Visible-Light-Induced GraphiticâC3N4@Nickel ... - ACS Publications

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

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Visible-Light-Induced Graphitic−C3N4@Nickel−Aluminum Layered Double Hydroxide Nanocomposites with Enhanced Photocatalytic Activity for Removal of Dyes in Water Ghazal Salehi, Reza Abazari, and Ali Reza Mahjoub* Department of Chemistry, Tarbiat Modares University, P.O. Box 14115−175, Tehran, Iran

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S Supporting Information *

ABSTRACT: One of the major challenges in photodegradation of organic dyes is designing a visible light active and highly efficient photocatalyst that can degrade both cationic and anionic dyes. To design such an ideal catalyst, this work synthesized graphitic−C3N4@NiAl layered double hydroxide nanocomposites (g-C3N4@NiAl−LDH NCPs) with various g-C3N4 contents through a convenient and high-yield method. The photocatalytic process was optimized by evaluating the impacts of type of dye (cationic and anionic), photocatalyst dosage, pH, and contact time. According to the results, the photocatalytic performance of g-C3N4@NiAl−LDH NCPs in degradation of cationic and anionic dyes is more noticeable than the photocatalytic activities of its discrete components. The observed improvement in the photocatalytic performance of the g-C3N4@NiAl−LDH NCPs can be attributed to the intimacy of their contact interfaces and a synergistic effect between pristine gC3N4 and NiAl−LDH, which results in effective mass transfer and separation of photogenerated charge carriers. The impact of some charge scavengers on the process was evaluated to define the role of each active species and propose a possible photodegradation mechanism. The g-C3N4@Ni−Al LDH NCPs could be reused for four cycles without any significant loss in efficiency.

1. INTRODUCTION Pollution of water resources by industrial dyes, natural organic matter, microorganisms, and heavy metals is a growing environmental concern.1 Among different wastewater pollutants, organic dyes are recognized as considerable contaminants that can result in serious health hazards even at low concentrations.2 Beyond direct health implications, the presence of dyes in wastewaters can reduce transparency of water, consume the existing oxygen molecules, intensify biochemical oxygen demand, and therefore, destroy aquatic life.3,4 Anionic dyes are classified as acidic, direct, and reactive dyes, whereas cationic dyes belong to the basic category of dyes.5 One of the well-known cationic dyes is rhodamine B (Rh B), which is usually used for wood, leather, paper, and silk.6 On the other hand, methyl orange (MO) is an anionic dye that has been extensively employed for printing, production of food and textiles, and scientific research, in addition to its use in pharmaceutical industries.7 Both Rh B and MO are toxic and can lead to gene mutations, cancer, and allergic dermatitis. If they were discharged into effluents, they would result in serious damage to the daily life of humans.8,9 Consequently, from an environmental perspective, it is crucial to eliminate organic dyes from wastewaters. For this purpose, the two methods of dye adsorption by an appropriate substrate and dye degradation into nontoxic metabolites through advance oxidation processes (AOPs) can be adopted.10 The adsorption method is associated with several disadvantages, © XXXX American Chemical Society

including incomplete dye adsorption, difficult adsorbent recovery, which reduces adsorbent reusability, and the problem of disposing the sludge produced after the transfer of dyes from wastewater to the solid phase.11 Semiconductor photocatalytic processes can be effective on removal of organic compounds. Moreover, they have a great potential for industrial applicability.12 In general, photocatalysis has attracted the interest of many researchers since this green technology can solve some global environmental and energy issues.13 The key to a successful photocatalysis process is employing a suitable photocatalyst that can provide high photocatalytic activity, stability, low cost, and nontoxicity.14 Layered semiconductors have attracted much attention due to their layered structures.15 In this context, layered double hydroxides (LDHs) have gained much interest for visible light photocatalysis applications, in recent years. The reason is that they are low-cost, and they can be prepared conveniently.16 In the past decade, different LDHs have been synthesized, and their photocatalytic properties have been investigated. For instance, Zhao et al. showed that MCr−X−LDH (M = Cu, Ni or Zn; X = NO3− or CO32−) is an effective and recyclable photocatalyst for degradation of organic dyes and colorless pollutants.17 Bhattacharyya and Roy Chowdhury reported that NiTi−LDH has noticeable photocatalytic activity for decomReceived: June 12, 2018

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DOI: 10.1021/acs.inorgchem.8b01636 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

transparent solution at room temperature, under magnetic stirring. This final solution was transferred to a 200 ml Teflon-lined stainless steel autoclave and heated for 48 h at 120 °C. After cooling the autoclaved solution to room temperature at a rate of 5 °C/h and drying the synthesized solid for 24 h in a vacuum at 80 °C, the solid was ground gently to obtain the final composite. To distinguish the composite samples, they are called g-C3N4−X@NiAl−LDH, where X refers to the weight percentage of g-C3N4 that is incorporated in the composites according to thermal weight analysis. On the basis of the described synthesis process, five g-C3N4−X@NiAl−LDH samples with 10, 20, 30, 40, and 50% g-C3N4 were prepared. For comparison purposes, several samples of pure NiAl−LDH with 1:1, 2:1, and 3:1 molar ratios of Ni/Al were synthesized through the same procedure in the three solvents of water, ethylene glycol and ethanol without adding g-C3N4.

position of MB under visible light, and it can result in 99% photodegradation within 100 min, which is beyond the photocatalytic activity of the Degussa P25 commercial product.18 Ni et al. synthesized a series of Zn−M−NO3 LDHs (M = Al, Fe, or Ti) to decompose Rh B, photocatalytically.19 They stated that the Zn−Ti−NO3 LDH displays the best photocatalytic performance due to its great specific surface area. However, the efficiency of pure LDHs is not sufficient for photocatalytic processes due to fact that photoexcited LDHs generate short-living electron−hole pairs.20,21 To overcome this problem, a solution is combining LDHs with other layered semiconductor materials, e.g., metal-free polymeric graphitic carbon nitride (g-C3N4), to design composite photocatalysts.22 The band gap of g-C3N4 is about 2.7 eV, which makes it a suitable material for visible light absorption.23 In addition to possessing a proper band structure and a strong ability of light absorption, g-C3N4 is associated with high mechanical, thermal, and chemical stability and convenient mass production.24 However, similar to pure LDHs, the photocatalytic efficiency of pure g-C3N4 is not high enough due to its low surface area and fast recombination of its photogenerated electron−hole pairs.25 However, due to the unique properties of g-C3N4, it has been extensively coupled with other semiconductor photocatalysts to achieve enhanced catalytic performance from the resultant composites. Specifically, coupling g-C3N4 with layered semiconductors has turned out to be an effective strategy for promoting the photocatalytic performance of layered materials.26 Many studies have used composites of LDHs and g-C3N4 since the synergic effects arising from the positive properties of these two materials are helpful for many applications. For example, Arif et al.27 employed g-C3N4@CoMn-LDH nanohybrid particles to obtain a high overall efficiency of photoelectrochemical water splitting. Therefore, combination of gC3N4 and LDH can be considered as an ideal choice for photocatalysis. Herein, g-C3N4 is coupled with NiAl−LDH to design a composite material, i.e., g-C3N4@NiAl−LDH, for photodegradation of MO and Rh B organic dyes. The effects of gC3N4 loading, pH, and photocatalyst dosage are evaluated to determine the optimum experimental conditions for photodegradation of cationic and anionic dyes. Recently, this NCP has been synthesized through other methods. However, the adopted methods have provided morphologies that are appropriate for other purposes, such as supercapacitive performance28 and photocatalytic reduction of CO2 into renewable fuels.29 To the best of the authors’ knowledge, this is the first attempt to investigate g-C3N4@NiAl−LDH NCPs for photocatalytic degradation of Rh B and MO dyes.

3. RESULTS AND DISCUSSION Characterization and Photocatalytic Activity Measurement. Figure 1 displays the FT−IR spectra of NiAl−LDH,

Figure 1. FT−IR spectra of the pure NiAl−LDH with molar ratio of 2:1 (a), g-C3N4−10@NiAl−LDH (b), g-C3N4−20@NiAl−LDH (c), g-C3N4−30@NiAl−LDH (d), g-C3N4−40@NiAl−LDH (e), gC3N4−50@NiAl−LDH (f), and pure g-C3N4 (g).

g-C3N4, and the g-C3N4−X@NiAl−LDH NCPs. The FT−IR spectrum of pure NiAl−LDH (Figure 1a) is associated with two broad peaks at 3432 and 1635 cm−1. The 3432 cm−1 peak refers to the symmetric and asymmetric stretching vibrations of O−H, while the 1635 cm−1 peak can be attributed to the symmetric and asymmetric bending vibrations of the water molecules that are positioned in the interlayer gallery of the LDH. In addition, the peak located at 1373 cm−1 corresponds to the vibrations of carbonate ion in the interlayer space of NiAl−LDH, and the peaks observed below 800 cm−1 are due to the M−O and M−OH vibrations.30,31 The FT−IR peaks of the g-C3N4−X@NiAl−LDH NCPs (Figure 1b to f) include the peaks of pure g-C3N4 and pure NiAl−LDH, simultaneously. This evidence confirms inclusion of both materials in the synthesized NCPs. The FT−IR spectrum of pure g-C3N4 (Figure 1g) contains the stretching vibrational modes of CN over the range of 1200 to 1700 cm−1, where the peaks of 1242 and 1637 cm−1 are related to C−N and CN stretching modes, respectively, and bending vibrations of heptazine rings at 809 cm−1.32,33 The FE−SEM images of the NiAl−LDHs synthesized using the Ni/Al molar ratios of 1:1, 2:1, and 3:1 are shown in Figure S1 (see Supporting Information) with a 1 μm scale. On the basis of this figure, the 1:1 ratio (Figure S1a) has resulted in a non-homogeneous morphology with a wide size distribution.

2. EXPERIMENTAL SECTION Synthesis of g-C3N4−X@NiAl−LDH NCPs. In this study, urea hydrolysis was used to prepare the Ni and Al containing LDH samples since this method is convenient and economically affordable and provides high yields. The g-C3N4@NiAl−LDH composites with various g-C3 N 4 contents were synthesized in situ through coprecipitation of LDH (with Ni/Al molar ratio of 2) onto the prepared g-C3N4 particles. For this purpose, a definite amount of gC3N4 (from 10 to 50%) was dissolved in 50 mL of deionized water and dispersed by ultrasonication. Then, 100 mL of an aqueous solution containing 0.48 mmol of Ni(NO3)2·6H2O and 0.24 mmol of Al(NO3)3·9H2O was added to the ultrasonicated g-C3N4 solution gradually, under vigorous mechanic stirring. This process gave a transparent solution. After that, 24 mmol of urea was dissolved in the B


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Inorganic Chemistry

Figure 2. FE−SEM images of the pure g-C3N4 (a), g-C3N4−10@NiAl−LDH (b), g-C3N4−20@NiAl−LDH (c), g-C3N4−30@NiAl−LDH (d), gC3N4−40@NiAl−LDH (e), g-C3N4−50@NiAl−LDH (f).

Figure 3. TEM images of the g-C3N4−40@NiAl−LDH NCPs in two different scales.

LDHs of like-hexagonal and cover them regularly. Also, the presence of the LDH prevents g-C3N4 agglomeration and distributes the g-C3N4 particles on the LDH nanoplates evenly. Also, a TEM image of the g-C3N4−40@NiAl−LDH NCPs is shown in Figure 3. In this figure, the bright and dark zones can be ascribed to the hexagonal-like layers of NiAl−LDH and the g-C3N4 particles, respectively. As Figure 3 indicates, NCP formation has induced a loss of stacking order to the LDH plates and has led to LDH delamination in the matrix of gC3N4. The particles of g-C3N4 and NiAl−LDH plates are pointed by arrow marks. Figure 4 shows the PXRD patterns of the pure g-C3N4, pure NiAl−LDH, and g-C3N4−X@NiAl−LDH NCPs over the 2θ range of 1 to 80°. The PXRD pattern of pure g-C3N4 (Figure 4a) outlines a peak at 27.4° that belongs to the (002) plane and is the characteristic peak of g-C3N4, consistent with the PXRD pattern reported for this material (JCPDS 871526).35,36 Figure 4b refers to the PXRD pattern of pure NiAl−LDH with a Ni/Al molar ratio of 2:1. The (003), (006), (012), (015), (018), (110), and (113) planes are in excellent agreement with the 15-0087 standard card (JCPDS 15-0087/ 01-089-1777).37,38 This sample is highly pure since no odd

However, the 2:1 and 3:1 ratios (Figure S1b and c) have presented a hexagonal-like morphology. These morphologies prove that there is a relationship between the employed MII/ MIII molar ratio and the resultant morphology. According to the literature,34 the hexagonal morphology of LDH provides more electron active sites for redox reactions, restricts the diffusion paths of ions, and enhances the specific capacity of LDHs. Figure S2 illustrates the FE−SEM images of the NiAl− LDH nanostructures prepared in ethanol and ethylene glycol. It can be seen that the morphology of the LDH nanostructures is less homogeneous in ethanol and ethylene glycol, compared with the water. Moreover, no additional or alternative particle morphology is observed by changing the solvent to ethanol and ethylene glycol. Therefore, among the three solvents, water, which is a simple, cheap, accessible, and ecofriendly solvent, was chosen as the optimal solvent for further synthesis trials. Figure 2 depicts the FE−SEM images of pure g-C3N4 and the g-C3N4−X@NiAl−LDH NCPs. As Figure 2a shows, pure g-C3N4 has an agglomerated morphology. In the mean time, according to Figure 2b−f, as the weight percentage of gC3N4 in the g-C3N4−X@NiAl−LDH NCPs increases from 10 to 50%, more g-C3N4 particles deposit on the surface of the C


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Inorganic Chemistry

which correlates with a bandgap energy of about 2.7 eV according to the following equation (eq 1): (αhv)1/ n = A(hv − Eg )

(1)

where α, h, n, Eg, and A are the absorption coefficient, Planck’s constant, light frequency, band gap energy, and a constant, respectively. Meanwhile, the optical absorption bands of pure NiAl−LDH are positioned in the UV and visible regions of light. The NiAl−LDH band that has appeared at about 421 nm is related to d−d transitions rising from the Ni2+ ions in an octahedral field.40,41 More specifically, the 421 nm band can be related to the spin forbidden 3A2g(F) →1T2g(D) transitions,42,43 whereas the UV region absorption bands are due to the spin−allowed 3A2g(F) → 3T1g(P) transitions, which result from the d8 configuration of the Ni2+ ions. On the other hand, the DRS spectra of the studied NCPs are accompanied by a red shift in their absorption edges. The red shift is due to the presence of g-C3N4 as a visible light material on the plates of NiAl−LDH. Also, the value of the valence band’s edge potential (EVB) of a semiconductor can be calculated using (eq 2)

Figure 4. PXRD patterns of the pure g-C3N4 (a), pure NiAl−LDH with a molar ratio of 2:1 (b), g-C3N4−10@NiAl−LDH (c), g-C3N4− 20@NiAl−LDH (d), g-C3N4−30@NiAl−LDH (e), g-C3N4−40@ NiAl−LDH (f), and g-C3N4−50@NiAl−LDH (g).

peak has been detected in its PXRD pattern. All the PXRD peaks of pure g-C3N4 and NiAl−LDH are present in the PXRD patterns of the g-C3N4−X@NiAl−LDH NCPs (Figure 4c to g). Upon increase of the g-C3N4 weight percentage, the characteristic peak of g-C3N4 intensifies, which declares correct synthesis of the NCPs. Also, through the application of Debye−Scherrer equation m = kλ/(β cos θ),39 the average crystallite size of the g-C3N4, NiAl−LDH, g-C3N4−10@NiAl− LDH, g-C3N4−20@NiAl−LDH, g-C3N4−30@NiAl−LDH, gC3N4−40@NiAl−LDH, and g-C3N4−50@NiAl−LDH NCPs was found to be about 48.14, 61.46, 82.65, 106.87, and 132.68 nm, respectively. In this equation, k is the so-called shape factor, which usually takes a value of about 0.9, β is the full width in radians at half-maximum intensity (fwhm), m is the mean particle size, λ is the wavelength of the X-ray source used in PXRD, and θ is the angle at maximum diffraction curve intensity. Since photocatalytic activity relies on optical absorption, the light response properties of the synthesized photocatalysts were evaluated by UV−vis DRS. As Figure 5 displays, the strong absorption edge of pure g-C3N4 is located at 459 nm,

E VB = XEe + 0.5Eg

(2)

where X is denoted as electronegativity of the semiconductor material estimated by geometrical averaging of the electronegativity values of its constituent atoms, Ee stands for the energy of free electrons on the hydrogen scale (∼4.5 eV), and Eg refers to the material’s bandgap energy. With respect to the EVB value, the value of the CB’s edge potential (ECB) can be obtained through ECB = EVB − Eg. The X values for pure gC3N4 and NiAl−LDH are obtained at about 0.049 and 0.195 eV, respectively. Therefore, the EVB values of pure g-C3N4 and NiAl−LDH can be determined as +1.57 and +2.35 eV, and the corresponding E CB values equal −1.22 and 0.59 eV, respectively. Figure 6 illustrates the photoluminescence (PL) spectra of the pure g-C3N4, pure NiAl−LDH, and g-C3N4−X@NiAl− LDH samples at 385 nm excitation wavelength. According to this figure, pure g-C3N4 exhibits a strong emission peak at about 470 nm, which can be related to the band−band PL induced by recombination of the photogenerated charge

Figure 6. PL spectra of the pure g-C3N4 (a), pure NiAl−LDH with molar ratio of 2:1 (b), g-C3N4−10@NiAl−LDH (c), g-C3N4−20@ NiAl−LDH (d), g-C3N4−30@NiAl−LDH (e), g-C3N4−40@NiAl− LDH (f), and g-C3N4−50@NiAl−LDH (g).

Figure 5. UV−vis diffuse reflectance of the pure g-C3N4 (a), pure NiAl−LDH with a molar ratio of 2:1 (b), and g-C3N4−40@NiAl− LDH NCPs (c). D


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Inorganic Chemistry

6 times the adsorption percentage observed for the cationic Rh B dye, under the same conditions. This observation unravels the idea that NiAl−LDH has a high tendency toward MO adsorption. However, as irradiation of the catalytic system by light prompts MO removal, both adsorptive and photocatalytic processes have roles in elimination of MO. The variations in the peak intensity of the Rh B and MO pollutants in the presence of the three Ni/Al ratios of NiAl−LDH are shown in Figures S7 and S8, respectively. On the basis of the results of these figures, intensities of the absorbance peaks decline with evolution of time, which is indicative of removal of the pollutants from the aqueous solutions. According to Figures S9 and S10, photocatalytic degradations of the Rh B and MO dyes by the optimum NCPs are second-order reactions. To determine the optimal weight percentage of g-C3N4 for degradation of the pollutants by the g-C3N4−X@NiAl−LDH NCPs, the effect of 10, 20, 30, 40, and 50% g-C3N4 and 10, 20, and 30% g-C3N4 were evaluated for removal of Rh B and MO, respectively. The results of the cationic and anionic dyes are displayed in Figures 8 (or Figure S11) and 9 (or Figure S12),

carriers. Despite the noticeable peak intensity of g-C3N4, the PL intensity of pure NiAl−LDH is weaker since NiAl−LDH produces a lower number of photogenerated electron−hole pairs under the same irradiation conditions. The PL peak of the g-C3N4−X@NiAl−LDH NCPs is less intense than that of the pure LDH and g-C3N4 samples, which reveals that the NCPs can present a lower rate of electron−hole recombination. The surface zeta potential (ξ) values of pure NiAl−LDH were measured as 62.7, 51.4, 18.3, 7.12, and −9.6 mV at the solution pH values of 3, 5, 7, 9 and 11, respectively. These values declare that NiAl−LDH possesses strong positive charges on its surface under neutral and acidic conditions. The reason is that the metal hydroxide layers of NiAl−LDH bear positive charges due to the replacement of some Ni2+ cations with Al3+, in the octahedral lattice of nickel hydroxide. When NiAl−LDH interacts and combines with g-C3N4 to give g-C3N4−40@NiAl−LDH, its ξ values shift significantly and result in the ξ values of 31.6, 17.5, −4.2, −14.9, and −28.0 mV at 3, 5, 7, 9, and 11 pH values, respectively (Figure 7). This

Figure 7. Effect of pH on the zeta potential of pure NiAl−LDH nanoplates and g-C3N4−40@NiAl−LDH NCPs. Figure 8. Photocatalytic activities of Rh B dye under dark conditions and visible-light irradiation after 240 min: g-C3N4−10@NiAl−LDH (a), g-C3N4−20@NiAl−LDH (b), g-C3N4−30@NiAl−LDH (c), gC3N4−40@NiAl−LDH (d), g-C3N4−50@NiAl−LDH (e), pure gC3N4 (f) under visible-light irradiation, pure g-C3N4 (g), and gC3N4−40@NiAl−LDH NCPs (h) under dark conditions, and without photocatalyst under visible-light irradiation (i).

finding suggests self-assembly of the g-C3N4−40@NiAl−LDH NCPs through electrostatic interactions. On the basis of the ξ measurements, the isoelectric points (IEPs) of the NiAl−LDH and g-C3N4−40/NiAl−LDH samples were determined as 9.83 and 6.65, respectively. Time evolution of Rh B and MO photodegradation by the NiAl−LDH nanostructures with different Ni/Al ratios is presented in Figures S3−S6. As can be seen, the optimal Ni/ Al ratio for both the anionic and cationic pollutants is 2:1. This metal ratio can provide the highest extent of dye removal. As shown in Figures S3 and S4, in degradation of Rh B dye, 56% of the pollutant is removed after 240 min using the 2:1 ratio, while the 1:1 and 3:1 ratios have resulted in 31% and 44% Rh B elimination, respectively, under the same conditions. Moreover, just 6% of the Rh B molecules adsorb on NiAl− LDH (2:1 Ni/Al molar ratio) after 240 min of contact time, under darkness and fixed conditions. Therefore, since removal of Rh B under irradiation is significant, it can be claimed that removal of Rh B follows a photocatalytic process rather than molecular adsorption. Also, based on Figures S5 and S6, the optimal Ni/Al ratio for elimination of MO by NiAl−LDH is 2:1, which results in 81% photodegradation within 180 min. Also, Figures S5 and S6 outline 65% MO adsorption on NiAl− LDH within 180 min of contact time, in the dark. This value is

respectively. In the case of Rh B, within 240 min of contact time, increasing the weight percentage of g-C3N4 from 10 to 40% enhances its photodegradation while a further increase from 40 to 50% declines degradation. After 240 min of contact time, the percentage of Rh B photodegradation using 10 to 50% g-C3N4 was found to be 58%, 69%, 80%, 93%, and 86%, respectively. In the case of MO, within 180 min of contact time, an increase of g-C3N4 percentage from 10 to 20% improves MO degradation, while a further increase, i.e., from 20 to 30%, decreases the degradation percentage. In this way, using 10 to 30% g-C3N4 gave 81%, 93%, and 76% MO photodegradation, respectively. To unravel further details about performance of the NCPs, it is necessary to investigate removal of the dyes by pure g-C3N4 in the presence and absence of light. According to Figure 8, 0.2 g of pure g-C3N4 provides 47% removal of Rh B (20 ppm and neutral pH) within 240 min of contact time under irradiation E


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Inorganic Chemistry

MO pollutants in the presence of the prepared NCPs are shown in Figures S21 and S22, respectively. Also, according to Figures S23 and S24, photocatalytic degradations of the Rh B and MO dyes by the optimum NCPs are first-order reactions. In the next step, the catalyst dosage was fixed to evaluate the effect of dye concentration on performance of the photocatalysts. The photocatalytic degradation of Rh B and MO declines about 47% and 61%, respectively, by increasing their starting concentrations from 20 to 40 ppm, at a constant reaction time, pH, and catalyst dosage. The reason can be attributed to the decrease of the number of available active sites on the surface of the catalyst, compared with the number of dye molecules. Another important parameter of photocatalytic processes is the pH of the reaction medium. In general, the difference in the activity of any semiconductor depends on pH due to various surface properties.44,45 In this study, three solutions with 20 ppm concentration and 100 mL volume were made using each dye, and their pH values were adjusted to the acidic, neutral, and basic pH values of 3, 7, and 10 to determine the optimum pH for photocatalytic degradation of Rh B and MO. The results are exhibited in Figures 10 (or Figure S25) and 11

Figure 9. Photocatalytic activities of MO dye under dark conditions and visible-light irradiation after 180 min: g-C3N4−10@NiAl−LDH (a), g-C3N4−20@NiAl−LDH (b), g-C3N4−30@NiAl−LDH (c), pure g-C3N4 (d) under visible-light irradiation, pure g-C3N4 (e), gC3N4−20@NiAl−LDH NCPs (f) under dark conditions, and without photocatalyst under visible-light irradiation (g).

conditions, whereas the optimal NCPs remove 93% of the Rh B molecules under the same conditions. Also, based on Figure 9, 0.2 g of g-C3N4 presents 61% MO removal (20 ppm and neutral pH) within 180 min of contact time under irradiation, while the optimal composite can eliminate 93% of the MO molecules under the same conditions. As Figure 8 shows, the optimal NCP, i.e., g-C3N4−40@NiAl−LDH, adsorbs 16% of 20 ppm Rh B after 240 min of contact time. Under similar conditions, pure g-C3N4 adsorbs 18% Rh B, in the dark. Also, according to Figure 9, the optimal NCP of g-C3N4−20@ NiAl−LDH can remove 47% of the MO pollutant molecules within 180 min, while pure g-C3N4 can provide 18% MO elimination under the same conditions, in the dark. Furthermore, Figures 8 and 9 demonstrate approximately zero pollutant degradation in the absence of any catalyst. This evidence clarifies the necessity of catalyst presence for photodegradation of the pollutants. The changes in the absorbance of the Rh B and MO solutions after application of pure g-C3N4 and the synthesized NCPs under irradiation and darkness conditions are illustrated in Figures S13 and S14, respectively. Also, according to Figures S15 and S16, photocatalytic degradations of the Rh B and MO dyes by the optimum NCPs are second-order reactions. One of the other important parameters in photocatalytic processes is the dosage of the utilized photocatalyst for elimination of the dye. When dosage of the catalyst exceeds a specific limit (saturation level), the coefficient of photon absorption decreases since the additional photocatalyst particles cover the active sites, reduce the total surface area that can be irradiated, and finally, decline decrease efficiency of the photocatalytic process. So, a definite amount of the photocatalyst can exhibit the best efficiency. In this research, the photocatalytic degradation of 20 ppm Rh B and MO solutions was conducted in the presence of 0.1, 0.2, 0.4, and 0.6 g g-C3N4−40@NiAl−LDH and 0.1, 0.2, and 0.4 g of gC3N4−20@NiAl−LDH (Figures S17 and S18 and Figures S19 and S20), respectively. With respect to the results, 0.2 g of gC3N4−X@NiAl−LDH was chosen as the optimal amount of the photocatalyst for Rh B and MO elimination. The variations in the absorbance of different concentrations of the Rh B and

Figure 10. Photocatalytic activities of Rh B dye under dark conditions and visible-light irradiation after 240 min at three different pH’s: pH = 3 (a), pH = 7 (b), and pH = 10 (c) under visible-light irradiation and pH = 10 under dark conditions (d).

(or Figure S26). With respect to Figure 10, the optimum pH for degradation of Rh B is pH = 10. Since Rh B is a cationic dye, Rh B and the free H+ ions repulse each other, experiencing electrostatic repulsion when they approach the positive surface of the LDH and compete for the negative nanoparticles of gC3N4, under acidic conditions. That is why degradation of Rh B is less considerable at pH 3, compared with the neutral and basic conditions. Figure 10 approves this statement by outlining the least efficiency of dye degradation at pH 3. Figure 10 exhibits two interesting points. First, adsorption of Rh B on the NCP is zero under acidic conditions due to the severe repulsion between the positive surface charges of the NCP, the cationic pollutant, and the protons, which prevent adsorption of Rh B on the catalyst. The second point is that degradation of Rh B in the irradiated acidic solution is so low, even lower than the darkness condition. The reason can be attributed to the presence of H+ ions in the acidic solution. Also, since MO is an anionic dye, the acidic solution is F


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Inorganic Chemistry

(Figure S31) indicated that solar irradiation gives 72% and 84% Rh B and MO degradation, respectively. These values are almost consistent with the degradation extents observed under irradiation of the Hg lamp. Moreover, to investigate the extent of the degradation process, the optimal NCP, i.e., g-C3N4− 40@NiAl−LDH, was adopted, and the total organic carbon (TOC) of the reaction solution was analyzed under optimized conditions. In this respect, photocatalytic mineralization of the MO and Rh B dyes by the optimal nanocomposites was tracked under visible light irradiation. The related results are depicted in Figure S32. As Figure S32 shows, the MO and Rh B dyes degrade within 180 and 240 min contact time; in the mean time, TOC removal of their reaction systems reaches 85.5% and 78%, respectively. From an economic perspective, the physical and chemical characteristics of each material should be known prior to its application. In this respect, thermal stability of the optimal photocatalyst and the effect of pH on its performance were studied. As Figure S33 shows, pH (4 and 9) does not affect the positions of the PXRD peaks, while it alters the intensity of the peaks. Thermal stability of the optimal NCP was also assessed. According to Figure S34, the TGA curve of the g-C3N4-40@ NiAl-LDH NCPs demonstrates three distinct regions of weight loss. The first region is located over the 25 to 190 °C temperature range and refers to the evaporation of the physisorbed and interlayer water molecules. The second weight loss is observed from 250 and 350 °C and represents dehydroxylation of the LDH layers and decomposition of the interlayer nitrates.46−48 The last region, which starts at 550 °C and ends around 700 °C, corresponds to the decomposition of the g-C3N4 nanosheets.49 Photocatalysts might deactivate due to the binding of secondary products or oxidized intermediates to their structures. It was observed that the photocatalytic performance of the g-C3N4−40@NiAl−LDH and g-C3N4−20@NiAl−LDH optimal NCPs for photocatalytic removal of the 20 ppm Rh B and MO dye solutions slightly declines after four consecutive runs, which indicates proper stability of the synthesized NCPs. The observed decrease of catalytic activity can be ascribed to the fact that a portion of the catalyst particles would be lost after each cycle of catalyst washing. The results of this evaluation for the g-C3N4−40@NiAl−LDH and g-C3N4−20@ NiAl−LDH catalysts are depicted in Figures 12 and 13, respectively. To acquire some details about the mechanism of Rh B and MO photodegradation, the main active oxidants of the process were identified. For this purpose, IPA, EDTA−2Na or BQ, which respectively act as hydroxyl radical, hole, and •O2− scavengers, were used to trap and measure the generated oxidants. As Figure 14 exhibits, the rate of Rh B and MO photodegradation by the optimal NCPs reduces greatly by adding EDTA−2Na, while BQ addition has an insignificant effect on the photocatalytic processes. In addition to EDTA− 2Na, the presence of IPA alters photocatalytic degradation of the two dyes. Consequently, both holes and hydroxyl radicals are the active species in catalytic photodegradation of the dyes, whereas the •O2− species have negligible roles. From an electronic perspective, the photodegradation mechanism initiates by irradiating the NCPs with some photons of light whose energies are greater than the bandgap energy of the NCPs. This incidence produces some pairs of electron and hole.50 Then, the generated electrons move their conduction band, and the formed holes remain in the valence

Figure 11. Photocatalytic activities of MO dye under dark conditions and visible-light irradiation after 180 min at three different pH’s: pH = 3 (a), pH = 7 (b), and pH = 10 (c) under visible−light irradiation and pH = 3 under dark conditions (d).

expected to provide the highest extent of MO degradation. In this respect, Figure 11 clarifies that pH = 3 is the optimal pH value for photodegradation of MO. Under basic conditions, repulsion and competition between the OH− anion and anionic MO for adsorption on the catalyst decreases MO degradation, compared with the neutral and basic pH conditions. Figure 11 confirms this reasoning and depicts the lowest degradation efficiency for pH = 10. To unravel the importance of adsorption in degradation of the pollutants, the optimal pH values, NCPs, and catalyst dosage were applied to the Rh B and MO solutions, in the dark. Figures 10 and 11 present the corresponding results, and comparing them illuminates the role of the positive surface charges of the NCPs on adsorption of the dyes. Under the optimal conditions, the adsorption percentages of Rh B (pH = 10) and MO (pH = 3) equal 45% and 83%, respectively. The greater adsorption of MO is an outcome of its anionic nature and existence of positive charges created by the presence of H+ ions, which facilitate the adsorption of anionic MO on the intrinsically positive surface of the NCP. The changes in the absorbance of the Rh B and MO solutions using the synthesized NCPs and the optimal pH values under irradiation and darkness conditions are shown in Figures S27 and S28, respectively. Also, to determine the reaction orders of photodegradation of Rh B by g-C3N4−40@NiAl−LDH NCPs at pH 10 and the MO dye by g-C3N4−20@NiAl− LDH NCPs at pH 3, the zero-, first-, and second-order curves of the reactions were plotted in Figures S29 and S30, respectively. Also, in the present work, to investigate if the photocatalyst can degrade both dyes simultaneously, a 10 ppm mixture of the two dyes was studied in the presence of the g-C3N4−40@ NiAl−LDH NCPs at pH 7. The degradation process does not complete within 240 min. The reason can be related to the stronger adsorption of the anionic dye, i.e., MO, onto the surface of the photocatalyst that influences degradation of Rh B negatively, in addition to the neutral pH of the reaction solution. Also, to simulate natural circumstances, the light source was replaced with natural solar radiation. In this respect, the photocatalytic reaction of dye degradation was carried out under solar irradiation, on a sunny day at 32 °C. The results G


Article

Inorganic Chemistry

and according to their bandgap energies (2.7 and 2.94 eV), their CB edges are positioned at −1.12 and 0.59 eV, respectively. Therefore, the CB of NiAl−LDH is located below the CB of g-C3N4, while the VB of g-C3N4 is above the VB of NiAl−LDH. On the basis of these electronic levels, a photocatalytic mechanism can be proposed for photodegradation of pollutants by the g-C3N4@NiAl−LDH NCPs, which is illustrated in Figure 15 schematically. According to the proposed mechanism, since the bandgap energies of both g-C3N4 and NiAl−LDH lie in the visible region of light, they can both get excited upon irradiation by visible light and produce electrons and holes in their CBs and VBs, respectively. The electrons that generate and move to the CB of g-C3N4 can pass through the heterojunction and transfer to the CB of NiAl−LDH, while the holes photogenerated on the VB of NiAl−LDH can surpass the heterojunction and reach the VB of g-C3N4. Such suppression of charge recombination and improved charge separation would contribute to the mutual activation of both g-C3N4 and NiAl−LDH. Further, the CB electrons can reduce the oxygen molecules that have bound to the surface of the NCPs to generate highly reactive superoxide anions followed by the production of reactive oxygen species. In a similar manner, the VB holes can oxidize the adsorbed water molecules to produce highly reactive hydroxyl radicals. Formation of such reactive species can prompt degradation of the pollutants. However, photocatalysis is associated with some disadvantages, including consumption of more time and energy. In summary, the holes generated in and transferred to the VB of g-C3N4 can degrade the dye molecules on the surface of the g-C3N4 sheets to form O2 and protons. Simultaneously, O2 molecules can trap the electrons photogenerated on the CB of NiAl−LDH to produce OH radicals by the assistance of the available protons through the following reactions (eqs 3−7):

Figure 12. Recycle and reuse of g-C3N4−40@NiAl−LDH NCPs for Rh B photocatalytic degradation.

g‐C3N4@NiAl−LDH + hυ

Figure 13. Recycle and reuse of g-C3N4−20@NiAl−LDH NCPs for MO photocatalytic degradation.

+ → g‐C3N4@NiAl−LDH (hVB + e−CB)

(3)

+ g‐C3N4@NiAl−LDH (hVB + e−CB) + → g‐C3N4(hVB )@NiAl−LDH (e−CB)

(4)

2H 2O → 4H+ + O2 + 4e− (E° = + 0.82 V)

(5)

CO2 + 2H+ + 2e− → CO + H 2O (E° = −0.53 V)

(6)

2H+ + 2e− → H 2 (E° = −0.41 V)

(7)

According to the recent literature, the complete photocatalytic degradation of Rh B involves the four steps of N-deethylation, chromophore cleavage, ring opening, and mineralization. Therefore, prolongation of the reaction increases the possibility of ring opening and enhances degradation of the aromatic intermediates into smaller compounds.51,52 Also, based on the findings of Lu et al.,53 the reaction of OH radicals with MO can be considered as the route of MO oxidation. This oxidative reaction can produce diverse products including benzenesulfonic acid, 4-hydroxy-N,N′-dimethylaniline, and N,N′-dimethylaniline. In general, degradation of organic dyes follows the three oxidative steps of bond cleavage, ring opening, and complete oxidation, and the progress of the degradation process can be determined by analyzing the total organic carbon of the reaction solution.

Figure 14. Effects of various active scavengers on the degradation of Rh B and MO dyes over g-C3N4−40@NiAl−LDH and g-C3N4−20@ NiAl−LDH NCPs under irradiation with visible light after 240 and 180 min, respectively.

bands of the NCPs. As previously mentioned, the VB edges of g-C3N4 and NiAl−LDH are positioned at 1.57 and 2.35 eV, H


Article

Inorganic Chemistry

Figure 15. Schematic possible mechanism for photocatalytic degradation of Rh B and MO dyes for the separation of photogenerated electron−hole pairs and the production of •O2− and •OH by g-C3N4@NiAl−LDH NCPs under visible-light irradiation.

composite, significantly. The presence of g-C3N4 restricts electron−hole recombination, while the interlayer of LDH separates the active sites from each other. Recyclability of the g-C3N4@NiAl−LDH NCPs was also examined. Several charge scavengers were employed to investigate their effects on the rate of dye photodegradation by the g-C3N4@NiAl−LDH NCPs and reveal the corresponding mechanism. In this way, the highest photocatalytic activity for degradation of Rh B and MO was observed by using 40% and 20% g-C3N4, respectively. While 0.2 g of catalyst could result in the best catalytic activity for degradation of both catalysts, the acidic pH of 3 and the basic pH of 10 were required as the optimum pH conditions for the anionic pollutant, i.e., MO, and the cationic dye, i.e., Rh B, respectively. According to these points of view, it can be proposed that the g-C3N4@NiAl−LDH NCPs can be used as a parent material to design some novel and sophisticated composite materials for photocatalysis purposes and solar energy utilization.

Table 1 compares the performance of the introduced gC3N4-X@NiAl-LDH NCPs with that of the other catalysts Table 1. Reported Photocatalytic Efficiency of Some Catalyst in Dyes Degradation dye

light

time

removal rate (%)

ref

MIL-100(Fe)

MO

7h

64

54

Cu2O/ZnAl-CLDH polyimide/ heterostructured Ni O−Fe2O3−ZnO SnO2 doped ZnO PS/CCA/TiO2 TiO2/MgZnAl LDH Co3(BPT)2(bpp)

MO MO

UV light visible visible

420 min 240 min

90 96.2

55 56

Rh Rh Rh Rh

360 300 9 120

49 78.7 93.73 90

57 58 59 60

g-C3N4-40@NiAl-LDH

Rh B

visible visible visible UV light visible

240 min

99

g-C3N4-20@NiAl-LDH

MO

visible

180 min

99

catalyst

B B B B

min min h min

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this work this work

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01636. Materials and physical techniques, FE−SEM images, UV/vis absorption spectra of degradation of poulutions, percent photodegradation and reaction kinetics for obtained materials (PDF)

proposed in the available literature. With respect to Table 1, the g-C3N4-X@NiAl-LDH NCPs provide a noticeable reaction yield in a shorter process time and under milder reaction conditions, compared with the other catalysts. Therefore, these NCPs can be accepted as a highly efficient photocatalyst for degradation of organic dyes.

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4. CONCLUSION In this research, the effect of the initial reactants’ concentrations and solvent type on the morphology and size of the NiAl−LDH nanostructures and, finally, their photocatalytic activity in degradation of anionic and cationic dyes were investigated. The results indicated that a Ni to Al molar ratio of 2:1, along with the green solvent of water, can present the best economic efficiency. To promote photodegradation of the dyes (RhB and MO), NiAl−LDH was modified with gC3N4 via a simple in situ method. The performance of the gC3N4@NiAl−LDH nanocatalyst in photodegradation of MO and Rh B dyes depends on the g-C3N4 content of the

AUTHOR INFORMATION

Corresponding Author

*Tel.: +98 2182883442. Fax: +98 2182883455. E-mail: [email protected]. ORCID

Ali Reza Mahjoub: 0000-0002-2542-0031 Author Contributions

Ghazal Salehi and Reza Abazari contributed equally to this work. Notes

The authors declare no competing financial interest. I


Article

Inorganic Chemistry

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

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