 O 7 nanoparticle creation by sonochemical route has been displayed in Scheme 1.

Preparation of g-C 3 N 4 /La 2 Sn 2 O 7 nanocomposites
The optimized La 2 Sn 2 O 7 nano-samples in terms of size and shape along with melamine were dispersed in ethanol in an ultrasonic bath for 30 min. then, the centrifuged precipitates were dried and heated at 500 • C for 4 h. The g-C 3 N 4 /LSO nanocomposites prepared in various weight ratios have been listed in Table 1.

Photocatalytic tests
The photodegradation process was accomplished in a homeproduced glass reactor set containing 100 mL aqueous solutions of 10 ppm of erythrosine and methyl violet. For every reaction, 10 mg La 2 Sn 2 O 7 nano-structures were dispersed in a dye solution. The achieved suspensions were stirred at room temperature and kept in dark circumstances for 30 min. To end, the organization set was irradiated under a UV lamp (Osram ULTRA-VITALUX 300 W) consisting of UVA (λ = 320 to 400 nm) and UVB (λ = 290-320 nm).

X-ray diffraction investigations
X-ray diffractograms of La-Sn-O sample prepared through sonochemical route in calcination condition of 800 • C for 3 h (LSO1) have been presented in Fig. 1a. This time and temperature of heat treatment are not sufficient for the preparation of La 2 Sn 2 O 7 nano-structures. By increasing temperature to 900 • C (LSO2), the peaks are still presented in non-crystallized form (Fig. 1b), but pure cubic La 2 Sn 2 O 7 structures (JCPDS No = 73-1686) were created at 900 • C for 5 h that have been illustrated in Fig. 1c (LSO3). It was concluded that a heat process of 900 • C for 5 h is essential for the fabrication of La 2 Sn 2 O 7 nanocrystals. Cubic La 2 Sn 2 O 7 structures are including four main peaks at 2 that of 28.88, 33.47, 48.06 and 57.04 related to (2 2 2), (4 0 0), (4 4 0) and (6 2 2) plans. The XRD pattern of La 2 Sn 2 O 7 crystals prepared in blank condition without ultrasonic irradiation has been demonstrated in Fig. 2a (LSO4). By using the precipitation route, La 2 Sn 2 O 7 powder along with little amount impurity of SnO 2 (JCPDS No = 03-0439) were formed. To the investigation of alkaline agent, ethylene diamine (en) was utilized instead of ammonia (Fig. 2b). The en has a lower release rate of hydroxide anion than NH 3 . In the presence of en, pure cubic lanthanum tin oxide nano-powder was fabricated (LSO5). In order to formation of structures with smaller dimensions, the presence of surfactant is vital. Fig. 2c represents XRD diffractogram of La 2 Sn 2 O 7 nano samples in existing of broccoli extract as a natural surfactant (LSO6) that indexed to pure La 2 Sn 2 O 7 nanostructures (JCPDS No = 73-1686). Moreover, one specimen was synthesized with an increasing ultrasonic irradiation time of 30 min (LSO7) that has been indicated in Fig. 2d. As seen in Fig. 2d, the peaks intensity of La 2 Sn 2 O 7 nanostructures has increased. The crystalline size of La-Sn-O nano-grains was calculated by the Scherer equation [37] and listed in Table 1.  (Fig. 3a, b) that by increasing temperature to 900 • C for 3 h (Fig. 3c, d), nanorods with the size of 30 nm in diameter and 500 nm in length were created besides aggregated particles. With the intensification of time to 5 h, regular particles were synthesized (Fig. 3e, f). La 2 Sn 2 O 7 structures prepared through precipitation technique without ultrasonic irradiation have large and irregular size that shown in Fig. 4a, b. ultrasonic waves help to formation uniform fine particles with accelerating chemical reaction. To investigate of precipitating operator on the final properties of LSO products, the ethylene diamine was applied instead of ammonia. As observed in Fig. 4c, d, uniform particles were synthesized with the size range of 20-40 nm. The en has a lower release rate of hydroxide anions (OH − ) than NH 3 and finally, helps to control of nucleation and growth of nanoparticles. Fig. 4e, f present SEM images of La 2 Sn 2 O 7 nanostructures prepared in presence of Broccoli extract. Spherical nanoparticles formed have homogeneous size and shape. Broccoli was utilized as both green surfactant and alkaline agent including active glucosinolate and polyphenol groups that can conjugate to created nuclei, control their growth and finally create favorable nanosized samples [31,32]. The influence of sonochemical time was evaluated on the appearance features of La 2 Sn 2 O 7 nano-products. By growing reaction time of ultrasonic to 30 min, larger particles were created with a range size of 20-100 nm (Fig. 4g, h) and reaction time of 15 min was designated as an ideal time.

Morphology investigation (SEM & TEM)
Particle size distribution diagrams of La 2 Sn 2 O 7 nanostructures obtained in different circumstances (LSO4, LSO5, LSO6 and LSO7) have been demonstrated in Fig. 5 that calculated by Digimizer software. With evaluating of particle size distribution diagrams was concluded that the lowest size distribution is related to La 2 Sn 2 O 7 nanostructures prepared via broccoli extract and 15 min ultrasonic irradiation (LSO6).

Ultrasonic formation mechanism
La 2 Sn 2 O 7 nano-products with optimized features are created through the simultaneous outcomes of ultrasonic irradiation and broccoli as a natural surfactant. Appropriate structures in the nano-sized form are produced via the generated cavitation with ultrasound waves. Excessive temperatures and pressures are created with huge energies according to hot-spot theory and make active components of radicals [28]. The development of preparation mechanism of samples via the sonochemical method stated as below: La 3+ + Sn(OH) 4 + OH − → La 2 Sn 2 O 7 nanoparticles

Optical features
UV-vis absorption mode was utilized to evident the optical features and energy structure in semiconductor materials. The UV-Vis outcomes for La 2 Sn 2 O 7 nano (LSO6) and bulk (LSO4) particles were exposed in Fig. 7a The luminescence characteristic of La 2 Sn 2 O 7 nano crystals prepared via sonochemical route in optimized circumstances has been illustrated in Fig. 8 under the excitation of 270 nm. As shown in Fig. 8, three peaks exist at 320, 450 and 650 nm. In the La 2 Sn 2 O 7 nanoparticle, the La and Sn cations with inversion symmetry are coordinated to eight and six oxygen, respectively in a geometry only slightly distorted from a regular octahedron [39]. Owing to the small intensity of CN peak versus great intensity of La 2 Sn 2 O 7 peaks, existing of LSO is not significantly observed. In Fig. 9a, a small peak in 2θ about 28 • is seen because of high weight ratios of LSO/CN 30:70. Fig. 9d present the XRD pattern of pristine g-C 3 N 4 component (JCPDS No. 75-2078).

Characterization of La 2 Sn 2 O 7 /g-C 3 N 4 (LSO/CN) nanocomposites
The SEM images of La 2 Sn 2 O 7 /g-C 3 N 4 (LSO/CN) nanocomposites obtained in different LSO:CN weight ratios of 30:70, 50:50 and 70:30 have been specified in Fig. 10a-c. The homogeneous distribution of LSO nanoparticles in ultra-thin sheets of g-C 3 N 4 is confirmed. Moreover, more amount CN sheets are observed in Fig. 10a.
The results of EDX analyses of pure LSO nanoparticles and divers LSO/CN nanocomposites have been represented in Fig. 11a-d, respectively. In Fig. 11a, the existence of La, Sn and O lines confirm the

BET analysis
The nitrogen adsorption-desorption isotherms and pore size distribution curves of LSO/CN nanocomposite with a weight ratio of 30:70 have been revealed in Fig. 12a, b. The hysteresis type of nanocomposite is related to aggregate particles with slit-like pores with broad size  distribution (type-IV and H3) [40]. The attained pore volume and average pore size for LSO/CN nanocomposite in ideal circumstances are 4.50 cm 3 g − 1 and 20.28 nm, respectively. Furthermore, the specific surface area evaluates 19.92 m 2 g − 1 . By evaluating obtained BET results, LSO/CN nanocomposite with a weight ratio of 30:70 is a favorable applicant for photocatalysis systems.

Photocatalytic degradation efficiency and kinetic studying
Due to suitable electronic band gap and high surface area properties of LSO nanoparticles and LSO/CN nanocomposites, the photocatalytic activity of nanostructures was evaluated under UV irradiation for degradation of water pollutants. The impact of various parameters of photocatalytic degradation was investigated in order to achieve the highest degradation percentage. The factors of catalyst type, different weight ratios of LSO:CN, nano-catalyst amount, dye type, dye concentration and different additives as the scavengers were considered. Fig. 13a indicates the photocatalytic performance of LSO nanoparticles prepared via ultrasonic irradiation and LSO bulk structures synthesized without ultrasonic irradiation. The details and results of the degradation efficiency of erythrosine dye under UV light have been presented in Fig. 13. As illustrated in Fig. 13a, degradation efficiency of LSO nanostructures (LSO6) is more than LSO bulk structures (LSO4) that is related to the higher surface area of nano into the bulk. The degradation efficiency of products is calculated as follows (Eq. (1)): Which A 0 and A t are absorbance amounts in times of start and t min. According to Eq. (1), the degradation percentage of nanoparticles and bulk products was obtained 84 and 75 % after 120 min. Also, in order to consider kinetic properties of samples according to Langmuir-Hinshelwood mechanism, the promising reaction rate coefficients can be gained from Eq. (2): Which C 0 and C t are dye concentration in times of start and t min and k is the pseudo-1st-order rate coefficient (min − 1 ) [41]. According to linear dependences of ln(C 0 /C t ) versus reaction time, the 1st-order rate constant k has been attained. As observed in Fig. 13b, the rate constant of k for nanoparticles is more than bulk structures that confirm catalytic degradation of erythrosine dye in presence of LSO nanoparticles is carried out with higher speed. As perceived in Fig. 13c, it is clear that the photo-degradation performance of LSO:CN nanocomposites is more than pristine La 2 Sn 2 O 7 (LSO6) and g-C 3 N 4 components. Also, by increasing g-C 3 N 4 amount in nanocomposite, the rate constant of k rise (Fig. 13d). The cause of advanced photocatalytic behavior of hybrid LSO/CN nanocomposite can be related to the accommodation of energy gaps and electronic bands of LSO and CN specimens and finally, formation of perfect electron migration routes, fast e − -h + separation and carrying [42,43].   The photocatalytic activity of LSO6 nanocatalyst was evaluated for degradation of anionic erythrosine and cationic methyl violet dyes to acquire developed efficiency. As indicated in Fig. 14a, b, the behavior of La 2 Sn 2 O 7 nanoparticles for degradation of erythrosine as an anionic dye is better than methyl violet as a cationic dye and photocatalytic efficiency was reported 100 and 82 %, respectively. Also, the degradation speed of erythrosine is higher than methyl violet dye. Fig. 14c, d compare the photocatalytic performance of LSO6 nanocatalyst in different erythrosine dye concentrations of 10 and 20 ppm. The outcomes exhibit optimized La 2 Sn 2 O 7 nanoparticles have better function in lower dye concentrations. Fig. 15  The schematic design of photo-degradation mechanism of LSO/CN nanocomposite has been illustrated in Scheme. 2.

Conclusions
In summary, we were productively designed binary La 2 Sn 2 O 7 /g-C 3 N 4 nanocomposites through ultrasonic waves with distinctive structural and optical features for photocatalytic activity under UV irradiations for elimination of drinking water pollutants. Effect of particles size, weight ratio of LSO:CN, type of dye, scavenger kind, dye and catalyst loading was designated on altering proficiency of nano-catalyst  function. As a result, La 2 Sn 2 O 7 /g-C 3 N 4 nanocomposites with 30% La 2 Sn 2 O 7 nanoparticle (ƞ=99%) have better efficiency than pristine La 2 Sn 2 O 7 nanoparticle (ƞ=72%) and g-C 3 N 4 specimen (ƞ=91%). Moreover, the photocatalytic activity is completely performed in presence of more amount of nanocatalyst. Also, the probable mechanism of removal dye by photocatalytic function was studied using three types of scavengers of EDTA, Benzoic acid and benzoquinone to trap h + , • OH and • O 2 − active specimens, respectively. Finally, it is found that • OH and