Ni–Ti Layered Double Hydroxide@Graphitic Carbon Nitride

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Ni−Ti Layered Double Hydroxide@Graphitic Carbon Nitride Nanosheet: A Novel Nanocomposite with High and Ultrafast Sonophotocatalytic Performance for Degradation of Antibiotics Reza Abazari,† Ali Reza Mahjoub,*,† Soheila Sanati,‡ Zolfaghar Rezvani,‡ Zhiquan Hou,§ and Hongxing Dai*,§ †

Department of Chemistry, Faculty of Basic Sciences, Tarbiat Modares University, Tehran 14115−175, Iran Department of Chemistry, Faculty of Basic Sciences, Azarbaijan Shahid Madani University, Tabriz, Iran § Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing on Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials, Education Ministry of China, Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China

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‡

S Supporting Information *

ABSTRACT: Pollution of water resources by antibiotics is a growing environmental concern. In this work, nanocomposites of g-C3N4@Ni−Ti layered double hydroxides (g-C3N4@Ni− Ti LDH NCs) with high surface areas were synthesized through an optimized hydrothermal method, in the presence of NH4F. Application of various characterization techniques unraveled that the prepared nanocomposites are composed of porous Ni−Ti LDH nanoparticles and hierarchical g-C3N4 nanosheets. Further, these NCs were employed for photocatalytic and sonophotocatalytic removal of amoxicillin (AMX), as a model antibiotic, from aqueous solutions. In addition, sonocatalysis was performed. It was found out that the g-C3N4@Ni−Ti LDH NCs outperform their pure g-C3N4 and Ni−Ti LDH components in photocatalytic degradation of AMX under visible light irradiation. Also, the following order was determined for efficiency of the three adopted processes: sonocatalysis < photocatalysis < sonophotocatalysis. Furthermore, variation of the sonophotocatalysis conditions specified 500 W light intensity, 9 s on/1 s off ultrasound pulse modem and 1.25 g/L g-C3N4-20@Ni−Ti LDH as the optimal conditions. In this way, optimization of the highly efficient sonophotocatalytic process resulted in 99.5% AMX degradation within 75 min. Moreover, a TOC analyzer was employed to estimate the rate of AMX degradation over the nanocomposites. In addition, formation of hydroxyl radicals (•OH) on the surface of the g-C3N4-20@Ni−Ti LDH particles was approved using the terephthalic acid probe in photoluminescence (PL) spectroscopy. No significant loss was observed in the sonophotocatalytic activity of the nanocomposites even after five consecutive runs. Also, a plausible mechanism was proposed for the sonophotocatalysis reaction. In general, our findings can be considered as a starting point for synthesis of other g-C3N4-based NCs and application of the resultant nanocomposites to environmental remediation.

1. INTRODUCTION Water pollution is one of the greatest concerns of many societies.1,2 This problem roots in the emission of various types of pollutants, including pharmaceutical compounds, to water resources.3,4 Particularly, the presence of pharmaceutics in water systems has turned into a great challenge because the number of drugs that are prescribed and consumed around the world, e.g., antibiotics, antipyretics, anticonvulsants, cytostatic agents, and hormones, is continuously growing.5,6 Under such condition, a considerable portion of drugs remain unused or expired and discharge into the environment. As an outcome, the chemicals that are meant to promote the health quality of humans affect the environment negatively and threaten human health.7,8 Even if the pharmaceutics can be consumed, they can © XXXX American Chemical Society

be discharged into the environment via urine and manure after metabolism in the human body. Therefore, it is common to find different medicines in wastewater and natural water resources.9,10 One of the pharmaceutics that is heavily consumed and might lead to water pollution is amoxicillin (AMX), which is the most common type of β-lactam antibiotics. The AMX content of wastewater treatment effluents, secondary water drainages and surface water depends on various factors and conditions. This medicine is chemically stable, highly toxic for the environment and poorly biodegradable.11 These features Received: September 10, 2018

A

DOI: 10.1021/acs.inorgchem.8b02575 Inorg. Chem. XXXX, XXX, XXX−XXX

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area for light absorption when they are synthesized through conventional methods. These shortcomings can be surpassed by exfoliation of the LDH particles and formation of a stable colloidal solution from the exfoliated layers.38,39 Consequently, exfoliation and composite formation are two important keys for improving the photocatalytic activity of LDHs.40 Furthermore, all the disadvantages of LDHs can be managed by combining them with a second material. A potential material for enhancing the photocatalytic performance of LDHs is metal-free polymer-like graphitic carbon nitride (gC3N4) because it is reported as an active material for visiblelight driven splitting of water molecules.2,27,41 In addition to its approved visible-light activity, g-C3N4 is nontoxic and low cost.42−44 However, like LDH, pure g-C3N4 should not be considered as an efficient photocatalyst because of the fast recombination of its charge carriers.45−47 On the other hand, coupling of layered semiconductors with g-C3N4 is introduced as an effective strategy for promoting the photocatalytic properties of g-C3N4.48−50 This strategy is an attractive option for the case of LDHs, e.g., NiTi LDHs, because LDH layers carry positive charges whereas the surface of g-C3N4 sheets is negatively charged.51,52 So that electrostatic attraction can help the assembly of exfoliated LDH layers and g-C3N4 sheets.53 However, to the best of our knowledge, no study has reported the synthesis of any NC from exfoliated Ni−Ti LDH and gC3N4 nanosheets. This study is devoted to the coupling of ultrasonication and catalysis treatments with the purpose of degrading AMX. Such combinatorial method is called sonocatalysis. This technique is recognized as a safe and economic method for removal of organic pollutants.54,55 To date, several research studies have applied sonocatalysis to degradation of some environmental pollutants. For example, Taghizadeh et al.56 investigated sonocatalysis of 2-hydroxyethyl cellulose degradation by different types of nanoparticles and obtained the following increasing order of sonocatalytic activity: montmorillonite clay < anatase-TiO2 < ZnO < rutile-TiO2 < Fe3O4. Also, in a different study, Zhang et al.57 focused on inactivation of E. coli upon simultaneous application of ultrasonication and photocatalysis by ZnO nanofluids. They observed a 20% increase in the inactivation efficiency of sonocatalysis and visible lightbased photocatalysis through coupling of these two techniques. Also, sonophotocatalytic degradation of rose bengal and disulfine blue dyes and the dyes of the vesuvine and trypan blue have been reported using HKUST-1-MOF-BiVO458 and Ag3PO4/Bi2S3−HKUST-1-MOF,59 respectively. With respect to the predicted positive impacts of g-C3N4LDH composite formation and sonocatalysis on activity of the two individual materials and degradation of chemicals, respectively, this study prepares g-C3N4@Ni−Ti LDH nanoparticles hydrothermally and employs the nanoparticles for sonophotocatalytic degradation of AMX under visible light. Also, this study evaluates the effect of various conditions including ultrasound pulse mode, light intensity and catalyst dose on the efficiency of the process and compares the performance of the sonocatalytic, sonochemical, sonophotocatalytic and photocatalytic approaches in degradation of AMX. It should be noted that no study has reported application of the g-C3N4@Ni−Ti LDH NC to sonophotocatalytic degradation of any organic pollutant, to date. The proposed nanoparticles were found to present improved activity, compared with their pure g-C 3 N 4 and Ni−Ti LDH components. Furthermore, the studied NC seems to be

result in the negative effect of AMX on the growth of some algal species.12 Meantime, the rate of AMX metabolism is slow and the rate of its excretion is about 80−90% in humans.13,14 Furthermore, conventional filtration techniques, such as reverse osmosis, adsorption onto activated carbon, and air flotation, are inefficient and incapable of removing high concentrations of such stable pollutant.15,16 Therefore, it is imperative to eliminate AMX but alternative treatments should be developed. Removal of AMX from water resources might be accomplished by ultrasonication. In general, ultrasound can enhance the performance of physical, chemical, and biological processes during material synthesis or wastewater treatment.17,18 The enhancement relies on the generation of high-temperature (about 1600 to 9000 K) and high-pressure (about 300 atm) cavitation bubbles in liquids, in addition to the creation of microjets, shock waves and shear forces, which impact the physiochemical properties of solutes.19,20 Moreover, ultrasonication is an ecofriendly, cost-efficient, and controllable approach for synthesis of nanostructures.21 The other advantageous features of ultrasonic treatment refer to the shock waves that high-intensity ultrasound induces in solutions and the intense collapsing of the resultant high-pressure and temperature bubbles, which lead to sonolysis of water molecules and production of the OH and H radicals.22,23 The generated radicals can contribute to the degradation of organic compounds. Therefore, ultrasonication can be adopted as a convenient and green method for degradation and elimination of water pollutants.24 In addition to ultrasonication, photocatalysis can be applied to degradation of AMX.25 The applied photocatalyst can be based on layered double hydroxides (LDHs) to take advantage from their noticeable adsorption capacity and visible light activity (420 < λ < 470 nm).2,26,27 LDHs are ionic clays with the [M2+1−xM3+x(OH)2]x+ (An−)x/n.mH2O formula, in which M2+ and M3+, respectively, represent bivalent and trivalent metal cations and An− is the interlayer anion. This unique formulation provides LDHs with a uniform distribution of the constituent metal cations, many surface hydroxyl groups, interlayer spaces and intercalated anions, which result in a unique structure with tunability, oxo-bridged linkages, swelling properties and high chemical stability.28 All these structural features have motivated the use of LDHs in many fields including photocatalyst substrates. However, they have not been widely used as photocatalysts since fast recombination of their electron−hole pairs lowers their photocatalytic efficiency.29 In this respect, some researchers have attempted to promote the photocatalytic activity of LDHs, specifically by coupling LDHs with other semiconductors.30−33 For instance, Nayak et al. combined NiFe-LDH with g-C3N4 and applied the obtained composite to water oxidation and reduction reactions under visible light.34 In another work, Mohapatra et al. used a molybdate/tungstate intercalated oxo-bridged ZnY LDH in photocatalytic degradation of organic contaminant.35 Both research groups observed an acceptable level of photocatalytic efficiency. Also, Sahoo et al. achieved an excellent adsorption and visible light-based photocatalytic performance for phenol oxidation by synthesis of a Ag@Ag3VO4/ZnCr LDH nanocomposite (NC).36 In another study, Seftel et al. observed a high degree of phenol degradation by the use of a CeO2/LDH heterojunction photocatalyst.37 The other shortcomings of LDH-based photocatalysts are aggregation of their particles and exhibition of a low surface B

DOI: 10.1021/acs.inorgchem.8b02575 Inorg. Chem. XXXX, XXX, XXX−XXX

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efficiency of the sonophotocatalytic process. For comparison purposes, degradation of AMX by the photocatalytic, sonochemical, and sonocatalytic methods was carried out, too.

applicable to photodegradation of other organic compounds. In general, the convenient synthesis route, high sonophotocatalytic activity and appropriate reusability of the g-C3N4@Ni− Ti LDH NC represent it as a potential candidate for elimination of other organic molecules.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Synthesized Nanostructures. The FT-IR spectra of the pure Ni−Ti LDH, pure gC3N4 and the g-C3N4@Ni−Ti LDH NC samples are shown in Figure 1. In the FT-IR spectrum of the g-C3N4 nanosheets, the

2. EXPERIMENTAL SECTION 2.1. Synthesis of the Ni−Ti LDH Nanoparticles. Ni(NO3)2.6H2O, titanium(IV) chloride, and NH4F with the 2:1:3 molar ratio were dissolved in 50 mL deionized water (solution A). In parallel, NaOH was dissolved in deionized water to give a 0.5 M NaOH solution (solution B). Then, under the inert atmosphere of N2, solution A was mixed with 100 mL of solution B while maintaining pH of the resultant solution mixture at about 5. After that, the obtained slurry was stirred vigorously for 15 min, transferred to a 180 mL Teflon-lined stainless-steel autoclave and aged for 48 h at 130 °C. The solid product was centrifuged, washed with ethanol and dried for 12 h under vacuum at 60 °C. The Ni−Ti LDH nanoparticles were synthesized in the presence of 0.5 g L−1 formamide under N2 atmosphere to prevent contamination of the product by carbonate anions. 2.2. Synthesis of Bulk g-C3N4 and g-C3N4 Nanosheets. Ten g urea was poured into a semiclosed crucible to decline its sublimation. The urea powder was then calcined for 4 h, at 550 °C, to obtain a light yellowish powder as bulk g-C3N4. The fluffy product was cooled down to room temperature, dispersed in isopropanol (IPA) to give a 1 g L−1 suspension of bulk g-C3N4 and ultrasonicated for 48 h to prepare g-C3N4 nanosheets. Lastly, the g-C3N4 nanosheets were centrifuged and freeze-dried. 2.3. Preparation of the g-C3N4@Ni−Ti LDH NCs. The g-C3N4X@Ni−Ti LDH NCs with different contents of g-C3N4 (X = 10, 15, 20, 30 wt %) were prepared using ultrasonication. At first, a predetermined amount of the g-C3N4 sheets and a fixed quantity of the exfoliated Ni−Ti LDH-formamide suspension were mixed with 50 mL distilled water and ultrasonicated for 45 min, at room temperature. Then, the resultant solution was filtered, washed with ethanol and distilled water several times, and dried for 12 h at 60 °C. The obtained g-C3N4-X@Ni−Ti LDH NCs with 10, 15, 20, and 30 wt % g-C3N4 nanosheets were labeled as g-C3N4-10@Ni−Ti LDH, gC3N4-15@Ni−Ti LDH, g-C3N4-20@Ni−Ti LDH, and g-C3N4-30@ Ni−Ti LDH, respectively. 2.4. Photocatalysis and Sonophotocatalysis. To investigate the photocatalytic performance of the synthesized NCs in removal of AMX from aqueous solutions, we dispersed a definite amount of the NCs in a 100 mL AMX solution with 1.0 g L−1 concentration. The reaction mixture was stirred for 30 min in the dark to permit the nanoparticles and AMX molecules reach an adsorption−desorption equilibrium status, before illumination. The concentration of the pollutant molecules that had adsorbed onto the NCs under darkness was determined to be 1 to 2 mg L−1 when the starting AMX concentration was 1.0 g L−1. Illumination of the reaction solution by visible light was performed using a 400 W Hg source with a 420 nm cutoff filter. Concentration of the AMX molecules was measured at 15 min reaction intervals to the end of the treatments by sampling the reaction mixture, separating the catalyst particles through centrifugation and recording absorbance of the supernatant using UV−vis spectroscopy at 237 nm. The extent of AMX degradation, i.e., the degradation ratio (D (%)) was calculated on the basis of eq 1, in which A0 and C0 are the initial absorbance and AMX concentration in the reaction mixture, and At and Ct are the absorbance and AMX content of the solution at time = t, respectively. ÄÅ ÉÑ ÄÅ ÉÑ ÅÅ ÅÅ A t ÑÑÑ Ct ÑÑÑ Å Å Å Ñ Å ÑÑ100 D = ÅÅ1 − Ñ100 = ÅÅ1 − ÅÅÇ ÅÅÇ A 0 ÑÑÑÖ C0 ÑÑÑÖ (1)

Figure 1. FT-IR spectra of pure g-C3N4, pure Ni−Ti LDH, and the gC3N4-X@Ni−Ti LDH NCs (X= 10, 15, 20, and 30).

peaks positioned over 1200 to 1700 cm−1 can be attributed to the stretching vibrations of the CN and C−N bonds, and the 810 cm−1 peak can be related to ring breathing of the triazine units.60,61 In the case of the Ni−Ti LDH, the two broad peaks located at 3432 and 1635 cm−1 can be ascribed to the symmetric and asymmetric stretching modes of the hydroxides and the bending vibrations of the water molecules positioned in the interlayer spaces of the LDH nanoparticles, respectively.62 Meantime, the peaks positioned below 800 cm−1 refer to the M−OH and M-O vibrational modes.63 Furthermore, based on the 1380 cm−1 peak, NO3− is the interlayer anion of the Ni−Ti LDH and no carbonate contamination exists in the sample due to the use of the N2 atmosphere.64 For the g-C3N4-X@Ni−Ti LDH NCs, the vibrational bands of both pure components, i.e., g-C3N4 and Ni−Ti LDH, can be seen, which confirms the presence of both materials in the prepared NCs. Also, as it can be observed, the increase of the weight percentage of g-C3N4 from 10 to 30% is accompanied by intensification of the g-C3N4 peaks in the spectra of the NCs. Because the photocatalytic properties of nanomaterials are influenced by their structures and morphologies, the FE-SEM images of the pure and composite samples were taken. The FESEM image corresponding to pure g-C3N4 is displayed in Figure 2a. According to this figure, the prepared g-C3N4 powder contains sheetlike particles with nanosized thickness. On the other hand, Figure 2b shows that pure Ni−Ti LDH is comprised of hierarchical particlelike structures with particle sizes ranging from 40 to 80 nm. When these two pure samples are combined to produce the g-C3N4-20@Ni−Ti LDH NCs, an assembly of uniform distribution of particle-like Ni−Ti LDH structures supported on a great number of welldeveloped g-C3N4 nanosheets is observed (Figure 2c). To gain more insight about the morphology of the NCs, the g-C3N4-20@Ni−Ti LDH NC was also analyzed by the highresolution transition electron microscopy (HR-TEM) techni-

The ultrasonication treatment was conducted by immersing a highintensity ultrasound probe with 20 kHz frequency and 200 W power in the reaction solutions. The applied ultrasound pulse mode, light intensity and catalyst dose were varied to evaluate their effects on C

DOI: 10.1021/acs.inorgchem.8b02575 Inorg. Chem. XXXX, XXX, XXX−XXX

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In addition to the morphological studies, the XRD patterns of the samples were analyzed to determine their crystalline structures and detect any phase impurity. The obtained XRD patterns of the pure Ni−Ti LDH, pure g-C3N4 and g-C3N4@ Ni−Ti LDH NC samples over the 2θ range of 5−75° are exhibited in Figure 4. In the case of pure g-C3N4, the only peak

Figure 2. FE-SEM images of (a) the pure g-C3N4 nanosheets, (b) the pure Ni−Ti LDH, and (c) g-C3N4-20@Ni−Ti LDH NC.

que. The resultant image is shown in Figure 3a, b. In this figure, the dark and bright zones can be ascribed to the particle-like Ni−Ti LDH and the g-C3N4 nanosheets, respectively. The nanosheets of g-C3N4 and the Ni−Ti-LDH nanoparticles are pointed by arrow marks. As this figure clarifies, the LDH nanoparticles are assembled on the nanosheets and the analyzed NC is pure. Moreover, utilizing image J software (Figure 3c) a particle size distribution histogram determined from the HR-TEM images. Based on this Figure, for the g-C3N4-20@Ni−Ti LDH NC, the size distribution is higher in the range of 50−150 nm. Furthermore, Figure 3d represents the chosen area electron diffraction (SAED) pattern of g-C3N4-20@Ni−Ti LDH NC. The nanocrystalline nature of the compound is indicated by a concentric ringlike pattern with randomly arranged uninterrupted bright spots over it.

Figure 4. PXRD patterns of pure g-C3N4, pure Ni−Ti LDH, and the g-C3N4-X@Ni−Ti LDH NCs (X = 10, 15, 20 and 30).

appears at 27.4° and refers to the (002) plane, which is known as the characteristic crystal plane of g-C3N4 nanosheets.65 In the meantime, the XRD pattern of pure Ni−Ti LDH outlines the (003), (006), (012), (015), (018), (110), and (113) planes, which are consistent with the common XRD pattern of

Figure 3. HR-TEM images for the g-C3N4-20@Ni−Ti LDH NC with two various scales of (a) 100 and (b) 10 nm, and (c) histogram of the particle size distribution for the g-C3N4-20@Ni−Ti LDH NC and (d) SAED pattern of the g-C3N4-20@Ni−Ti LDH NC. D

DOI: 10.1021/acs.inorgchem.8b02575 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. XPS spectra of g-C3N4-X@Ni−Ti LDH NCs.

LDH structures.66 The LDH sample is of high purity and includes no odd peak. On the other hand, Figure 4 indicates that the XRD patterns of the g-C3N4-X@Ni−Ti LDH NCs are composed of all XRD peaks of the pure LDH and g-C3N4 samples. As expected, enhancing the contribution of the gC3N4 nanosheets intensifies the peak related to the (002) plane of g-C3N4, which confirms the success of the NC synthesis method. Similar to the LDH sample, the synthesized NCs have no impurity. Figure 5a indicates the examination XPS spectra of g-C3N420@Ni−Ti LDH NC. On the basis of the measurement result, it is revealed that Ni, Ti, C, N, and O happen for g-C3N4-20@ Ni−Ti LDH NC. On the basis of Figure 5b, for the typical nickel peaks of Ni−OH, four 2p peaks of nickel centered at 856.0 eV (Ni 2p3/2), 861.7 eV(Ni 2p3/2, sat), 873.6 eV (Ni 2p1/2), and 880.0 eV (Ni 2p1/2, sat) are assigned.67 The XPS spectra of Ti 2p provided in Figure 5c show two peaks centered at 464.1 eV (Ti 2p1/2) and 458.3 eV (Ti 2p3/2), equivalent to the binding energies of Ti4+ in Ti(OH)62−.67 Principally three carbon species exist in the C 1s spectrum (Figure 5d), and the equivalent binding energies are calculated as 285.0 eV, 288.0 and 288.5 eV. Carbon contamination can be responsible for the peak at lower binding energy (BE), centered at 285.0 eV, for instance, C−C, and/or graphite C−C bonds. The key noticeable peak at 288.0 eV is associated with sp2 carbon−nitrogen (N−C−N) bondings of the aromatic ring

system, indeed, the s-triazine unit and pyridine-like structure. The peak at higher BE at 288.5 eV is allocated to tertiary nitrogen N−(C)3 groups, indeed, the nitrogen which is trigonally bonded to three sp2 carbon atoms in the C−N network (graphitic-like nitrogen structure).68 The N 1s XPS spectra of the -C3N4-20@Ni−Ti LDH NC (Figure 5e) indicates that the peaks at binding energies of 398.3 and 400.9 eV are associated with pyridine N and to the N−H structure, respectively.69 The O 1s peak can be observed into peak centered at 531.5 eV equivalent to oxygen peak in hydroxyl bonding with Ti and Ni (Figure 5f).67 The existence of Ni−Ti LDH and g-C3N4 nanosheets on the surfaces of the -C3N4-20@Ni−Ti LDH NC XPS is confirmed further by the results. After analysis of the physical features of the samples, their light absorption characteristics were explored by UV−vis DRS spectroscopy. The associated results (Figure 6 and Figure S1) point to a red shift in the absorbance of the NCs under visible light irradiation, compared with the absorbance of pure Ni−Ti LDH. This shift is resulted from the difference between the optical absorbance of the pure and heterojunction g-C3N4 nanosheets at wavelengths above 450 nm and improves the efficiency of the NCs in absorption of visible light.70,71 In fact, heterojunction of the two pure materials can restrain the corresponding contact barrier, improve electronic coupling of the two semiconductors with correlating energy levels and E

DOI: 10.1021/acs.inorgchem.8b02575 Inorg. Chem. XXXX, XXX, XXX−XXX

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In eq 2, α, h, n, Eg, and A are the coefficient of absorption, the Planck’s constant, frequency of light, band gap energy, and a constant, respectively. Also, Figure 6 shows that the absorption bands of the prepared Ni−Ti LDH sample are observed over the visible and UV regions of light. In this figure, the band observed around 423 nm can be ascribed to the d−d transitions of Ni−Ti LDH. As aforementioned, generation of the composite materials induces a red shift in the DRS spectra of the g-C3N4-20@Ni−Ti LDH NC due to the presence of the visible-light-active g-C3N4 nanosheets. On the basis of the absorption edges determined in the DRS spectra of pure Ni− Ti LDH, g-C3N4-10@Ni−Ti LDH, g-C3N4-15@Ni−Ti LDH, g-C3N4-20@Ni−Ti LDH, and g-C3N4-30@Ni−Ti LDH, the band gap energies of these materials are 2.95, 2.86, 2.82, 2.76, and 2.78 eV, respectively (Figure 7). Consequently, accompanying the Ni−Ti LDH nanoparticles with the g-C3N4 nanosheets and formation of the NCs is a proper option for promoting the visible light activity of the Ni−Ti LDH nanoparticles and providing a higher level of photocatalytic performance. Furthermore, as the band gap energy values declare, the g-C3N4-20@Ni−Ti LDH NC can present the highest extent of charge carrier photogeneration during photocatalytic processes. To gain more insight about the photocatalytic performance of the samples, the edge potentials of their valence bands (EVB) were calculated by following:27

Figure 6. UV−vis diffuse reflectance of pure g-C3N4, pure Ni−Ti LDH, and the g-C3N4-20@Ni−Ti LDH NC.

present maximum absorption of visible light.72−74 Also, inclusion of g-C3N4 into the NC materials can enhance the NC’s surface charge. All these improvements and the observed red shift result from the synergistic effects of the Ni−Ti LDH and g-C3N4 components75 and motivate consideration of the g-C3N4-X@Ni−Ti LDH NCs as appropriate visible-light-active photocatalysts. According to Figure 6, pure g-C3N4 presents a strong absorption edge at 464 nm. The corresponding bandgap energy, i.e., 2.69 eV, can be calculated using the Tauc relationship (eq 2). (αhv)1/ n = A(hv − Eg )

E VB = XEe + 0.5Eg

(3)

where X, Ee, and Eg are electronegativity of the material, energy of free electrons on the hydrogen scale (∼4.5 eV) and bandgap energy of the sample, respectively. It should be added that the approximate value of X can be determined by geometrical averaging of the electronegativity values of its constituent atoms. By having the EVB values of the samples, the edge potentials of their conduction bands (ECB) can be calculated as ECB = EVB − Eg. Here, the electronegativities of pure g-C3N4

(2)

Figure 7. Specific absorption band edges calculated using the Tauc relationship of (a) pure g-C3N4, (b) pure Ni−Ti LDH, (c) g-C3N4-10@Ni−Ti LDH, (d) g-C3N4-15@Ni−Ti LDH, (e) g-C3N4-20@Ni−Ti LDH, and (f) g-C3N4-30@Ni−Ti LDH. F

DOI: 10.1021/acs.inorgchem.8b02575 Inorg. Chem. XXXX, XXX, XXX−XXX

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electron−hole pairs. As can be seen in Figure 8, excitation of the samples with the 355 nm wavelength, results in the lowest PL intensity in the case of g-C3N4-20@Ni−Ti LDH. It means that, among all samples, the g-C3N4-20@Ni−Ti LDH NC is the best in suppressing radiative recombination of the photoinduced charge carriers. The suppression capability of this NC can be associated with the synergetic effect of Ni−Ti LDH and 20%g-C3N4, which can present more active sites for participation of the photoinduced holes and electrons in surface reactions and prevention of their recombination. The studied process relies on generation of charge carriers and adsorption of the AMX molecules. Consequently, the synergic effect between the adsorption activity and the photocatalytic performance of the pure and NC samples was studied by measuring their surface Zeta potential (ξ) over the pH range of 3 to 11. At pH 3, 5, 7, 9 and 11, the ξ value of the LDH equals to 60.43, 48.71, 19.20, 6.95, and −10.36 mV, respectively, which means that the Ni−Ti LDH nanoparticles carry strong positive charges on their surfaces at neutral and acidic conditions. The positive charges on the LDH structure are the direct outcomes of Ni2+ replacement with Ti4+. Further, when the Ni−Ti LDH nanoparticles mix with the g-C3N4 nanosheets and yield the g-C3N4-20@Ni−Ti LDH NC, the ξ value reduces to 28.49, 16.52, −5.33, −13.87, and −29.64 mV at pH 3, 5, 7, 9 and 11, respectively (Figure 9), which indicates

and Ni−Ti LDH are about 0.049 and 0.204 eV, respectively. Therefore, the EVB (ECB) values of the pure g-C3N4 and Ni−Ti LDH samples are +1.60 (−1.16) and +2.413 (−0.577) eV, respectively. Another important issue in photocatalytic processes is recombination of the photoexcited charge carriers. In this respect, fluorescence spectroscopy at the excitation wavelength of 375 nm was carried out on the pure and composite samples. As Figure 8 illustrates, the pure g-C3N4 nanosheets give rise to

Figure 8. PL spectra of pure g-C3N4, pure Ni−Ti LDH, and the gC3N4-X@Ni−Ti LDH NCs.

an intense emission peak around 476 nm that can be attributed to the band−band photoluminescence (PL) induced by recombination of the photoexcited electron−hole pairs. Among the six pure and NC samples, the least peak intensity corresponds to the g-C3N4-20@Ni−Ti LDH NC since it has the highest potential of capturing the photogenerated electrons. Therefore, the g-C3N4-20@Ni−Ti LDH NC can reduce the recombination rate of the photogenerated charge carriers more efficiently to demonstrate the best lightharvesting potential and provide the highest photocatalytic activity.76,77 In other words, the lowest PL intensity and therefore the lowest rate of electron−hole recombination and the highest photocatalytic activity refer to the g-C3N4-20@Ni− Ti LDH NC. Also, Figure 8 shows that the PL emission bands of the gC3N4-X@Ni−Ti LDH NCs are blue-shifted relative to pure gC3N4. This shift roots in the interaction of the g-C3N4 nanosheets and the Ni−Ti LDH nanoparticles, which results in the observation of the Burstein−Moss effect.78,79 Also, the reason might be related to the ionized oxygen vacancies of the NCs’ valence band. To be more specific, the photogenerated holes might recombine with the electrons occupying the oxygen vacancies to radiate visible light.80 In addition to fluorescence spectroscopy, PL spectroscopy was performed to assess the behavior of the pure and composite samples in separation of the photogenerated charge carriers. During the PL measurements, the photons absorbed by the samples could excite electron from their valence band to their conduction band. Then, the excited electrons could relax to the ground state of the samples by emitting a PL photon. According to this sequence of events, a high PL intensity implies a high recombination rate of the photoexcited

Figure 9. Effect of pH on the zeta potential of pure Ni−Ti LDH, pure g-C3N4, and g-C3N4-20@Ni−Ti LDH NC.

that the g-C3N4-20@Ni−Ti LDH NCs are self-assembled via electrostatic interactions. According to the ξ values, the isoelectric points of the pure Ni−Ti LDH sample and the gC3N4-20@Ni−Ti LDH NC are 9.86 and 6.42, and the LDH and NC are negatively and positively charged at pH values that are above and below these values, respectively. These results demonstrate that the g-C3N4 nanosheets carry negative charges in alkaline and neutral solutions. This finding was further approved by obtaining the ξ value of pure g-C3N4 nanosheets as 1.07, 0.64, −15.43, −22.7, and −39 mV at pH 3, 5, 7, 9 and 11, respectively (Figure 9). Therefore, the ξ values suggest that the opposite surface charges of the g-C3N4 nanosheets and the Ni−Ti LDH nanoparticles help them to assemble through electrostatic interactions and form the g-C3N4-X@Ni−Ti LDH NCs. The curves of pore-size distribution and the N2 adsorption− desorption isotherms of pure g-C3N4, pure Ni−Ti LDH and the g-C3N4-20@Ni−Ti LDH NC are depicted in Figure 10. G

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studied under visible-light illumination. Also, the degradation reaction was followed under the same reaction conditions using no photocatalyst. However, no AMX loss was detected in the absence of the examined photocatalysts. Figure 11 shows the observed rates of AMX degradation for all samples, as a function of irradiation time. Before starting the catalyzed degradation reactions, the AMX solutions were stirred for 30 min in the dark to establish an adsorption−desorption equilibrium between the photocatalyst particles and the AMX molecules. As shown in Figure 11, g-C3N4 and Ni−Ti LDH can respectively provide 35 and 26% AMX degradation within 150 min, whereas the g-C3N4-X@Ni−Ti LDH NCs exhibit a greater photocatalytic activity over the same irradiation period. In other words, combination of the two individual materials leads to the excellent photocatalytic performance of the resultant composites. Moreover, the weight percentage of the g-C3N4 nanosheets has a deterministic role in the photocatalytic activity of the NCs. So that, increasing the gC3N4 content from 10 to 20% improves the efficiency of the NC in removal of AMX while further increase of the g-C3N4 content to 30% does not lead to any further enhancement in the NC’s efficiency. Consequently, 20 wt % is the optimal amount of g-C3N4 in the NC and the g-C3N4-20@Ni−Ti LDH NC is the optimal photocatalyst. In fact, the g-C3N4-20@Ni− Ti LDH NC is so efficient that it can give 83% AMX degradation within 150 min visible light irradiation. In order to confirm the effect of the optimal catalyst on the observed photocatalytic activity, an experiment was conducted in its absence. Without g-C3N4-20@Ni−Ti LDH NC, the concentration of AMX did not decrease even when light was employed. Accordingly, it was concluded that both light and the catalyst are important for degradation of AMX. It should be added that the high efficiency of this NC relies on its high surface area, low optical energy band gap and mesoporous structure, in addition to the nature of its p−n junction at its gC3N4-@Ni−Ti LDH interface. To gain more insight about the photocatalytic performance of the g-C3N4-20@Ni−Ti LDH NC (as the optimum photocatalyst), the reaction of AMX degradation was

Figure 10. Nitrogen adsorption−desorption isotherms and pore-size distribution curves of (a) pure g-C3N4, (b) pure Ni−Ti LDH, and (c) g-C3N4-20@Ni−Ti LDH.

Both the pure and composite samples represent IV-type isotherms, which are indicative of mesoporous materials. Despite similarity of their isotherms, the BET surface area of the NC (85.44 m2/g) is 16.5 times higher than that of pure Ni−Ti LDH (5.16 m2/g). It means that incorporation of the gC3N4 nanosheets (323.89 m2/g) into the matrix of Ni−Ti LDH can enhance the adsorption capacity of the Ni−Ti LDH nanoparticles, noticeably. The small pore radius and the high specific surface area of the g-C3N4-20@Ni−Ti LDH NC can enhance its adsorption and absorption abilities to provide enhanced performance in sonophotocatalytic degradation of AMX. 3.2. Photocatalysis and Sonophotocatalysis. Photocatalytic degradation of AMX in the presence of pure g-C3N4, pure Ni−Ti LDH and the g-C3N4-X@Ni−Ti LDH NCs was

Figure 11. Photocatalytic activity of pure g-C3N4, pure Ni−Ti LDH, and g-C3N4-X@Ni−Ti LDH (X = 5, 10, 20, and 30 wt %) in degradation of AMX under darkness and visible-light irradiation after 150 min. H

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Figure 12. UV−visible absorption spectra of AMX during degradation by the g-C3N4-20@Ni−Ti LDH NC under (a) darkness and (b) visible-light irradiation.

reveals that simultaneous application of ultrasonication and the optimal NC is effective on elimination of AMX and represents the g-C3N4-20@Ni−Ti LDH NC as a sonocatalyst for improving sonochemical AMX removal, under darkness. A further improvement in AMX degradation can be achieved by simultaneous application of ultrasonication, the optimal catalyst and visible light irradiation (Figure 14). This

investigated under the darkness and visible-light illumination conditions, at room temperature. For this purpose, the NCs were removed from the reaction medium by centrifugation and, then, absorbance of the AMX solution was recorded at 237 nm. Figure 12a displays the changes in the UV−vis absorption spectrum of the reaction solution in the presence of the optimal NC, in the dark. As Figure 12a shows, no significant reduction is observed in the concentration of AMX under darkness. Meanwhile, Figure 12b indicates a decrease in the concentration of the AMX molecules under visible light irradiation. So that, if the reaction solution be illuminated for 150 min by visible light, then the optimal NC would result in about 83% degradation, which unravels a considerable photocatalytic activity (Figure 11). Therefore, the NCs can catalyze the AMX degradation reaction but they cannot proceed the reaction without light irradiation. Figure 13 displays degradation of AMX under visible light irradiation, catalysis, ultrasound irradiation and combinations

Figure 14. Degradation of AMX by g-C3N4-20@Ni−Ti LDH NC under different conditions and physical mixture of the g-C3N4 nanosheets and the Ni−Ti LDH nanoparticles.

improvement roots in the effect of light on the NC particles. When the rays of light with hv ≥ Eg impact the surface of the NC, the NC’s valence band electrons transit to its conduction band. This process leaves some holes (h+) and electrons (e−) in its valence and conduction band, respectively. The generated holes play the role of a highly oxidizing agent while the produced electrons act as a strong reducing agent. After photogeneration of the electron−hole pairs, the valence band holes can transfer to the NC’s surface and oxidize the adsorbed −OH anions and water molecules to OH radicals.81 Then, the produced OH radicals can degrade the present AMX molecules. Figure 14 displays the changes in the concentration of AMX under different conditions and outlines the following order of degradation rate: sonocatalysis < photocatalysis < sonophotocatalysis. As this figure shows, degradation of AMX

Figure 13. Degradation of AMX under different conditions.

of these treatments. As it can be observed, illumination of the reaction mixture by visible light is not sufficient for degradation of AMX, in the absence of ultrasound waves. Therefore, AMX photodegradation is insignificant under visible light irradiation. It is while the AMX molecules degrade using ultrasonication in the absence of visible light and any photocatalyst due to the reaction of AMX with the OH radicals that generate through water sonolysis. Also, comparison of the curves of Figure 13 I

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Figure 15. Influence of (a) light intensity, (b) ultrasound pulse mode, and (c) catalyst dose on sonophotocatalytic removal of AMX by the g-C3N420@Ni−Ti LDH NC.

and 9 s on/1 s off ultrasound pulse mode. As it can be seen, increasing the light intensity from 200 to 500 W results in the promotion of AMX degradation from 56 to 99%. It means that higher intensities of light can induce generation of a greater number of charge carriers and, therefore, a more noticeable number of OH radicals per unit time and area. Figure 15b exhibits photodegradation of AMX using 500 W light intensity, 1.0 g/L g-C3N4-20@Ni−Ti LDH, 150 min reaction time, and various pulse modes (3/7, 5/5, 7/3 and 9 s on/1 s off). According to this figure, prolonging the ultrasound pulse mode increases the rate of AMX degradation. The reason is that longer pulse lengths introduce a greater ultrasound power to the solution. In agreement with our finding, previous studies have stated that the pulse length and the ratios of the on/off modes influence cavitation activities.82 Lastly, Figure 15c demonstrates the effect of catalyst dose (0.25, 0.50, 0.75, 1.0, and 1.25 g/L g-C3N4-20@Ni−Ti LDH) on elimination of AMX at 500 W light intensity, 150 min reaction time and 9 s on/1 s off ultrasound pulse mode. As expected, higher doses of the photocatalyst promote degradation of AMX. So that, 1.25 g/L g-C3N4-20@Ni−Ti LDH can present 99.5% AMX removal within 75 min reaction time. These results indicate that light intensity, catalyst dose, and ultrasound pulse length

increases with reaction time and reaches to 99% after 150 min sonophotocatalysis. It is while the extent of degradation is limited to 53 and 87% that the AMX solution is respectively subjected to sonolysis and photocatalysis over the same period of reaction time. Therefore, sonophotocatalysis, which provides about 99% degradation within 150 min, is the most efficient process for removal of AMX from aqueous solutions. Further, a control experiment was carried out to evaluate degradation of AMX using a physical mixture of the g-C3N4 nanosheets and the Ni−Ti LDH nanoparticles. As the blue line in Figure 14 illustrates, the efficiency of the prepared mixture is almost equal to that of the pure LDH nanoparticles. This result implies that the interaction between the two components is not efficient and effective in the case of the physical mixture and the inefficiency restricts appropriate transfer of electron and hole so that the mixture exhibits an activity equivalent to those of the pure components. The only difference is that application of the mixture instead of the pure materials leads to a longer degradation time due to the adsorption of a high AMX concentration. Figure 15a depicts the effect of various light intensities (200, 300, 400, and 500 W) on sonophotocatalytic removal of AMX using 1.0 g/L g-C3N4-20@Ni−Ti LDH, 150 min reaction time J

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Inorganic Chemistry are the factors that control the extent of AMX removal during the sonophotocatalytic treatment. To analyze the OH radicals generated using pure g-C3N4, pure Ni−Ti LDH, and the g-C3N4-20@Ni−Ti LDH NC, we employed the terephthalic acid PL (TA-PL) probing method.83 The TA-PL spectra obtained by irradiation of visible light with 315 nm wavelength on the samples for 150 min are shown in Figure 16, in which the PL peak observed at

Figure 17. Sonophotocatalytic degradation of AMX over g-C3N4-20@ Ni−Ti LDH under visible-light irradiation during five consecutive runs.

Figure S2 depicts the variations in the total organic content (TOC) of the AMX solutions treated by visible-light-based sonophotocatalysis in the presence of pure g-C3N4, pure Ni− Ti LDH and g-C3N4-20@Ni−Ti LDH, as a function of reaction time. This figure shows that sonophotocatalysis by the pure g-C3N4, pure Ni−Ti LDH and g-C3N4-20@Ni−Ti LDH photocatalysts mineralizes about 38, 24, and 73% of the total organic content of the AMX solutions within 150 min. This result shows that the AMX molecules oxidize into CO2, H2O, and other smaller organic molecules, rapidly. 3.3. Reaction Kinetics and Mechanism. Kinetics of the sonophotocatalytic reaction was explored using 1.25 g/L gC3N4-20@Ni−Ti LDH, 500 W visible light, and the 9 s on/1 s off ultrasound pulse mode. Figure 19 illustrates the corresponding rate of AMX degradation and displays a linear relationship between the natural logarithm of the changes in the AMX concentration, i.e., −ln(Ct/C0), and irradiation time (t). The observed linear relationship suggests that the degradation process can be described as a pseudo-first-order reaction and its associated rate constant (k) can be calculated using −ln(Ct/C0) = kt.27 Accordingly, the rate constants of the sonophotocatalysis, photocatalysis and sonolysis processes of AMX degradation equal to 0.0476, 0.0108, and 0.002 min−1, respectively. It means that the sonophotocatalytic degradation reaction is 4.4 and 23.8 times faster than the photocatalytic and sonochemical reactions, respectively. Also, the rate constants of sonophotocatalytic AMX removal using pure g-C3N4, pure Ni−Ti LDH and the g-C3N4-20@Ni−Ti NC equal to 0.0026, 0.0018, and 0.0476 min−1, respectively. These values show that the sonophotocatalytic degradation reaction using the g-C3N420@Ni−Ti NC is 18.3 and 26.4 times faster than the sonophotocatalytic degradation reaction using pure g-C3N4 and pure Ni−Ti LDH, respectively. In general, sonophotocatalysis of organic pollutant degradation relies on the synergistic effect of the photocatalysis and sonolysis processes. The synergic effect enhances the efficiency of the sonophotocatalysis treatment by85,86 (i) increasing generation of oxidizing agents through sonolytic hydrolysis, (ii) improving transport of the pollutant molecules to the surface of the catalyst with the help of the propagated shock waves, (iii) prevention of catalyst aggregation and maintenance of high active surface area as an outcome of acoustic cavitation, and (iv) continuous cleaning of the surface of the catalyst by the ultrasound waves, which improves the catalytic activity of

Figure 16. •OH-trapping photoluminescence spectra of pure g-C3N4 (150 min), pure Ni−Ti LDH (150 min), g-C3N4-20@Ni−Ti LDH (150 min), and g-C3N4-20@Ni−Ti LDH (0 min).

430 nm refers to 2-hydroxyterephthalic acid. This figure shows that irradiation of pure g-C3N4 and Ni−Ti LDH leads to the formation of almost no •OH specie because of the fast recombination of the photogenerated charge carries. On the other hand, the g-C3N4-20@Ni−Ti LDH NC gives a PL intensity higher than that of the LDH as a consequence of Zscheme charge transfer.84 This kind of charge transfer causes hole enrichment in the VB of the LDH nanoparticles, production of more •OH radicals, and a high PL intensity. In other words, intensification of the 430 nm band shows that the •OH radicals are the main drivers of the photodegradation process, in addition to approving the photocatalytic performance of the g-C3N4-20@Ni−Ti LDH NC. To evaluate the practical applicability of the developed photocatalyst, we assessed the stability and reusability of the optimal photocatalyst, i.e., g-C3N4-20@Ni−Ti LDH, by performing five consecutive catalytic cycles. The obtained results (Figure 17) indicated that the sonophotocatalyst is able to conserve its activity even after 5 catalytic runs with no significant loss of its sonophotocatalytic efficiency. Therefore, this NC should be highly stable and durable, in the nature. The observed negligible loss of sonophotocatalytic activity can be attributed to the loss of active particles during the washing and separation processes. To ensure that the structure of the NCs does not change during the sonophotocatalytic treatment, we analyzed the XRD pattern and the FT-IR spectrum of the NC particles employed in the fifth cycle. Figure 18a, b show the FT-IR spectra and the XRD patterns of the NC before and after the fifth cycle of AMX degradation, respectively. As is evident, performance of the five sonophotocatalysis cycles does not alter the crystal structure of the g-C3N4-20@Ni−Ti LDH NC. K

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Figure 18. (a) FT-IR spectra and (b) XRD patterns of the g-C3N4-20@Ni−Ti LDH NC before and after five cycles of sonophotocatalytic AMX degradation.

Figure 19. Kinetics of AMX degradation through (a) sonophotocatalysis, photocatalysis and sonolysis, and (b) using pure g-C3N4, pure Ni−Ti LDH, and the g-C3N4-20@Ni−Ti NC.

Scheme 1. Sonophotocatalytic Mechanism Proposed for Degradation of AMX over the g-C3N4@Ni−Ti LDH NC

bubbles in the solution. On the other hand, the applied heterogeneous catalyst improves cavitation by presenting additional nuclei for the bubbles. The improvement in cavitation enhances water pyrolysis and •OH production since collapsing of the bubbles generates highly active H and OH radicals. In addition, association of the generated H radicals with the dissolved O2 molecules gives HO2 (eqs 4 and

the photocatalyst. With respect to the results obtained from analysis of the different aspects of the degradation process, the reaction mechanism of Scheme 1 can be proposed for sonophotocatalytic removal of AMX. According to the proposed mechanisms, the AMX molecules adsorbed on the surface of the g-C3N4-20@Ni−Ti LDH nanoparticles react with the photogenerated active radicals meantime the ultrasound waves create high-temperature and pressure L

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Inorganic Chemistry 5).87 After cavitation, the produced radicals interact with the AMX molecules and degrade them. H 2O + ultrasound →• H +• OH ·

H+O2 → HO2

·

Table 1. Sonophotocatalytic Degradation of Organic Pollutants Using Nanocomposites

(4)

catalyst hedgehoglike FTiO2(B)/CNT HKUST-1-MOF−Bi VO4

(5)

Addition of suitable powders to the reaction solution has been found effective on increasing the rate of cavitation bubble formation and, consequently, the amount of OH radicals.88 For example, Iida et al.89 reported an enhancement in the sonochemical removal of methyl orange by adding a proper α-alumina powder and Nakui et al. observed an increase in the rate of phenol removal by adding a porous and rough sample of coal ash to their reaction medium.90 Therefore, the g-C3N420@Ni−Ti LDH nanoparticles can be considered as extra nuclei for creation of bubbles, in the sonocatalysis process. On the other hand, the mechanism of the photocatalysis reaction is bound to photogeneration of charge carriers. Photoinduced formation of holes can lead to the transfer of the holes from the valence band of the g-C3N4-20@Ni−Ti LDH NC to its surface, oxidation of the adsorbed −OH and H2O species to the OH and O2− radicals and, finally, degradation of the AMX molecules that have adsorbed onto the surface of the photocatalyst. The corresponding reactions follow eqs 6−12. g‐C3N4‐20@Ni − Ti LDH NC + hv → h+ + e−

(6)

e− + O2 → ·O2 −

(7)

h+ + H 2O →• OH +• H

(8)

h+ +− OH →• OH

(9)

·

O2− + AMX → H 2O + CO2

(10)

h+ + AMX → H 2O + CO2

(11)

·

OH + AMX → H 2O + CO2

Ce:Cu-1,4-BDOAH2 Ag3PO4/Bi2S3− HKUST-1-MOF ZnO/CNTs Bi2O3/TiZrO4 TiO2− montmorillonite/ polythiophene-SDS Au−TiO2 g-C3N4@Ni−Ti LDH

pollutant

ultrasonic frequency

.ultrasonic power (W)

malachite green

45−55 Hz

285

91

disulfine blue and rose bengal diazinon trypan blue and vesuvine rhodamine B 4-chlorophenol rhodamine 6G

25 kHz

50

92

25 kHz 25 kHz

95 60

93 94

35 kHz 20 kHz 500 kHz

200 51 30

95 96 97

simazine AMX

42 kHz 20 kHz

50 200

98 this work

ref

4. CONCLUSION In this study, uniform g-C3N4 nanosheets, nonaggregated Ni− Ti LDH nanoparticles, and g-C3N4@Ni−Ti LDH nanocomposites with high purity were prepared through an optimized hydrothermal method in the presence of NH4F. Further, the synthesized materials were applied to sonophotocatalytic removal of AMX from aqueous solutions. Among the nanocomposites, g-C3N4-20@Ni−Ti LDH exhibited the highest sonophotocatalytic efficiency. The observed enhancement in the sonophotocatalytic activity of the nanocomposites can be related to their higher specific surface areas, the intimacy of the contact interfaces of their individual components, i.e., pristine g-C3N4 and Ni−Ti LDH, the synergistic effect between these components and restriction of electron−hole recombination. In addition to sonophotocatalysis, the sonolysis and photocatalysis processes were adopted to degrade AMX. Our findings specified the following order of efficiency for degradation of AMX by these approaches: sonocatalysis < photocatalysis < sonophotocatalysis. In this respect, 500 W light intensity, 9 s on/1 s off ultrasound pulse mode, and 1.25 g/L g-C3N4-20@Ni−Ti LDH were determined as the optimum sonophotocatalysis conditions. These conditions led to 99.5% AMX removal within 75 min. According to the proposed mechanism and the total organic content of the examined solutions, sonophotocatalysis results in the generation of different active species, e.g., OH radicals and holes, that can oxidize and mineralize the AMX molecules. Finally, reusability and stability of g-C3N4-20@Ni− Ti LDH was evaluated by performing five consecutive cycles of sonophotocatalysis. This investigation demonstrated a negligible loss of catalytic efficiency, insignificance of photocorrosion and a high level of photostability. The promoted sonophotocatalytic performance and the low cost of preparing the nanocomposite make us recommend application of the proposed degradation process to treatment of environmental pollutants.

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Therefore, during the sonophotocatalytic process, the photocatalytic and sonochemical degradation of AMX should proceed simultaneously. By referring to Figure 19, one can notice that the rate of the sonophotocatalytic reaction is higher than sum of the rates of the sonochemical and photocatalytic reactions. This observation can be explained by relying on the fact that the created cavitation bubbles prompt transfer of the AMX molecules from the bulk solution to the surface of the photocatalyst. Moreover, the cavitation bubbles help to remove any degradation products from the surface of the photocatalyst, which presents more active sites for adsorption and degradation of the other AMX molecules on the surface of the NC. Therefore, coupling of ultrasonication and photocatalysis (sonophotocatalysis) seems to promote the rate of pollutant degradation by improving OH production. Table 1 compares the performance of the devised sonophotocatalytic process with the efficiency of some other sonophotocatalytic processes that have been employed to degrade various organic pollutants.91−98 As Table 1 presents, the g-C3N4-20@Ni−Ti LDH NC might have the potential of acting as an efficient catalyst for degradation of other pollutants. Therefore, it is suggested its sonophotocatalytic activity is evaluated in the degradation of more organic compounds.

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02575. M

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UV−vis diffuse reflectance and total organic carbon for obtained materials (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +98 2182883442. Fax: +98 2182883455 (A.R.M.). *E-mail: [email protected]. Tel.: +8610-6739-6118. Fax: +8610-6739-1983 (H.X.D.). ORCID

Ali Reza Mahjoub: 0000-0002-2542-0031 Hongxing Dai: 0000-0003-1738-0348 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Tarbiat Modares University, Iran. The authors are grateful for the financial support. REFERENCES

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