Facile Color Tuning, Characterization and Application of Acid Green

Article pubs.acs.org/IECR

Facile Color Tuning, Characterization, and Application of Acid Green 25 and Acid Yellow 25 Co-intercalated Layered Double Hydroxides Tingwei Chen,† Pinggui Tang,*,†,‡ Yongjun Feng,† and Dianqing Li*,†,‡ †

State Key Laboratory of Chemical Resource Engineering, and ‡Beijing Engineering Center for Hierarchical Catalysts, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: Acid Green 25 (AG25) has been cointercalated with Acid Yellow 25 (AY25) into the interlayer of ZnAl layered double hydroxides (LDH) via a coprecipitation method to tune the color of hybrid pigments based on LDHs. The prepared hybrids were analyzed by X-ray diffraction, scaning electron microscopy, Fourier transform infrared microscopy, inductively coupled plasma−emission spectroscopy, thermogravimetry−differential thermal analysis, UV−vis, and CIE 1976 L*a*b* color scales, which show that AG25 and AY25 have been cointercalated into LDH and the color of the prepared LDH can be easily tuned from greenish blue to bluish green and green by adjusting the molar ratio of AY25/AG25. There exists host−guest and guest−guest interactions in the hybrids, and the intercalation into LDH significantly improves the thermal stability of the AY25. The hybrids were used as colorant to prepare green coatings and films, showing their potential application in the fields of paints and plastics.

1. INTRODUCTION Organic dyes have received much attention because of their wide range of colors, excellent photosensitivity, and high color strength, and have been widely used as colorant in the fields of textiles, inks, and electronic devices. Nevertheless, they are seldom found in applications in the area of paints because of their undesirable solubility in water and organic solvents, severely limiting their use as pigment in industry. It is thereby desirable to find ways to decrease their solubility in order to expand their applications as pigments. Maya blue, an ancient pigment, has attracted a great deal of interest in recent years because of its excellent stability under the conditions of dilute mineral acid and alkaline solutions, solvent treatment, humidity, and so on,1−3 inspiring researchers to immobilize organic dyes in inorganic matrixes of zeolites, metal phosphates, and clay minerals to expand their application.4−8 The insertion of organic dyes into inorganic matrixes can produce a kind of organic−inorganic hybrid pigments with excellent performance by taking the advantage of the excellent mechanical strength and high stability of inorganic matrixes. Owing to their swelling and soft-matter-related properties, clays are considered as a good candidate to immobilize dye molecules to prepare multifunctional hybrid materials.9 Layered double hydroxides (LDH), a special class of anionic layered clays, has a general formula of [M2+1−xM3+x(OH)2]x+(An−)x/n·mH2O, where M2+ and M3+ stand for various divalent and trivalent metal cations, respectively, and An− represents the interlayer anions in the hydrated interlayer galleries.10−16 Positive charges are generated due to the partial substitution of divalent cations by trivalent cations in the octahedral hydroxide layers, which is balanced by the interlayer negative anions. Recently, a large number of © XXXX American Chemical Society

attention has been spent on LDH as a result of their controllable chemical composition and charge density, anion exchange ability, good thermal stability, and easy production. They also have wide applications in the fields of catalysis,17−20 adsorption,21,22 sensors,23,24 electrochemistry,25,26 biochemistry,27,28 polymer additives,29−31 and so on. In recent years, dye anions intercalated LDH materials have attracted considerable interest because of the existence of the synergistic effect between the LDH matrices and the organic dyes, giving rise to novel functionalities beyond the individual components alone.32−37 On one hand, the LDH host layers offer thermal and optical stabilization and protection to organic dyes by immobilizing them in the interlayer. On the other hand, the organic dyes contribute the optical function, such as color, fluorescence, and nonlinear optical properties.38−42 Organic dyes intercalated LDH composites have potential application as luminescent devices and pigments.8,34,36,43−45 For example, Yan et al. prepared bis(2-sulfonatostyryl)biphenyl anion intercalated MgAl LDH film through a layer-by-layer method, and the obtained film showed thermochromic property.46 Shi et al. synthesized 1,3,6,8-pyrenetetrasulfonate acid anion intercalated ZnAl LDH films on a quartz substrate via a layer-by-layer method as well, and found that the obtained film can be used as a fluorescence chemosensor for the detection of Cu2+ in water.47 The present authors prepared Acid Green 28 dye intercalated ZnAl-LDH which can be used as a pigment to color plastics.48 Though several dyes have been Received: Revised: Accepted: Published: A

January 19, 2017 April 14, 2017 April 27, 2017 April 27, 2017 DOI: 10.1021/acs.iecr.7b00279 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Structural formulas of AG25 (a) and AY25 (b).

Table 1. Detailed Preparation Condition of the Hybrid Pigments samples

Zn(NO3)2·6H2O (mol)

Al(NO3)3·9H2O (mol)

NaOH (mol)

AG25 (mol)

AY25 (mol)

LDH-1 LDH-2 LDH-3 LDH-4 LDH-5 AG25-LDH AY25-LDH

0.02 0.02 0.02 0.02 0.02 0.02 0.02

0.01 0.01 0.01 0.01 0.01 0.01 0.01

0.057 0.057 0.057 0.057 0.057 0.057 0.057

3.67 × 10−3 2.75 × 10−3 2.2 × 10−3 1.83 × 10−3 1.57 × 10−3 5.5 × 10−3 0

3.67 × 10−3 5.5 × 10−3 6.6 × 10−3 7.32 × 10−3 7.86 × 10−3 0 0.011

intercalated into the interlayer of LDH and some hybrid pigments have been obtained, the color of the prepared hybrid pigments can hardly be controlled and sometimes undesirable color is obtained. Acid Green 25 (CAS No. 403-90-1, denoted as AG25) shown in Figure 1a is a green dye and is widely used to dye silk, wool, leather, textiles, and cosmetics. However, it cannot be used as pigment to color paints and polymers due to its solubility in solvent. The immobilization of AG25 into the interlayer of LDH will overcome this drawback. Nevertheless, it was found that the color of AG25 intercalated LDH is blue instead of green, which significantly restricts its use as green pigment. Therefore, it is necessary to find ways to tune the color of hybrid pigments based on AG25 intercalated LDH. Herein, we reported the facile color tuning of hybrid pigments based on AG25 intercalated LDH by cointercalation with Acid Yellow 25 (shown in Figure 1b and denoted as AY25) through a coprecipitation method. The thermal stability of AY25 was improved by intercalation into the interlayer of ZnAl LDH, and the obtained hybrids can tolerate at least 250 °C. The color of

the prepared hybrids can be easily controlled by adjusting the molar ratio of AY25/AG25 in the interlayer of LDH, and the guest−guest interactions between AG25 and AY25 anions have effects on the color of the hybrids. The obtained hybrid materials can uniformally disperse in coatings and plastics, indicating their potential application as green colorant in the fields of paints and plastics.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Zn(NO3)2·6H2O, Al(NO3)3·9H2O, NaOH, ethylene glycol, and ethanol were all of analytical grade and used without any further purification. Deionized water with a conductivity less than 10−6 S·cm−1 was freshly decarbonated by boiling before use. AG25 and AY25 with a purity of about 94% were purchased from Hangzhou Aronda Chemicals Co. Ltd. and were recrystallized three times in water before use. 2.2. Synthesis of AG25/AY25-LDH. AG25 and AY25 cointercalated ZnAl LDH (AG25/AY25-LDH) was synthesized by the coprecipitation method. In a typical procedure, Zn(NO3)2·6H2O (0.02 mol) and Al(NO3)3·9H2O (0.01 mol) B

DOI: 10.1021/acs.iecr.7b00279 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research were dissolved in 50 mL water. AG25 (3.67 × 10−3 mol) and AY25 (3.67 × 10−3 mol) were dissolved in 50 mL ethylene glycol, and then this solution was added to the water solution. An alkaline solution was prepared by dissolving NaOH (0.057 mol) in water (50 mL) and then added dropwise into the mixed solution above with vigorous agitation under N2 atmosphere. The obtained slurry was then heated at 95 °C in a water bath with violent stirring under a N2 stream for 15 h. The obtained precipitate was centrifuged and washed for six cycles with deionized water and two cycles with anhydrous ethanol, and then dried in an oven at 100 °C for 12 h. AG25/ AY25-LDH with different AY25/AG25 molar ratios, AG25LDH, and AY25-LDH were synthesized according to the same process above by changing the molar amounts of AG25 and AY25. The amounts of chemical materials used for hybrid pigments synthesis are listed in Table 1, and the obtained AG25/AY25-LDH materials were denoted as LDH-n (n = 1, 2, 3, 4, and 5, representing the molar ratio of AY25/AG25 used). 2.3. Preparation of Coatings and Films. The obtained hybrid pigments were used to color coatings and polypropylene (PP) films. Typically, 0.02 g of hybrid pigment was added to 1.98 g of amino resin varnish first, and the mixture was thoroughly ground in a mortar. The mixture was coated on a white tile with a size of 5 cm × 5 cm, and then a colored coating on the tile was obtained by heating it at 105 °C for 1 h in an oven. For polypropylene film, 0.4 g of hybrid pigment were mixed with 19.6 g of PP resin in an SSR-Z4 double roller mixer at 165 °C for about 15 min to prepare a composite. The resulting composite was molded at 160 °C under pressure, and film with a thickness of about 0.08 mm was obtained. 2.4. Characterization. The crystal phases of the samples were analyzed by X-ray diffraction on a Shimadzu XRD-6000 diffractometer using monochromatic Cu Kα radiation (λ = 0.15406 nm) operating at 40 kV and 30 mA, with a step of 0.02° and a scan speed of 5° min−1. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vector 22 infrared spectrophotometer with a resolution of 2 cm−1 using the KBr disk method with a weight ratio of sample/KBr of 1:100. The morphology of samples was investigated on a field-emission scanning electron microscope (SEM; Zeiss Supra 55) operated at a voltage of 20 kV. Samples were sputtered with gold before SEM observation. The metal element contents in the LDH powder were analyzed on an ICPS-7500 inductively coupled plasma emission spectrometer (ICP-ES) which was calibrated by Zn and Al standard solutions. Carbon, nitrogen, and hydrogen were analyzed on Elementar vario EL analyzer. Thermogravimetric−differential thermal analyses (TG−DTA) were carried out on a PCT−IA instrument in the range of 30− 800 °C with a heating rate of 10 °C·min−1 under air atmosphere. The ultraviolet−visible (UV−vis) absorbance and diffuse reflectance spectra were recorded on a Shimadzu UV-2501 PC instrument. The L*a*b* values, ΔE values, and color coordinates of samples were analyzed in terms of CIE 1976 L*a*b* with a TC-P2A automatic colorimeter (Xinao Yike Optic-Electronic Co., Beijing).

Figure 2. XRD patterns of AG25-LDH, AY25-LDH, and AG25/AY25LDH.

Therefore, AG25 and AY25 have been cointercalated into the interlayer of ZnAl LDH and uniformly distributed in the interlayer room, forming a homogeneous crystalline phase. The 003 reflections of AG25-LDH, LDH-1, LDH-2, LDH-3, LDH4, LDH-5, and AY25-LDH appear at the 2θ value of 4.8, 4.8, 3.25, 3.25, 3.25, 3.25, and 3.25°, corresponding to basal spacing (d003) of 1.84, 1.91, 2.74, 2.74, 2.74, 2.74, and 2.74 nm, respectively. By subtracting the layer thickness of 0.48 nm from the basal spacing (d003), the gallery height is about 1.36, 1.43, 2.26, 2.26, 2.26, 2.26, and 2.26 nm for AG25-LDH, LDH-1, LDH-2, LDH-3, LDH-4, LDH-5, and AY25-LDH, respectively. The larger gallery height of AG25/AY25-LDH and AY25-LDH than that of AG25-LDH can be ascribed to the larger size of the AY25 anion. Taking the size of AG25 (1.37 nm) and AY25 (2.11 nm) into account, it is supposed that AG25 and AY25 anions form a monolayer and bilayer in AG25-LDH and AY25LDH, respectively. For AG25/AY25-LDH, AG25 and AY25 anions may form a monolayer in LDH-1; AY25 anions form a bilayer and AG25 anions are dispersed in the bilayer in LDH-2, LDH-3, LDH-4, and LDH-5. The 00l reflections become stronger and more symmetrical as the molar ratio of AY25/ AG25 increases, indicating that the crystallinity of AG25/AY25LDH increases with the increase of the molar ratio of AY25/ AG25. The (110) diffractions of all the prepared LDH samples, appearing at 2θ = 60.8°, are the same, indicating that all the LDH samples have the same lattice parameter a (a = 2d110 = 0.30 nm) and the compositions of the prepared LDH layers are similar to each other. Figure 3 shows the SEM images of AG25-LDH and AG25/ AY25-LDH. All samples have platelet-like morphology, and the size of AG25-LDH is larger than that of AG25/AY25-LDH, but the thickness of the platelet is thinner. The size distribution cuves of LDH samples are obtained by measuring sizes of about 100 platelets, indicating that the average sizes of AG25-LDH, LDH-1, LDH-2, LDH-3, LDH-4, and LDH-5 are about 80, 60, 50, 64, 52, and 59 nm, respectively. The average sizes of AG25/ AY25-LDH are similar to each other, but smaller than that of AG25-LDH, probably because AG25 more easily forms a crystalline structure with LDH layers than AY25. Figure 4 shows the FT−IR spectra of AG25, AY25, AG25LDH, AY25-LDH, and AG25/AY25-LDH in the range of 4000−800 cm−1 and 1300−1000 cm−1. The broad absorption band centered at around 3450 cm−1 in the spectrum of AG25 (Figure 4a) can be assigned to the stretching vibrations of N−

3. RESULTS AND DISCUSSION 3.1. Structure, Morphology, and Composition. The XRD patterns of the synthesized hybrid materials are shown in Figure 2. All of the diffraction peaks of obtained samples are in good agreement with the characteristic peaks of LDH-like materials, and only one series of 00l reflections appear, suggesting that only one crystalline phase has been formed.13,39 C


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Figure 3. SEM images (A) and size distribution curves (B) of AG25-LDH (a), LDH-1 (b), LDH-2 (c), LDH-3 (d), LDH-4 (e), and LDH-5 (f).

Figure 4. FT-IR spectra of AG25 (a), AY25 (b), AG25-LDH (c), AY25-LDH (d), LDH-1 (e), LDH-2 (f), LDH-3 (g), LDH-4 (h), and LDH-5 (i) in the range of 4000−800 cm−1 (A) and 1300−1000 cm−1 (B).

antisymmetric and symmetric stretching vibrations of the −SO3− groups appear at 1151 and 1036 cm−1, respectively.32 In the spectrum of AG25-LDH, both the characteristic absorption bands of LDH-like materials and AG25 are observed. The broad band centered at around 3440 cm−1 is attributed to the O−H stretching vibrations of interlayer water, hydroxyl groups on the layers and dye anions, and the N−H stretching vibration of AG25. The bands at 1590, 1500, 1176, and 1025 cm−1 could be ascribed to the characteristic peaks of AG25 anions. It can be seen that the symmetric stretching vibration of the −SO3− groups of AG25 shifts from 1030 to

H groups and O−H stretching vibration of adsorbed water molecules. The absorption bands at 1590, 1546, and 1502 cm−1 could be ascribed to the stretching vibrations of phenyl groups,49 and the peaks at 1174 and 1030 cm−1 can be attributed to antisymmetric and symmetric stretching vibrations of the −SO3− groups, respectively.50 As shown in Figure 4b, the broad band centered at around 3438 cm−1 could be ascribed to the N−H stretching vibration of AY25 and O−H stretching vibration of adsorbed water molecules. The absorption bands at 1598, 1555, and 1499 cm−1 could be assigned to the stretching vibrations of benzene rings, and the peaks attributed to D


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Industrial & Engineering Chemistry Research Table 2. Chemical Compositions (wt %) and Chemical Formulae of the Samples samples

Zn

Al

C

N

S

chemical formulas

AG25-LDH LDH-1 LDH-2 LDH-3 LDH-4 LDH-5 AY25-LDH

24.66 25.18 24.38 23.97 23.10 24.25 25.25

5.75 5.85 5.61 5.59 5.34 5.66 5.81

25.41 28.98 28.57 30.05 30.30 28.79 31.86

2.13 4.32 5.39 5.91 6.30 6.47 7.92

4.56 5.80 6.01 6.41 6.56 6.42 7.24

Zn0.64Al0.36(OH)2(AG25)0.18·0.67H2O Zn0.64Al0.36(OH)2(AG25)0.125(AY25)0.111·0.63H2O Zn0.64Al0.36(OH)2(AG25)0.083(AY25)0.194·0.55H2O Zn0.64Al0.36(OH)2(AG25)0.075(AY25)0.217·0.56H2O Zn0.64Al0.36(OH)2(AG25)0.061(AY25)0.238·0.57H2O Zn0.64Al0.36(OH)2(AG25)0.048(AY25)0.264·0.58H2O Zn0.64Al0.36(OH)2(AY25)0.36·0.47H2O

Figure 5. TG-DTA curves of AG25, AY25, LDH-1, and LDH-4.

1025 cm−1, indicating there may exist host−guest interactions between the ZnAl LDH layers and AG25 anions. The spectrum of AY25-LDH also shows the characteristic absorption bands of LDH-like materials (3440 cm−1 for O−H) and AY25 anions (1600, 1552, 1499, 1158, and 1035 cm−1). It can be seen that the antisymmetric stretching vibration of the −SO3− groups of AY25 shifts from 1151 to 1158 cm−1, which could be attributed to the host−guest interactions between the ZnAl LDH layers and AY25 anions. In the FT−IR spectra of AG25/AY25-LDH, all the characteristic absorption bands of LDH-like materials, AG25 and AY25 are observed, but there are some differences among them as the molar ratios of AY25/AG25 are different from each other. The intensity of the characteristic absorption bands of C−N at 1088 and 1072 cm−1 of AG25 decreases, and the intensity of the characteristic absorption band of NN at 1657 cm−1 of AY25 gradually increases with the increase of the molar ratio of AY25/AG25 in the interlayer of ZnAl LDH. In comparison with pristine AG25 and AY25, the shifts of symmetric and antisymmetric absorption peaks of the −SO3− groups of AG25 and AY25 anions in AG25/AY25-LDH are also observed, respectively, suggesting the presence of host−guest interactions in the composites.13 It can be found that the symmetric stretching vibration of the −SO3− groups of AG25 shifts to lower wavenumber, but the antisymmetric stretching

vibration of the −SO3− groups of AY25 shifts to a higher wavenumber. This reverse effect can be explained as follows: AY25 has one −SO3− group, but AG25 has two −SO3− groups; the restriction level of AG25 is higher than that of AY25 in the interlayer of LDH, resulting in a different combination structure of the −SO3− group for AG25 and AY25 with the LDH layer. These results together with the XRD analysis confirm the successful cointercalation of AG25 and AY25 into the interlayer of LDH and the formation of hybrid materials. The ICP-ES and C, N, S elemental analysis data for AG25LDH, AY25-LDH, and AG25/AY25-LDH are listed in Table 2. The Zn/Al and N/S molar ratios are calculated according to their weight percentage contents. On the basis of the N/S molar ratio and charge balance, the molar contents of AG25 and AY25 anions are numerated. The molar ratios of AY25/ AG25 in AG25/AY25-LDH are similar to the theoretical values, suggesting that the molar ratio of AY25/AG25 in the interlayer spacing of LDH can be easily controlled. The amount of interlayer water calculated from the mass loss from 100 to 200 °C (Table S1 in the Supporting Information) in the TG curves of AG25-LDH, LDH-1, LDH-2, LDH-3, LDH-4, LDH-5, and AY25-LDH is about 6%, 5%, 4%, 4%, 4%, 4%, and 3%, respectively. E


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Industrial & Engineering Chemistry Research 3.2. Thermal Stability. The thermal stability of the prepared hybrid pigments was investigated by TG-DTA and the thermal aging test. The TG-DTA curves of AG25, AY25, AG25-LDH, AY25-LDH, and AG25/AY25-LDH are present in Figure 5 and Figure S1 in the Supporting Information. The first and second mass loss stage in the TG curve of AG 25 can be ascribed to the removal of physically adsorbed water and the oxidative thermal decomposition of AG25, respectively. The mass loss stage in the range of 280−340 °C with an exothermic peak centered at 293 °C in the TG-DTA curves can be ascribed to the oxidative thermal decomposition of AY25, and the mass loss stage from 340 to 700 °C can be attributed to the combustion of the residue. The TG-DTA curves of of LDH-1 and LDH-4 are significantly different from those of AG25 and AY25. There are approximately three mass loss stages in their TG curves. The first mass loss stage in the range of 40−200 °C can be assigned to the removal of physically adsorbed water and interlayer water molecules.13,48 The second mass loss stage from 200 to 320 °C with an exothermic peak centered at about 278/280 °C results from the dehydroxylation of the ZnAl LDH layers. The third mass loss stage from 320 to 800 °C in the TG curves can be ascribed to the oxidative thermal decomposition of AG25 and AY25 anions in the interlayer of ZnAl LDH and further combustion of their residue. It can be found that the temperature for oxidative thermal decomposition of AY25 anions in the interlayer of LDH is a little higher than that for AY25 by comparing their first exothermic peaks, which may result from the protection of the ZnAl LDH layers. The mass percentages of the end products (Table S1 in the Supporting Information) for AG25-LDH, LDH-1, LDH-2, LDH-3, LDH-4, LDH-5, and AG25-LDH after calcination at 800 °C are about 35, 33, 32, 33, 26, 29, and 33%, respectively. The corresponding values calculated from the chemical formulas in Table 2 are 34, 31, 29, 28, 28, 27, and 32%, which are in good agreement with the values obtained from the TG data, further suggesting the calculated formulas are rational. The color difference values (ΔE) of AG25, AY25, and LDH4 after heating at 100, 150, 200, 250, and 300 °C are present in Figure 6A. The ΔE values of AG25, AY25, and LDH-4 are very small when the heating temperature is below and at 200 °C, indicating they are stable at 200 °C. The ΔE value of AY25 increases sharply from 2.5 to 42 when the temperature increases from 200 to 250 °C, suggesting the decomposition of AY25 at 250 °C. However, the ΔE values of AG25 and

LDH-4 nearly maintain unchanged, indicating AG25 and LDH4 are very stable at 250 °C. Therefore, the thermal stability of AY25 is largely improved by intercalating into the interlayer of ZnAl LDH, and LDH-4 can tolerate at least 250 °C. As shown in Figure 6B, the color of AY25 changed from yellow to dark brown after heating at 300 °C for 30 min. In contrast, LDH-4 still maintains the green color, suggesting that it has much better thermal stability than AY25. Therefore, LDH-4 can be used as pigments to color plastics at relatively high temperature. 3.3. UV−visible Spectra of Samples. The UV−visible spectra of AG25-LDH, AY25-LDH, and AG25/AY25-LDH dispersed in water are shown in Figure 7A. AG25-LDH shows a broad absorption band centered at a wavelength of 385 nm and nearly no absorption in the wavelength range of 550−800 nm. In contrast, AY25-LDH shows strong absorption with two peaks centered at 605 and 651 nm in the wavelength range of 550−700 nm. In comparison with AY25-LDH, the spectra of AG25/AY25-LDH are somewhat different in the wavelength range of 550−700 nm. The absorption peaks of AY25 shift from 605 and 651 nm for AY25-LDH to 597 and 646 nm for AG25/AY25-LDH, respectively, indicating the environment experienced by AY25 is different from each other. Therefore, AG25 and AY25 have guest−guest interactions in the confined interlayer of ZnAl LDH.35 The diffuse reflectance UV−visible spectra of the powdered samples are shown in Figure 7B. The spectra of AG25/AY25-LDH differ from that of AY25-LDH in the range of 500−750 nm, further indicating that there exists guest−guest interactions between AG25 and AY25 in the interlayer of ZnAl LDH. In other words, the guest−guest interactions have effects on the visible light absorption property of the hybrid pigment. 3.4. Color Analysis and Application. The digital photos of the obtained powdered samples are shown in Figure 8A. It is apparent that the color of AG25-LDH is greenish blue. The color of the prepared hybrid turns greener when AG25 and AY25 were cointercalated into the interlayer of ZnAl LDH with a AY25/AG25 molar ratio of 1. The color of AG25/AY25-LDH becomes greener and greener as the molar ratio of AY25/AG25 increases from 1 to 4. The color of the samples were further investigated by CIE 1976 L*a*b* color scales (Figure 8B). The L*, a*, and b* values indicate the level of lightness or darkness, redness or greenness, and yellowness or blueness, respectively. The L* value increases with the increase of AY25/AG25 molar ratio, suggesting the lightness of AG25/AY25-LDH can be improved by increasing the content AY25. On the contrary, the a* value decreases sharply from −3.9 to −13.9 when the molar ratio of AY25/AG25 increases from 0 to 1, and further decreases to −24 as the AY25/AG25 molar ratio increases to 4, indicating the color of AG25/AY25-LDH becomes greener. However, upon further increasing the molar ratio to 5, the a* value will increase. The b* value increases with the increase of AY25/AG25 molar ratio and changes from positive to negative when the molar ratio of AY25/AG25 increases to 3, implying that the color hue of the LDH materials changes from blue to yellow. The color coordinates of the samples are shown in Figure 8C, which clearly illustrate that the color of AG25/ AY25-LDH can be facilely tuned from greenish blue to bluish green and green by adjusting the molar ratio of AY25/AG25 in the interlayer of ZnAl LDH. To demonstrate their potential application in the fields of paints and plastics, the prepared hybrid pigments were used as colorant to color coatings and PP films. The digital photos of the prepared coatings and films are shown in Figure 9. It can be

Figure 6. (A) ΔE values of AG25, AY25, and LDH-4 after thermal aging at different temperatures; and (B) digital photos of AG25, AY25, and LDH-4 before and after thermal aging at 300 °C. F


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Figure 7. (A) UV−vis spectra of samples dispersed in water; and (B) diffuse reflectance UV−vis spectra of powdered samples.

Figure 8. (A) Digital photos of the prepared pigments; (B) L*a*b* values of the hybrid pigments; and (C) color coordinates of the hybrid pigments: 1, AG25-LDH; 2−6, LDH-n, n = 1, 2, 3, 4, and 5, respectively; 7, AY25-LDH.

found that the coating prepared with AG25-LDH shows the color of blue (Figure 9A). However, the color of coatings prepared with AG25/AY25-LDH is significantly different. The color turns to greenish blue for coating prepared with LDH-1, and bluish green for coating prepared with LDH-2, and becomes green for coating prepared with LDH-3 and LDH-4. The color of the PP films differs from that of the coating, which may result from the higher treatment temperature of PP films. The color of PP film prepared with AG25-LDH is greenish blue, and the color turns to green when LDH-2 and LDH-3 were used and somewhat yellowish green for the film prepared with LDH-4. Figure 10 shows the SEM image with EDS mapping of LDH-4/PP. It can be seen that Zn, Al, N, O, and S elements are uniformly dispersed in PP, demonstrating the

good dispersibility of LDH-4 in PP. Therefore, the prepared AG25/AY25-LDH hybrid pigments can be used as green pigments to color coatings and plastics.

4. CONCLUSIONS AG25 and AY25 have been cointercalated into the interlayer of ZnAl LDH through the coprecipitation method. The color of AG25 and AY25 cointercalated LDH can be easily controlled by adjusting the molar ratio of AY25/AG25 in the interlayer of ZnAl LDH and can be facilely tuned from greenish blue to bluish green and green. The intercalation into the interlayer of ZnAl LDH improved the thermal stability of AY25, and the obtained hybrids can tolerate at least 250 °C. The characterization results revealed the presence of host−guest interactions G


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between LDH layers and interlayer anions and guest−guest interactions between AG25 and AY25 anions in the interlayer of LDH. The guest−guest interactions have effects on the visible light absorption property of the hybrid materials; in other words, they affect the color of the hybrids. The prepared AG25/AY25-LDH hybrid have good dispersibility in coating and PP, suggesting that they can be used as green pigments to color coatings and plastics. Therefore, the cointercalation of AG25 and AY25 into the interlayer of LDH gives rise to a kind of green hybrids, which can expand the application of AG25 and AY25 in the field of pigment.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00279. TG-DTA curves of AG25-LDH, AY25-LDH, LDH-2, LDH-3, and LDH-5; mass percentages of physical water and end products (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax and Tel: + 86-10-64436992. E-mail: [email protected]. edu.cn. *E-mail: [email protected]. ORCID

Pinggui Tang: 0000-0003-1866-7527 Yongjun Feng: 0000-0001-9254-6219 Dianqing Li: 0000-0001-6761-8946

Figure 9. (A) Digital photos of the prepared colored coatings and (B) digital photos of PP films.

Notes

The authors declare no competing financial interest.

Figure 10. SEM image with EDS mapping of LDH-4/PP composite film. H


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (21206005, U1507119, 21371022), National Key Research and Development Program of China (2016YFB0301601, 2016YFB0301602), National Major Scientific Instruments Development Project of China (21627813), and the Fundamental Research Funds for the Central Universities (YS1406).

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