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methylene [email protected]2 Maya Blue-like pigment
Microporous and Mesoporous Materials 211 (2015) 124e133
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Learning from ancient Maya: Preparation of stable palygorskite/ methylene blue@SiO2 Maya Blue-like pigment Yujie Zhang a, b, c, Ling Fan a, b, Hao Chen d, Junping Zhang a, b, *, Yuan Zhang a, c, Aiqin Wang a, b, * a
Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China R&D Center of Xuyi Palygorskite Applied Technology, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, China Graduate University of the Chinese Academy of Sciences, 100049 Beijing, PR China d School of Pharmaceutical and Chemical Engineering, Taizhou University, Linhai 317000, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 1 March 2015 Accepted 3 March 2015 Available online 10 March 2015
A facile method for the preparation of stable palygorskite/methylene blue@SiO2 (PAL/MB@SiO2) Maya Blue-like pigment was reported. The PAL/MB pigment was prepared via adsorption of MB by PAL, which was further coated with a layer of SiO2 by polycondensation of tetraethoxysilane (TEOS) with acetic acid (HAc) as the catalyst to form the PAL/MB@SiO2 pigment. The weight ratio of MB to PAL, ball milling time and heating temperature play important roles in affecting stability of the PAL/MB pigment. The MB content in the PAL/MB pigment is up to 12%, which is higher than all the state-of-the-art Maya Blue-like pigments. The MB molecules can only be adsorbed onto the external surface, the grooves and the openings of the channels of PAL, but cannot enter the channels according to the BET, zeta potential, FTIR and XRD analyses. Owing to the fact that the stability of the PAL/MB pigment is not very high, a layer of SiO2 is introduced to shield the MB molecules and further improve the stability. The PAL/MB@SiO2 pigment shows excellent stability against elution, thermal aging and intensive UV irradiation. A TEOS/ HAc/H2O molar ratio of 1/2/140 in forming the SiO2 layer is greatly helpful to improve the stability. © 2015 Elsevier Inc. All rights reserved.
Keywords: Palygorskite Methylene blue Maya Blue Silica
1. Introduction Maya Blue, a well-known artificial pigment, was widely used by Mayan in mural paintings and ceramic pieces in Yucatan [1]. Maya Blue is an organic/inorganic hybrid composed of palygorskite (PAL) clay and indigo dye [2e11]. PAL is a phyllosilicate clay with nanochannels filled by zeolitic H2O [12]. The ideal formula of PAL is [(OH2)4(Mg, Al,Fe)5(OH)$2Si8O20]$4H2O [13]. Different from other clays, PAL has a unique fibrous or rod-like microstructure due to inversion of oxygen atoms on the edge of siliconeoxygen tetrahedral layers, which results in discontinuous arrangement of the aluminum-oxide octahedral layers [14]. The large surface area, moderate cation exchange capacity and excellent adsorption property of PAL are of great benefit for its applications in various fields, especially in the adsorption of guest molecules [15,16] and
* Corresponding authors. Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China. E-mail addresses: [email protected] (J. Zhang), [email protected] (A. Wang). http://dx.doi.org/10.1016/j.micromeso.2015.03.002 1387-1811/© 2015 Elsevier Inc. All rights reserved.
ions [17], and the preparation of organic/inorganic nanocomposites [2,18,19]. A particularly fascinating property of Maya Blue is that it does not fade even in an environment of high humidity and high temperature for thousands of years. Maya Blue shows unprecedented stability when exposed to acids, alkalis, organic solvents and UV irradiation. Thus, Maya Blue has attracted much attention of researchers in the fields of material, chemistry and archeology in the past decades. The interaction between PAL and indigo is the origin of great stability of Maya Blue, but the nature of the interaction still remains controversial. Some researchers claimed that water molecules are eliminated from the channels of PAL as a result of heating and the entrance of indigo molecules into the channels [20e23]. Whereas the others concluded that the dye molecules cannot enter the channels but fix to the opennings of the channels [24,25]. Lima et al. revealed that synthetic Maya Blue stabilized the indigo molecule probably at the clay surface and may be in the grooves, but the natural ones stabilized the indoxyl molecules in the tunnels and over time oxidized to indigo [2]. The hydrogen bonding between
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indigo and PAL was widely suggested to be the main reason for the unusual stability of Maya Blue. The hydrogen bonding hypothesis include: 1) carbonyl and amino of indigo with edge silanols of PAL [25], 2) carbonyl of indigo with structural water inside the microchannels of PAL [22,23,26,27], 3) carbonyl of indigo with structural water accompanied by direct interaction between indigo and octahedral cations of PAL as well as Van der Waals interaction [23]. In addition, for some researchers, the stability of Maya Blue depends more on steric shielding than on hydrogen bonding [28]. Inspired by the excellent stability of Maya Blue, researchers have tried to prepare Maya Blue-like pigments by dry-grinding clay powder with 0.5%e2.0% of dye/pigment, and then heating for a period of time. Various clays and dyes/pigments have been used to prepare novel Maya Blue-like pigments of different color according to such an approach. Giustetto et al. reported preparetion of a redpurplish Maya Blue-like pigment by grinding and heating PAL with Acid Red 2 [29e31]. A sepiolite/indigo pigment could also be prepared according to a similar procedure [4,32]. Also, some thioMayan-like compounds could be obtained by the combination of PAL and indigo sulfur derivatives in aqueous phase [33]. Such approaches could produce various Maya Blue-like pigments with limited toxicity and low production costs. However, the uniformity and stability of the so-obtained Maya Blue-like pigments remain to be improved, especially when other dyes/pigments are used instead of indigo. Here we report a facile method for the fabrication of stable PAL/ methylene blue@SiO2 (PAL/MB@SiO2) Maya Blue-like pigment. We have reported preparation of stable PAL/cationic red X-GRL pigments by forming a layer of SiO2 on the surface of the pigments via the polycondensation of TEOS in ethanol with ammonia as the catalyst [34]. However, it is impossible to obtain stable PAL/ MB@SiO2 pigments via the same approach because of the severe discoloration of PAL/MB in ethanol. Thus, the PAL/MB@SiO2 pigment were prepared by adsorption of the guest MB molecules onto the host PAL in the aqueous phase, which were then coated with a thin layer of SiO2 by polycondensation of tetraethoxysilane (TEOS) in the acidic aqueous solution to further improve the stability. The uniform PAL/MB@SiO2 pigment features excellent stability against elution, thermal aging and intensive UV irradiation. 2. Experimental section 2.1. Materials and reagents Natural PAL, obtained from Xuyi, Jiangsu Province, China, was crushed and purified by 2% H2SO4 to remove quartz and other impurities, such as carbonates, etc. The purified PAL was filtered by passing through a 180 mesh sieve. MB (C16H18ClN3S$3H2O) was purchased from Shanghai Reagent Factory (Shanghai, China) and used without further purification. TEOS was purchased from Gelest. Acetic acid (HAc, 99.5%), NaOH, HCl (37.5%) and ethanol were provided by China National Medicines Corporation Ltd. All the reagents were used without further purification and all solutions were prepared with distilled water. 2.2. Preparation of PAL/MB pigment The PAL/MB pigment was prepared according to the following procedure. 2.00 g of PAL was dispersed in a proper amount of MB solution (20,000 ppm) and distilled water to maintain a solid/liquid weight ratio of 1/10, and then magnetically stirred for 4 h to reach the adsorption equilibrium. Subsequently, the mixture was centrifuged at 10,000 rpm for 10 min to remove the supernatant and the precipitate was dried at 60 C for 2 h. The precipitate was
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milled for several minutes in an agate ball mill, and then heated at different temperature (25e240 C) for a period of time. 2.3. Preparation of PAL/MB@SiO2 pigment The polycondensation of TEOS on the surface of the PAL/MB pigment was carried out following the method proposed by Karmakar et al. [35]. A mixture of 1.00 g of the PAL/MB pigment and a given amount of distilled water was magnetically stirred for 5 min at room temperature. Then TEOS was added to the dispersion during stirring, followed by dropwise addition of a proper amount of HAc. The mixture was magnetically stirred at room temperature for 1 h. Then, the resulting mixture was centrifuged at 10,000 rpm for 10 min. The precipitate was washed with distilled water until the supernatant was colorless and its pH was close to 7. The obtained pigment was dried to a constant weight at 60 C for 0.5 h. 2.4. Stability tests Chemical stability of pigments was carried out by elution using 1 M HCl, 1 M NaOH and ethanol. The supernatant of the pigments after centrifugation were analyzed using a UVeVis spectrophotometer after 1, 4, 8, 24, 48 and 72 h immersion at room temperature to check possible color variations and structural rearrangement. The images of the supernatants were also taken using a Nikon D800 camera. Analyses of the thermal stability of samples were carried out in the air atmosphere using an STA 6000 imultaneous thermal analyzer (PerkinElmer Instrument Co., Ltd. USA). The temperature program was 10.0 C/min from 20 to 900 C. The simulated aging process of the pigments were also tested in a UV accelerated weathering tester. Before UV aging, a pretreatment of the sample is required. 0.3 g of each sample was dispersed in 40 mL of acetone. The suspension was spray coated onto an aluminum plate, and then dried at room temperature. Then, the aluminum plates were placed in a UV Accelerated Weathering Tester (ZN-P, Xinlang, Shanghai, China) with eight UV-B (280e315 nm) bulbs (40 w). The temperature was maintained at 60 C under UV-B exposure with a radiation intensity of 0.6 w/m2. Digital images of the samples were taken every two days. 2.5. Characterization UVeVis spectra were obtained using a UVeVis spectrophotometer (Specord 200, Analytik Jena AG). The micrographs of samples were obtained using a field emission scanning electron microscope (SEM, JSM-6701F, JEOL) and a field emission transmission electron microscope (TEM, TECNAI-G2-F30, FEI). Before SEM observation, a drop of the pigments dispersion in ethanol was put on an aluminum stub and dried in the open atmosphere, and then coated with a thin layer of gold film (~7 nm). For TEM observation, the samples were prepared as follows. A drop of the pigments dispersion in ethanol was put on a copper grid and dried in the open atmosphere. The SBET of samples were analyzed with an Accelerated Surface Area and Porosimetry System (Micromeritics, ASAP2020, Atlanta, USA) using N2 as an adsorbate at 196 C. The zeta potentials of samples were collected on a Malvern Zetasizer Nano system with irradiation of a 633 nm HeeNe laser (ZEN3600) at 25 C, using a folded capillary cell. 0.50 g of sample was fully dispersed in 100 mL of distilled water and then sonicated for 5 min before measurement of the zeta potentials. Three parallel measurements were conducted, and the averages were reported. FTIR spectra of samples were collected on a Thermo Nicolet NEXUS TM spectrophotometer (Thermo, Madison, USA) in the range of 4000e400 cm1 using KBr pellets. XRD patterns were obtained on X'pert PRO diffractometer with working conditions Cu Ka, 30 mA
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and 40 kV (l ¼ 1.54060 Å). The scanning was made at room temperature between 3 and 50 in 2q with a scanning speed of 0.02 per second. 3. Results and discussion 3.1. Design of PAL/MB@SiO2 pigment The PAL/MB pigment was prepared by the adsorption of MB onto PAL in aqueous solutions, and then milling for several minutes and finally heating for 12 h. Subsequently, the PAL/MB@SiO2 pigment could be formed by coating a layer of the amorphous SiO2 on the surface of the PAL/MB pigment via the polycondensation of TEOS under acidic condition (Fig. 1). The effects of various parameters in the adsorption of MB (e.g., weight ratio of MB to PAL, ball milling time and temperature of heat treatment) and in the coating of PAL/MB with SiO2 (e.g., dosage of TEOS, HAc and H2O) on stability of the pigments were investigated in detail in order to obtain the PAL/MB@SiO2 pigment with excellent stability. The stability of the pigments can be spectroscopically evaluated by the absorbance of the supernatants after stability tests. A lower absorbance of the supernatants means higher stability of the pigment against elution. 3.2. Preparation of PAL/MB pigment The weight ratio of MB to PAL, mMB/mPAL, was investigated in advance to find an appropriate MB content in the pigment. The PAL microparticles were charged into different amounts of the MB aqueous solution (20,000 ppm) to change the weight ratio of MB to PAL. The UVeVis spectra, concentration and digital images of the MB aqueous solution after adsorption by PAL were shown in Fig. 2. The concentration of residual MB in the aqueous solution after adsorption was determined according to its absorbance at 673 nm using a UVeVis spectrophotometer. The MB aqueous solution becomes colorless and the characteristic absorbance of MB at 673 nm is close to zero after adsorption by PAL when mMB/mPAL 8%. The concentration of residual MB in the aqueous solution slightly increases to 0.56 ppm with increasing mMB/mPAL to 12%. This means about 99.995% of MB was adsorbed by PAL at an mMB/mPAL of 12%. Evident increase in the concentration of MB after adsorption was observed with further increasing mMB/mPAL to 16% and 20%. The solutions must be diluted with distilled water to an appropriate concentration before used for recording the UVeVis spectra. The concentration of residual MB in aqueous solution after adsorption increases from 0.20 ppm to 819.05 ppm with increasing mMB/mPAL from 4% to 20%. Meanwhile the percentage of adsorbed MB decreases from 99.995% to 95.91%. The free MB attached on the PAL/ MB pigment when mMB/mPAL 16% must be washed away in order
to acquire stable Maya Blue-like pigment, whereas this procedure is tedious and time-consuming. The residual MB is negligible when mMB/mPAL 12% and does not need washing after adsorption by PAL. The PAL/MB hybrids with a mMB/mPAL of 4%, 8% and 12% were dried at 60 C for 2 h, and then milled and heat-treated for a period of time to form the Maya Blue-like PAL/MB pigment. The MB content in the so-obtained PAL/MB pigment is up to 12%, which is higher than all the state-of-the-art Maya Blue-like pigments. A guest molecule content, e.g., indigo, of about 0.5%e2.0% is the most frequently reported value according to the previous literature. The stability of the PAL/MB pigment is closely related to the dye content, milling and heat treatment conditions [29,36e39]. The influences of mMB/mPAL, time of ball milling and heating temperature on stability of the PAL/MB pigment were investigated via attack of 1 M HCl and anhydrous ethanol. Since no color differences can be observed from the solid pigments after elution, the minimum absorbance of the supernatant after chemical attack means less removal of MB from the pigments and is defined to be the optimal condition. The representative UVeVis spectra and images of the supernatants corresponding to 24 h of immersion are shown in Fig. 3. The supernatant after attack of 1 M HCl is nearly colorless and the absorbance at 673 nm is below 0.02 when mMB/mPAL 8% (Fig. 3a). The stability of the pigment with an mMB/mPAL of 12% is lower. The absorbance of the supernatant increases to 0.2 and a noticeable blue color can be observed. The PAL/MB pigment are not very stable in ethanol since both the absorbance and the chromaticity of the supernatants are very high, especially for those with a higher mMB/mPAL. The supernatant of the PAL/MB pigment with a mMB/mPAL of 12% after immersion in ethanol for 24 h is black blue. Solid-state grinding is a very important procedure for preparing Maya Blue pigments and other functional materials by promoting the interactions between guest and host species [37e39]. Here ball milling was selected to improve stability of the PAL/MB pigment. Samples with an mMB/mPAL of 2% were used to study the effect of ball milling time and the results are shown in Fig. 3b. Since the content of MB is low, differences can hardly be observed from the images of supernatants with different ball milling time after attack of 1 M HCl. Also, there is no absorbance in the visible region for the supernantants with an mMB/mPAL of 2%. Thus, analysis of the spectra is based on the absorbance at 340 nm which is the characteristic absorption peak of benzene ring in MB molecules in the UV region. The UVeVis spectra reveal that the sample milled for 4 min shows the highest tolerance against acid attack. The results of ethanol attack further confirm the effect of ball milling in improving the stability. Both the spectra and images show that the stability of pigment against elution is enhanced by milling for 4 min. However, a longer milling time results in decrease in stability of the pigments.
Fig. 1. Schematic illustration of preparation of the Maya Blue-like PAL/MB@SiO2 pigment and the corresponding images.
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Fig. 2. (a) UVeVis spectra and digital images of the MB solutions after adsorption by PAL with an mMB/mPAL of 4%e20%, and (b) variations of concentration of residual MB after adsorption by PAL and percentage of adsorbed MB with increasing mMB/mPAL from 4% to 20%.
The absorbance at 610 nm increases from 0.28 to 0.72 with increasing the milling time from 4 min to 60 min. This is because the fibers of PAL are broken during long time milling [40]. Consequently, a part of the adsorbed MB molecules are exposed, which can be easily attacked by acid and ethanol. In the preparation of Maya Blue, it has been proposed that a partial or perhaps total evacuation of the zeolitic H2O is essential to allow indigo to either penetrate in the channels or block the entrance of the channels [22,27,41,42]. It is well agreed that a
moderate thermal treatment can enhance the chemical interaction between PAL and indigo [24]. Thus, heat treatment is another important factor besides grinding in improving the stability [29,36]. Samples with an mMB/mPAL of 12% were used to study the effect of heat temperature and the results are shown in Fig. 3c. The supernatants after ethanol attack show different UVeVis spectra. According to the UVeVis spectra and images, the stability of the PAL/MB pigment is enhanced when the temperature reaches 120 C, which is due to the loss of zeolitic water and the interaction
Fig. 3. Variations of UVeVis spectra and digital images of the supernatants of PAL/MB with (a) mMB/mPAL, (b) time of ball milling and (c) heating temperature after attacks of 1 M HCl and ethanol for 24 h.
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Fig. 4. Variation of UVeVis spectra and images of the supernatants of PAL/MB@SiO2 with dosages of (a) TEOS (mol, 0.082 mol of HAc and 0.089 mol of H2O), (b) HAc (mol, 0.002 mol of TEOS and 0.222 mol of H2O) and (c) H2O (mol, 0.002 mol of TEOS and 0.004 mol of HAc) after attacks of 1 M HCl and ethanol for 24 h.
between PAL and MB. The supernatants after attacks of both acid and ethanol are dark blue when the temperature is over 180 C. The structure of MB was changed once the temperature exceeds 180 C as the characteristic absorption peaks of MB in 1 M HCl and ethanol showed evident shift in the visible region. It can be seen from the spectra and images of the supernatants after attacks of acid and ethanol that the PAL/MB pigment shows moderate stability against elution although the preparation procedure has been optimized. Thus, a process of surface modification is chosen to further improve stability of the pigment. 3.3. Preparation of PAL/MB@SiO2 pigment Silica nano-/micro-spheres can be formed by hydrolysis and polycondensation of TEOS [43e46]. Dense and glassy silica spheres could be obtained directly by hydrolysis and condensation of TEOS in the presence of inorganic or organic acid and water at room temperature [35,47]. Because of the severe discoloration of PAL/MB in ethanol, the hydrolysis and condensation of TEOS on the surface of the PAL/MB pigment was carried out in the aqueous solution using HAc as the catalyst in order to enhance the stability. The dosages of all the reagents were varied to find the optimal condition to enhance stability of the PAL/MB pigment. According to the UVeVis spectra and images of the supernatants after chemical attack, the optimal TEOS/HAc/H2O molar ratio for the surface modification of PAL/MB is 1/2/140. With a TEOS/HAc/H2O molar
ratio of 1/2/140, a homogeneous sol can be obtained as a result of hydrolysis and polycondensation of TEOS [35,48]. The UVeVis spectra and images in Fig. 4a reveal that the stability of the PAL/MB pigment has been improved after forming a SiO2 layer on the surface owing to its shielding effect to MB molecules. The stability of PAL/MB@SiO2 decreased once the dosage of TEOS exceeded 0.002 mol. This is probably due to the fact that superfluous TEOS may hinder the condensation of hydrolyzed TEOS into SiO2 [34]. Once the dosage of HAc exceeded 0.004 mol, the stability of the pigments decreased and more dye were removed during chemical attack. Also, superfluous H2O dilutes TEOS and HAc, and then affects the modification effect. Thus, the TEOS/H2O/HAc molar ratio is the key factor governing stability of the PAL/MB@SiO2 pigment. 3.4. Analyses of PAL/MB and PAL/MB@SiO2 pigments The SEM and TEM micrographs of PAL, PAL/MB and PAL/ MB@SiO2 pigments are shown in Fig. 5. The PAL/MB pigment was prepared by dispersing 2.0 g of PAL in 12 mL of MB solution for 4 h, which was followed by ball milling for 4 min and heating at 120 C for 12 h. The PAL/MB@SiO2 pigment was prepared by dispersing 1.0 g of PAL/MB in 0.278 mol of distilled water and modified by the addition of 0.002 mol of TEOS and 0.004 mol of HAc for 1 h, and then centrifugated and dried. These samples were also used for BET, zeta potentials, FTIR, XRD and TGA analyses as well as stability tests. The rod-like crystals of PAL develop very well and tightly pack
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Fig. 5. SEM and TEM images of (a, d, g) PAL, (b, e, h) PAL/MB and (c, f, i) PAL/MB@SiO2 pigments. mMB/mPAL is 12% in the PAL/MB and PAL/MB@SiO2 pigments.
together due to the Van der Waals' force and hydrogen bonding (Fig. 5a, d, g). The crystals of PAL are about 1 mm in length and 20e30 nm in diameter. For the PAL/MB pigment (Fig. 5b, e, h), some rod-like crystals of PAL become shorter due to the ball milling procedure to enhance the interaction between PAL and MB. In addition, the surface of the PAL crystals in the PAL/MB pigment becomes rough compared with that of the pristine PAL, which indicates the successful binding of MB onto PAL. No noticeable change can be observed via SEM after further modification with TEOS
(Fig. 5c, f, i), whereas a thin layer of amorphous SiO2 can be seen via TEM (Fig. 5i). The binding sites of PAL locate at the surface of the fibrous clay mineral according to Shariatmadar et al. [49]. So the surface area is an important parameter influencing the adsorption of PAL for guest molecules. The effects of MB adsorption and subsequent TEOS modification on surface area and total pore volume (Vtotal) of PAL are shown in Table 1. The specific surface area (SBET) is calculated by the BET method. The surface area of micropores (Smicro) and external surface area (Sext) are estimated by the t-plot method. Vtotal is obtained from the volume of N2 held at the relative pressure
Table 1 BET data of PAL, PAL/MB and PAL/MB@SiO2 pigments. Samples
SBET (m2/g)
Smicro (m2/g)
Sext (m2/g)
Vtotal (cm3/g)
PAL PAL/MB PAL/MB@SiO2
262.09 121.80 129.28
92.60 1.05 2.73
169.49 120.75 126.56
0.41 0.26 0.28
Table 2 Zeta potentials of PAL, PAL/MB and PAL/MB@SiO2 pigments. Samples
PAL
MB
PAL/MB
PAL/MB@SiO2
Zeta potentials (mV)
17.73
0.624
17.43
21.80
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Fig. 6. FTIR spectra of PAL, MB, MB treated at 120 C for 12 h, PAL/MB and PAL/ MB@SiO2 pigments.
P/P0 ¼ 0.95. As can be seen in Table 1, the SBET of pure PAL is 262.09 m2/g, which decreases evidently to 121.80 m2/g for the PAL/ MB pigment. The Smicro drastically decreases from 92.60 m2/g to 1.05 m2/g after adsorption of MB, which means 98.87% decrease of the Smicro. Meanwhile the Sext also decreases from 169.49 m2/g to 120.75 m2/g. Since the molecule dimension of MB (16.32 Å 5.64 Å 5.41 Å) is larger than that of the nano-pore of PAL (3.7 Å 6.4 Å) [5,14], it is impossible for MB molecules to enter the channels of PAL. Thus, the MB molecules can only be adsorbed onto the external surface, the grooves and the openings of the channels of PAL. The adsorption of MB at the openings blocks the channels of PAL, which is the main reason for the drastic decrease in the Smicro. The adsorption of MB on the external surface of PAL results in partly decrease of Sext to 120.75 m2/g. The loading of MB onto PAL also decreases the Vtotal from 0.41 cm3/g to 0.26 cm3/g. The further modification with TEOS slightly increases SBET, Smicro, Sext
Fig. 7. TGA curves of PAL, PAL/MB and PAL/MB@SiO2 pigments in the air atmosphere. The insert is the TGA curve of MB in the air atmosphere. mMB/mPAL is 12% in the PAL/MB and PAL/MB@SiO2 pigments.
and Vtotal, which is attributed to the amorphous SiO2 layer on the surface of PAL/MB. The zeta potentials of PAL, MB, PAL/MB and PAL/MB@SiO2 are shown in Table 2. PAL displays a zeta potential of 17.73 mV, which matches well with the published data [50]. The PAL particles can be considered as anions with large size and high charge density [51], which attract ions of opposite charge and repel ions of the same charge [52]. The zeta potential of cationic MB is 0.624 mV. Therefore, the MB molecules can be adsorbed onto PAL via electrostatic interaction when PAL is dispersed in the MB solution. Consequently, the surface negative charge of PAL was partly neutralized and the PAL/MB pigment shows a slight increase in zeta potential compared with PAL. The zeta potential decreases significantly to 21.8 mV after introduction of the amorphous SiO2 layer. This proves again successful surface modification of the PAL/MB pigment via polycondensation of TEOS. The results of zeta potentials match very well with the morphology and BET analyses. FTIR spectra of PAL, MB, MB treated at 120 C for 12 h, PAL/MB and PAL/MB@SiO2 pigments are shown in Fig. 6. In the spectrum of MB, the band at 1599 cm1 is attributed to the vibration of the aromatic ring and the bands in the region of 1300 cm1 to 1700 cm1 are consistent with previous studies [53,54]. The peaks at 1490 cm1 and 1392 cm1 are assigned to the CeN stretching vibrations [55]. An obvious bathochromic shift from 1490 cm1 to 1482 cm1 can be seen in the spectrum of MB after heated at 120 C for 12 h, which indicates the structure changes of MB. The characteristic absorption bands of MB in the range of 1300 cm1 to 1600 cm1 also can be seen in the spectra of PAL/ MB and PAL/MB@SiO2, which confirms formation of the PAL/MB and PAL/MB@SiO2 complexes. No bathochromic shifts for the characteristic bands of MB can be seen in the spectrum of PAL/ MB indicating no structure changes of MB and the thermal stability of MB is improved once formed complex with PAL. Oppositely, hypochromatic shifts (from 1599 cm1 to 1604 cm1, from 1482 cm1 to 1494 cm1, from 1331 cm1 to 1336 cm1) can be observed for the characteristic bands of MB after immobilized onto PAL. The changes in the FTIR spectra should be attributed to the electrostatic interaction and hydrogen bonding between PAL and MB. In the FTIR spectrum of PAL, the absorption band at 3615 cm1 is attributed to the AleAleOH stretching vibration [56]. Also, three strong bands attributed to HeOeH (coordinated and zeolitic water) of PAL appear at 3551, 3414 and 1651 cm1. The absorption bands at 1028 cm1 and 983 cm1 are attributed to the stretching vibration of SieO [57]. In the spectrum of PAL/MB, the intensity of the bands at 1651 cm1 and 3551 cm1 attributed to HeOeH decreases evidently, which is due to the loss of zeolitic water and adsorbed water during adsorption of MB and heat treatment at 120 C. The polycondensation of TEOS on the surface of PAL/MB enhances the intensity of the bands at 3551 cm1 (silanol) and 1028 cm1 (SieO). Fig. S1 shows the XRD patterns of PAL, PAL/MB and PAL/ MB@SiO2 pigments. The diffraction peaks at 2q of 8.38 , 13.74 , 16.34 , 19.83 , 27.54 and 34.38 are the characteristic peaks of PAL [58]. The strong peak of PAL at 2q ¼ 8.38 (d ¼ 10.6 Å) can be seen for all the samples, indicating high order-disorder structural degree of PAL [58]. The peaks at 2q of 26.63 and 30.97 are ascribed to the characteristic peaks of quartz and carbonate impurity, respectively [58]. No change of the characteristic peaks of PAL in the XRD patterns can be seen after forming a complex with MB and polycondensation of TEOS, which reveal that the crystal structure of PAL is well preserved. The XRD result is consistent with the BET analyses that the MB molecules can only be adsorbed onto the external surface, the grooves and the openings of the channels of PAL, but cannot enter the channels. Amorphous SiO2 from the
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Fig. 8. Variations of UVeVis spectra and images of the supernatants of PAL/MB and PAL/MB@SiO2 pigments after chemical attacks for 3 d using (a) 1 M HCl, (b) 1 M NaOH and (c) ethanol, digital images of (d) PAL/MB and (e) PAL/MB@SiO2 pigments before and after attack of 1 M HCl, 1 M NaOH and ethanol.
polycondensation of TEOS often gives a broad peak in the 2q range of 20 e25 [59]. Whereas the SiO2 peak is not observable for the PAL/MB@SiO2 pigment, which may be caused by the overlap with the peaks of PAL in this region. 3.5. Stability of PAL/MB@SiO2 pigment The thermal stability of PAL, MB, PAL/MB and PAL/MB@SiO2 pigments were evaluated by means of thermogravimetric analysis (TGA) and the results are shown in Fig. 7. The slight weight loss of MB before 200 C (2.11%) is mainly attributed to the removal of the adsorbed water. MB exhibits a gradual weight loss starting from 200 C. The weight of MB decreases linearly after 400 C and only 2.27% of the initial weight is reserved with further increasing the temperature to 600 C. The final weight of the dye is 0.12% indicating the complete decomposition of MB at 900 C. The TGA curve of PAL is consistent with previous studies [13,58] and can be
divided in four sections: (i) in the low temperature region (<150 C), a 1.85% weight loss can be observed and is attributed to the loss of both superficially adsorbed and loosely bound zeolitic OH2; (ii) the second region (150e300 C), a further 2.07% weight loss is related to the loss of residual zeolitic OH2 and the first fraction of structural OH2; (iii) the third region (300e580 C), residual first half of structural OH2 and the second half of the structural OH2 are removed from PAL; (iv) the last region (580e800 C), a further progressive weight loss corresponds to the decomposition of structural OH2 and hydroxyl groups. During this process, PAL decomposes into an amorphous phase and 90.18% of the initial weight is reserved. The weight loss of PAL/MB and PAL/MB@SiO2 is obviously less than that of PAL in the range of 100e327 C. This means the thermal stability of PAL/MB and PAL/MB@SiO2 is higher than that of PAL. This is probably because the openings of the channels of PAL are blocked by the dye molecules which partly mitigates the loss of
Fig. 9. Digital images of (a) PAL/MB and (b) PAL/MB@SiO2 after UV irradiation at 60 C for 24 d.
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zeolitic OH2. This result is consistent with the BET analyses. The temperatures of maximum weight loss rate of MB are 400 C, 415 C and 434 C, respectively, for MB, PAL/MB and PAL/MB@SiO2. MB is completely decomposed and the weight loss of PAL is 8.74% at 600 C. Meanwhile the weight loss of PAL/MB and PAL/MB@SiO2 are 13.69% and 12.72%, respectively, which is 4e5% more than that of PAL. This means there are some residual partly decomposed MB in PAL/MB and PAL/MB@SiO2 since the MB content in PAL/MB and PAL/MB@SiO2 is about 12%. All the data indicate that the thermal stability of MB is obviously improved after forming complex with PAL and surface modification with SiO2. This is because of the hosteguest interaction between MB and PAL, and the shielding effect of PAL and the SiO2 layer. In order to evaluate the effect of the SiO2 layer on stability of the pigments, the pigments were immersed in 1 M HCl, 1 M NaOH and ethanol at room temperature for 72 h. The supernatants after centrifugation were measured by UVeVis spectrophotometer and photographed every 24 h (Fig. 8). It can be observed from Fig. 8a that the supernatants of PAL/MB@SiO2 after acidic attack show distinctively lower absorbance (the maximum absorbances are 0.86 (1 d), 1.30 (2 d) and 1.56 (3 d), respectively) than those of the PAL/ MB pigment (the maximum absorbances are 1.11 (1 d), 1.54 (2 d) and 1.77 (3 d)). In Fig. 8b, the same trends were observed for the supernatants attacked by alkaline solution. Also, the supernatants show color (in the web version) change from light yellow to light purple, which probably due to the structure change of MB molecules in the supernatants after attack by alkaline solution. In ethanol, the supernatants of PAL/MB@SiO2 pigment can be measured directly, whereas the supernatants of PAL/MB pigment must be diluted to proper concentration in order to record the UVeVis spectra. Thus, it is obvious that the stability of the pigment against chemical elution was improved after TEOS modification, especially in ethanol. The color differences between PAL/MB and PAL/MB@SiO2 after chemical attack are also shown in Fig. 8d, e. The PAL/MB@SiO2 pigment shows higher stability against chemical attack compared with the PAL/MB pigment. The simulated aging process of the PAL/MB and PAL/MB@SiO2 pigments were also studied in a UV accelerated weathering tester. The representative images of the pigments after the simulated aging tests are shown in Fig. 9. Slight fading of the PAL/MB pigment was observed after 24 d of UV irradiation at 60 C. Whereas the PAL/ MB@SiO2 pigment exhibits excellent stability to UV aging, and no fading of the pigment was observed in 24 d of UV irradiation. 4. Conclusions Stable organic/inorganic hybrid PAL/MB@SiO2 Maya Blue-like pigment has been prepared by adsorption of MB on PAL and polycondensation of TEOS. The weight ratio of MB to PAL in the pigment can be up to 12%. Proper ball milling time and heating temperature could enhance the interaction between PAL and MB, and then improve stability of the pigment. The polycondensation of TEOS on the surface of PAL/MB could further improve stability of the pigment. The MB molecules are adsorbed onto the external surface, the grooves and the openings of the channels of PAL, but cannot enter the channels. The PAL/MB@SiO2 pigment shows excellent stability against chemical elution, thermal aging and UV irradiation. The adsorption-shield of dye molecules reported herein by PAL and the SiO2 layer may pave the way for preparing stable Maya Blue-like pigments of various colors to meet practical applications. Acknowledgments The authors are grateful for financial support of the “Hundred Talents Program” of the Chinese Academy of Sciences, the open
funding (CASXY2013-02) the Xuyi Center of Attapulgite Applied Technology Research Development & Industrialization of the Chinese Academy of Sciences, and the Key Technology R&D Program of Jiangsu (BE2014102). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2015.03.002. References [1] H. Van Olphen, Science 154 (1966) 645. n, M. Vera, J.L. Rivera, J. Fraissard, J. Phys. Chem. C 116 (2012) [2] E. Lima, A. Guzma 4556. lez, A. Magazzú, C. Cavallari, M. Su [3] C. Mondelli, M.S.d. Río, M.A. Gonza arez, E. García-Romero, P. Romano, J. Phys. Conf. Ser. 340 (2012) 012109. [4] J. Raya, J. Hirschinger, S. Ovarlez, F. Giulieri, A.M. Chaze, F. Delamare, Phys. Chem. Chem. Phys. 12 (2010) 14508. [5] R. Giustetto, O. Wahyudi, I. Corazzari, F. Turci, Appl. Clay Sci. 52 (2011) 41. [6] P. Vandenabeele, S. Bode, A. Alonso, L. Moens, Spectrochim. Acta A Mol. Biomol. Spectrosc. 61 (2005) 2349. -Yacam n, J. Arenas, M.C.S. Puche, Science 273 (1996) 223. [7] M. Jose an, L. Rendo [8] A. Domenech, M.T. Domenech-Carbo, C. Vidal-Lorenzo, M.L. Vazquez de Agredos-Pascual, Angew. Chem. Int. Ed. 51 (2012) 700. [9] M.M. Lezhnina, T. Grewe, H. Stoehr, U. Kynast, Angew. Chem. Int. Ed. 51 (2012) 10652. nech, M.T. Dome nech-Carbo , M.L. Va zquez de Agredos-Pascual, [10] A. Dome Angew. Chem. Int. Ed. 50 (2011) 5741. [11] A. Sheppard, H. Gottlieb, Am. Antiq. 27 (1962) 565. [12] W.F. Bradley, Am. Mineral. 25 (1940) 405. [13] R.L. Frost, Z. Ding, Thermochim. Acta 397 (2003) 119. [14] H. Chen, J. Zhao, A. Zhong, Y. Jin, Chem. Eng. J. 174 (2011) 143. € an, M. Alkan, Microporous Mesoporous Mater. 96 (2006) [15] Y. Ozdemir, M. Dog 419. € an, Y. Ozdemir, [16] M. Dog M. Alkan, Dyes Pigments 75 (2007) 701. [17] X. Zou, J. Pan, H. Ou, X. Wang, W. Guan, C. Li, Y. Yan, Y. Duan, Chem. Eng. J. 167 (2011) 112. [18] C. Sanchez, B. Julian, P. Belleville, M. Popall, J. Mater. Chem. 15 (2005) 3559. nech-Carbo , M.T. Dome nech-Carbo , L. Osete-Cortina, F.M. Valle[19] A. Dome Algarra, D. Buti, Microporous Mesoporous Mater. 194 (2014) 135. [20] R.R. Chianelli, M. Perez De la Rosa, G. Meitzner, M. Siadati, G. Berhault, A. Mehta, J. Pople, S. Fuentes, G. Alonzo-Nunez, L.A. Polette, J. Synchrotron. Radiat. 12 (2005) 129. [21] L.A. Polette, G. Meitzner, M. Jose Yacaman, R.R. Chianelli, Microchem. J. 71 (2002) 167. [22] G. Chiari, R. Giustetto, G. Ricchiardi, Eur. J. Mineral. 15 (2003) 21. [23] E. Fois, A. Gamba, A. Tilocca, Microporous Mesoporous Mater. 57 (2003) 263. [24] M.S. del Río, E. Boccaleri, M. Milanesio, G. Croce, W. van Beek, C. Tsiantos, rez, J. Mater. Sci. 44 (2009) G.D. Chyssikos, V. Gionis, G.H. Kacandes, M. Sua 5524. [25] B. Hubbard, W. Kuang, A. Moser, G.A. Facey, C. Detellier, Clay Clay Min. 51 (2003) 318. s i Xamena, G. Ricchiardi, S. Bordiga, A. Damin, [26] R. Giustetto, F.X. Llabre R. Gobetto, M.R. Chierotti, J. Phys. Chem. B 109 (2005) 19360. [27] R. Giustetto, D. Levy, G. Chiari, Eur. J. Mineral. 18 (2006) 629. [28] C. Dejoie, E. Dooryhee, P. Martinetto, S. Blanc, P. Bordat, R. Brown, F. Porcher, M.S. Del Rio, P. Strobel, M. Anne, arXiv preprint arXiv:1007.0818, 2010. [29] R. Giustetto, K. Seenivasan, D. Pellerej, G. Ricchiardi, S. Bordiga, Microporous Mesoporous Mater. 155 (2012) 167. [30] R. Giustetto, O. Wahyudi, Microporous Mesoporous Mater. 142 (2011) 221. [31] R. Giustetto, J.G. Vitillo, I. Corazzari, F. Turci, J. Phys. Chem. C 118 (2014) 19322. [32] S. Ovarlez, F. Giulieri, A.M. Chaze, F. Delamare, J. Raya, J. Hirschinger, Chem. Eur. J. 15 (2009) 11326. rgio Seixas de Melo, J. Phys. Chem. C 117 (2013) 603. [33] R. Rondaeo, J. Se [34] L. Fan, Y. Zhang, J. Zhang, A. Wang, RSC Adv. 4 (2014) 63485. [35] B. Karmakar, G. De, D. Kundu, D. Ganguli, J. Non-cryst. Solids 135 (1991) 29. [36] F.S. Manciu, L. Reza, L.A. Polette, B. Torres, R.R. Chianelli, J. Raman Spectrosc. 38 (2007) 1193. [37] S. Deguchi, S. Mukai, M. Tsudome, K. Horikoshi, Adv. Mater. 18 (2006) 729. [38] G. Koshkakaryan, L.M. Klivansky, D. Cao, M. Snauko, S.J. Teat, J.O. Struppe, Y. Liu, J. Am. Chem. Soc. 131 (2009) 2078. [39] Y.M. Wang, Z.Y. Wu, H.J. Wang, J.H. Zhu, Adv. Funct. Mater. 16 (2006) 2374. [40] Y. Liu, W. Wang, A. Wang, Powder Technol. 225 (2012) 124. [41] G. Chiari, R. Giustetto, J. Druzik, E. Doehne, G. Ricchiardi, Appl. Phys. A 90 (2008) 3. [42] S. Ovarlez, A.-M. Chaze, F. Giulieri, F. Delamare, Comptes Rendus Chim. 9 (2006) 1243. [43] G. Bogush, C. Zukoski IV, J. Colloid Interf. Sci. 142 (1991) 19. [44] G. Bogush, C. Zukoski IV, J. Colloid Interf. Sci. 142 (1991) 1.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Learning from ancient Maya: Preparation of stable palygorskite/ methylene blue@SiO2 Maya Blue-like pigment Yujie Zhang a, b, c, Ling Fan a, b, Hao Chen d, Junping Zhang a, b, *, Yuan Zhang a, c, Aiqin Wang a, b, * a
Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China R&D Center of Xuyi Palygorskite Applied Technology, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou 730000, China Graduate University of the Chinese Academy of Sciences, 100049 Beijing, PR China d School of Pharmaceutical and Chemical Engineering, Taizhou University, Linhai 317000, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 1 March 2015 Accepted 3 March 2015 Available online 10 March 2015
A facile method for the preparation of stable palygorskite/methylene blue@SiO2 (PAL/MB@SiO2) Maya Blue-like pigment was reported. The PAL/MB pigment was prepared via adsorption of MB by PAL, which was further coated with a layer of SiO2 by polycondensation of tetraethoxysilane (TEOS) with acetic acid (HAc) as the catalyst to form the PAL/MB@SiO2 pigment. The weight ratio of MB to PAL, ball milling time and heating temperature play important roles in affecting stability of the PAL/MB pigment. The MB content in the PAL/MB pigment is up to 12%, which is higher than all the state-of-the-art Maya Blue-like pigments. The MB molecules can only be adsorbed onto the external surface, the grooves and the openings of the channels of PAL, but cannot enter the channels according to the BET, zeta potential, FTIR and XRD analyses. Owing to the fact that the stability of the PAL/MB pigment is not very high, a layer of SiO2 is introduced to shield the MB molecules and further improve the stability. The PAL/MB@SiO2 pigment shows excellent stability against elution, thermal aging and intensive UV irradiation. A TEOS/ HAc/H2O molar ratio of 1/2/140 in forming the SiO2 layer is greatly helpful to improve the stability. © 2015 Elsevier Inc. All rights reserved.
Keywords: Palygorskite Methylene blue Maya Blue Silica
1. Introduction Maya Blue, a well-known artificial pigment, was widely used by Mayan in mural paintings and ceramic pieces in Yucatan [1]. Maya Blue is an organic/inorganic hybrid composed of palygorskite (PAL) clay and indigo dye [2e11]. PAL is a phyllosilicate clay with nanochannels filled by zeolitic H2O [12]. The ideal formula of PAL is [(OH2)4(Mg, Al,Fe)5(OH)$2Si8O20]$4H2O [13]. Different from other clays, PAL has a unique fibrous or rod-like microstructure due to inversion of oxygen atoms on the edge of siliconeoxygen tetrahedral layers, which results in discontinuous arrangement of the aluminum-oxide octahedral layers [14]. The large surface area, moderate cation exchange capacity and excellent adsorption property of PAL are of great benefit for its applications in various fields, especially in the adsorption of guest molecules [15,16] and
* Corresponding authors. Center of Eco-material and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China. E-mail addresses: [email protected] (J. Zhang), [email protected] (A. Wang). http://dx.doi.org/10.1016/j.micromeso.2015.03.002 1387-1811/© 2015 Elsevier Inc. All rights reserved.
ions [17], and the preparation of organic/inorganic nanocomposites [2,18,19]. A particularly fascinating property of Maya Blue is that it does not fade even in an environment of high humidity and high temperature for thousands of years. Maya Blue shows unprecedented stability when exposed to acids, alkalis, organic solvents and UV irradiation. Thus, Maya Blue has attracted much attention of researchers in the fields of material, chemistry and archeology in the past decades. The interaction between PAL and indigo is the origin of great stability of Maya Blue, but the nature of the interaction still remains controversial. Some researchers claimed that water molecules are eliminated from the channels of PAL as a result of heating and the entrance of indigo molecules into the channels [20e23]. Whereas the others concluded that the dye molecules cannot enter the channels but fix to the opennings of the channels [24,25]. Lima et al. revealed that synthetic Maya Blue stabilized the indigo molecule probably at the clay surface and may be in the grooves, but the natural ones stabilized the indoxyl molecules in the tunnels and over time oxidized to indigo [2]. The hydrogen bonding between
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indigo and PAL was widely suggested to be the main reason for the unusual stability of Maya Blue. The hydrogen bonding hypothesis include: 1) carbonyl and amino of indigo with edge silanols of PAL [25], 2) carbonyl of indigo with structural water inside the microchannels of PAL [22,23,26,27], 3) carbonyl of indigo with structural water accompanied by direct interaction between indigo and octahedral cations of PAL as well as Van der Waals interaction [23]. In addition, for some researchers, the stability of Maya Blue depends more on steric shielding than on hydrogen bonding [28]. Inspired by the excellent stability of Maya Blue, researchers have tried to prepare Maya Blue-like pigments by dry-grinding clay powder with 0.5%e2.0% of dye/pigment, and then heating for a period of time. Various clays and dyes/pigments have been used to prepare novel Maya Blue-like pigments of different color according to such an approach. Giustetto et al. reported preparetion of a redpurplish Maya Blue-like pigment by grinding and heating PAL with Acid Red 2 [29e31]. A sepiolite/indigo pigment could also be prepared according to a similar procedure [4,32]. Also, some thioMayan-like compounds could be obtained by the combination of PAL and indigo sulfur derivatives in aqueous phase [33]. Such approaches could produce various Maya Blue-like pigments with limited toxicity and low production costs. However, the uniformity and stability of the so-obtained Maya Blue-like pigments remain to be improved, especially when other dyes/pigments are used instead of indigo. Here we report a facile method for the fabrication of stable PAL/ methylene blue@SiO2 (PAL/MB@SiO2) Maya Blue-like pigment. We have reported preparation of stable PAL/cationic red X-GRL pigments by forming a layer of SiO2 on the surface of the pigments via the polycondensation of TEOS in ethanol with ammonia as the catalyst [34]. However, it is impossible to obtain stable PAL/ MB@SiO2 pigments via the same approach because of the severe discoloration of PAL/MB in ethanol. Thus, the PAL/MB@SiO2 pigment were prepared by adsorption of the guest MB molecules onto the host PAL in the aqueous phase, which were then coated with a thin layer of SiO2 by polycondensation of tetraethoxysilane (TEOS) in the acidic aqueous solution to further improve the stability. The uniform PAL/MB@SiO2 pigment features excellent stability against elution, thermal aging and intensive UV irradiation. 2. Experimental section 2.1. Materials and reagents Natural PAL, obtained from Xuyi, Jiangsu Province, China, was crushed and purified by 2% H2SO4 to remove quartz and other impurities, such as carbonates, etc. The purified PAL was filtered by passing through a 180 mesh sieve. MB (C16H18ClN3S$3H2O) was purchased from Shanghai Reagent Factory (Shanghai, China) and used without further purification. TEOS was purchased from Gelest. Acetic acid (HAc, 99.5%), NaOH, HCl (37.5%) and ethanol were provided by China National Medicines Corporation Ltd. All the reagents were used without further purification and all solutions were prepared with distilled water. 2.2. Preparation of PAL/MB pigment The PAL/MB pigment was prepared according to the following procedure. 2.00 g of PAL was dispersed in a proper amount of MB solution (20,000 ppm) and distilled water to maintain a solid/liquid weight ratio of 1/10, and then magnetically stirred for 4 h to reach the adsorption equilibrium. Subsequently, the mixture was centrifuged at 10,000 rpm for 10 min to remove the supernatant and the precipitate was dried at 60 C for 2 h. The precipitate was
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milled for several minutes in an agate ball mill, and then heated at different temperature (25e240 C) for a period of time. 2.3. Preparation of PAL/MB@SiO2 pigment The polycondensation of TEOS on the surface of the PAL/MB pigment was carried out following the method proposed by Karmakar et al. [35]. A mixture of 1.00 g of the PAL/MB pigment and a given amount of distilled water was magnetically stirred for 5 min at room temperature. Then TEOS was added to the dispersion during stirring, followed by dropwise addition of a proper amount of HAc. The mixture was magnetically stirred at room temperature for 1 h. Then, the resulting mixture was centrifuged at 10,000 rpm for 10 min. The precipitate was washed with distilled water until the supernatant was colorless and its pH was close to 7. The obtained pigment was dried to a constant weight at 60 C for 0.5 h. 2.4. Stability tests Chemical stability of pigments was carried out by elution using 1 M HCl, 1 M NaOH and ethanol. The supernatant of the pigments after centrifugation were analyzed using a UVeVis spectrophotometer after 1, 4, 8, 24, 48 and 72 h immersion at room temperature to check possible color variations and structural rearrangement. The images of the supernatants were also taken using a Nikon D800 camera. Analyses of the thermal stability of samples were carried out in the air atmosphere using an STA 6000 imultaneous thermal analyzer (PerkinElmer Instrument Co., Ltd. USA). The temperature program was 10.0 C/min from 20 to 900 C. The simulated aging process of the pigments were also tested in a UV accelerated weathering tester. Before UV aging, a pretreatment of the sample is required. 0.3 g of each sample was dispersed in 40 mL of acetone. The suspension was spray coated onto an aluminum plate, and then dried at room temperature. Then, the aluminum plates were placed in a UV Accelerated Weathering Tester (ZN-P, Xinlang, Shanghai, China) with eight UV-B (280e315 nm) bulbs (40 w). The temperature was maintained at 60 C under UV-B exposure with a radiation intensity of 0.6 w/m2. Digital images of the samples were taken every two days. 2.5. Characterization UVeVis spectra were obtained using a UVeVis spectrophotometer (Specord 200, Analytik Jena AG). The micrographs of samples were obtained using a field emission scanning electron microscope (SEM, JSM-6701F, JEOL) and a field emission transmission electron microscope (TEM, TECNAI-G2-F30, FEI). Before SEM observation, a drop of the pigments dispersion in ethanol was put on an aluminum stub and dried in the open atmosphere, and then coated with a thin layer of gold film (~7 nm). For TEM observation, the samples were prepared as follows. A drop of the pigments dispersion in ethanol was put on a copper grid and dried in the open atmosphere. The SBET of samples were analyzed with an Accelerated Surface Area and Porosimetry System (Micromeritics, ASAP2020, Atlanta, USA) using N2 as an adsorbate at 196 C. The zeta potentials of samples were collected on a Malvern Zetasizer Nano system with irradiation of a 633 nm HeeNe laser (ZEN3600) at 25 C, using a folded capillary cell. 0.50 g of sample was fully dispersed in 100 mL of distilled water and then sonicated for 5 min before measurement of the zeta potentials. Three parallel measurements were conducted, and the averages were reported. FTIR spectra of samples were collected on a Thermo Nicolet NEXUS TM spectrophotometer (Thermo, Madison, USA) in the range of 4000e400 cm1 using KBr pellets. XRD patterns were obtained on X'pert PRO diffractometer with working conditions Cu Ka, 30 mA
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and 40 kV (l ¼ 1.54060 Å). The scanning was made at room temperature between 3 and 50 in 2q with a scanning speed of 0.02 per second. 3. Results and discussion 3.1. Design of PAL/MB@SiO2 pigment The PAL/MB pigment was prepared by the adsorption of MB onto PAL in aqueous solutions, and then milling for several minutes and finally heating for 12 h. Subsequently, the PAL/MB@SiO2 pigment could be formed by coating a layer of the amorphous SiO2 on the surface of the PAL/MB pigment via the polycondensation of TEOS under acidic condition (Fig. 1). The effects of various parameters in the adsorption of MB (e.g., weight ratio of MB to PAL, ball milling time and temperature of heat treatment) and in the coating of PAL/MB with SiO2 (e.g., dosage of TEOS, HAc and H2O) on stability of the pigments were investigated in detail in order to obtain the PAL/MB@SiO2 pigment with excellent stability. The stability of the pigments can be spectroscopically evaluated by the absorbance of the supernatants after stability tests. A lower absorbance of the supernatants means higher stability of the pigment against elution. 3.2. Preparation of PAL/MB pigment The weight ratio of MB to PAL, mMB/mPAL, was investigated in advance to find an appropriate MB content in the pigment. The PAL microparticles were charged into different amounts of the MB aqueous solution (20,000 ppm) to change the weight ratio of MB to PAL. The UVeVis spectra, concentration and digital images of the MB aqueous solution after adsorption by PAL were shown in Fig. 2. The concentration of residual MB in the aqueous solution after adsorption was determined according to its absorbance at 673 nm using a UVeVis spectrophotometer. The MB aqueous solution becomes colorless and the characteristic absorbance of MB at 673 nm is close to zero after adsorption by PAL when mMB/mPAL 8%. The concentration of residual MB in the aqueous solution slightly increases to 0.56 ppm with increasing mMB/mPAL to 12%. This means about 99.995% of MB was adsorbed by PAL at an mMB/mPAL of 12%. Evident increase in the concentration of MB after adsorption was observed with further increasing mMB/mPAL to 16% and 20%. The solutions must be diluted with distilled water to an appropriate concentration before used for recording the UVeVis spectra. The concentration of residual MB in aqueous solution after adsorption increases from 0.20 ppm to 819.05 ppm with increasing mMB/mPAL from 4% to 20%. Meanwhile the percentage of adsorbed MB decreases from 99.995% to 95.91%. The free MB attached on the PAL/ MB pigment when mMB/mPAL 16% must be washed away in order
to acquire stable Maya Blue-like pigment, whereas this procedure is tedious and time-consuming. The residual MB is negligible when mMB/mPAL 12% and does not need washing after adsorption by PAL. The PAL/MB hybrids with a mMB/mPAL of 4%, 8% and 12% were dried at 60 C for 2 h, and then milled and heat-treated for a period of time to form the Maya Blue-like PAL/MB pigment. The MB content in the so-obtained PAL/MB pigment is up to 12%, which is higher than all the state-of-the-art Maya Blue-like pigments. A guest molecule content, e.g., indigo, of about 0.5%e2.0% is the most frequently reported value according to the previous literature. The stability of the PAL/MB pigment is closely related to the dye content, milling and heat treatment conditions [29,36e39]. The influences of mMB/mPAL, time of ball milling and heating temperature on stability of the PAL/MB pigment were investigated via attack of 1 M HCl and anhydrous ethanol. Since no color differences can be observed from the solid pigments after elution, the minimum absorbance of the supernatant after chemical attack means less removal of MB from the pigments and is defined to be the optimal condition. The representative UVeVis spectra and images of the supernatants corresponding to 24 h of immersion are shown in Fig. 3. The supernatant after attack of 1 M HCl is nearly colorless and the absorbance at 673 nm is below 0.02 when mMB/mPAL 8% (Fig. 3a). The stability of the pigment with an mMB/mPAL of 12% is lower. The absorbance of the supernatant increases to 0.2 and a noticeable blue color can be observed. The PAL/MB pigment are not very stable in ethanol since both the absorbance and the chromaticity of the supernatants are very high, especially for those with a higher mMB/mPAL. The supernatant of the PAL/MB pigment with a mMB/mPAL of 12% after immersion in ethanol for 24 h is black blue. Solid-state grinding is a very important procedure for preparing Maya Blue pigments and other functional materials by promoting the interactions between guest and host species [37e39]. Here ball milling was selected to improve stability of the PAL/MB pigment. Samples with an mMB/mPAL of 2% were used to study the effect of ball milling time and the results are shown in Fig. 3b. Since the content of MB is low, differences can hardly be observed from the images of supernatants with different ball milling time after attack of 1 M HCl. Also, there is no absorbance in the visible region for the supernantants with an mMB/mPAL of 2%. Thus, analysis of the spectra is based on the absorbance at 340 nm which is the characteristic absorption peak of benzene ring in MB molecules in the UV region. The UVeVis spectra reveal that the sample milled for 4 min shows the highest tolerance against acid attack. The results of ethanol attack further confirm the effect of ball milling in improving the stability. Both the spectra and images show that the stability of pigment against elution is enhanced by milling for 4 min. However, a longer milling time results in decrease in stability of the pigments.
Fig. 1. Schematic illustration of preparation of the Maya Blue-like PAL/MB@SiO2 pigment and the corresponding images.
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Fig. 2. (a) UVeVis spectra and digital images of the MB solutions after adsorption by PAL with an mMB/mPAL of 4%e20%, and (b) variations of concentration of residual MB after adsorption by PAL and percentage of adsorbed MB with increasing mMB/mPAL from 4% to 20%.
The absorbance at 610 nm increases from 0.28 to 0.72 with increasing the milling time from 4 min to 60 min. This is because the fibers of PAL are broken during long time milling [40]. Consequently, a part of the adsorbed MB molecules are exposed, which can be easily attacked by acid and ethanol. In the preparation of Maya Blue, it has been proposed that a partial or perhaps total evacuation of the zeolitic H2O is essential to allow indigo to either penetrate in the channels or block the entrance of the channels [22,27,41,42]. It is well agreed that a
moderate thermal treatment can enhance the chemical interaction between PAL and indigo [24]. Thus, heat treatment is another important factor besides grinding in improving the stability [29,36]. Samples with an mMB/mPAL of 12% were used to study the effect of heat temperature and the results are shown in Fig. 3c. The supernatants after ethanol attack show different UVeVis spectra. According to the UVeVis spectra and images, the stability of the PAL/MB pigment is enhanced when the temperature reaches 120 C, which is due to the loss of zeolitic water and the interaction
Fig. 3. Variations of UVeVis spectra and digital images of the supernatants of PAL/MB with (a) mMB/mPAL, (b) time of ball milling and (c) heating temperature after attacks of 1 M HCl and ethanol for 24 h.
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Fig. 4. Variation of UVeVis spectra and images of the supernatants of PAL/MB@SiO2 with dosages of (a) TEOS (mol, 0.082 mol of HAc and 0.089 mol of H2O), (b) HAc (mol, 0.002 mol of TEOS and 0.222 mol of H2O) and (c) H2O (mol, 0.002 mol of TEOS and 0.004 mol of HAc) after attacks of 1 M HCl and ethanol for 24 h.
between PAL and MB. The supernatants after attacks of both acid and ethanol are dark blue when the temperature is over 180 C. The structure of MB was changed once the temperature exceeds 180 C as the characteristic absorption peaks of MB in 1 M HCl and ethanol showed evident shift in the visible region. It can be seen from the spectra and images of the supernatants after attacks of acid and ethanol that the PAL/MB pigment shows moderate stability against elution although the preparation procedure has been optimized. Thus, a process of surface modification is chosen to further improve stability of the pigment. 3.3. Preparation of PAL/MB@SiO2 pigment Silica nano-/micro-spheres can be formed by hydrolysis and polycondensation of TEOS [43e46]. Dense and glassy silica spheres could be obtained directly by hydrolysis and condensation of TEOS in the presence of inorganic or organic acid and water at room temperature [35,47]. Because of the severe discoloration of PAL/MB in ethanol, the hydrolysis and condensation of TEOS on the surface of the PAL/MB pigment was carried out in the aqueous solution using HAc as the catalyst in order to enhance the stability. The dosages of all the reagents were varied to find the optimal condition to enhance stability of the PAL/MB pigment. According to the UVeVis spectra and images of the supernatants after chemical attack, the optimal TEOS/HAc/H2O molar ratio for the surface modification of PAL/MB is 1/2/140. With a TEOS/HAc/H2O molar
ratio of 1/2/140, a homogeneous sol can be obtained as a result of hydrolysis and polycondensation of TEOS [35,48]. The UVeVis spectra and images in Fig. 4a reveal that the stability of the PAL/MB pigment has been improved after forming a SiO2 layer on the surface owing to its shielding effect to MB molecules. The stability of PAL/MB@SiO2 decreased once the dosage of TEOS exceeded 0.002 mol. This is probably due to the fact that superfluous TEOS may hinder the condensation of hydrolyzed TEOS into SiO2 [34]. Once the dosage of HAc exceeded 0.004 mol, the stability of the pigments decreased and more dye were removed during chemical attack. Also, superfluous H2O dilutes TEOS and HAc, and then affects the modification effect. Thus, the TEOS/H2O/HAc molar ratio is the key factor governing stability of the PAL/MB@SiO2 pigment. 3.4. Analyses of PAL/MB and PAL/MB@SiO2 pigments The SEM and TEM micrographs of PAL, PAL/MB and PAL/ MB@SiO2 pigments are shown in Fig. 5. The PAL/MB pigment was prepared by dispersing 2.0 g of PAL in 12 mL of MB solution for 4 h, which was followed by ball milling for 4 min and heating at 120 C for 12 h. The PAL/MB@SiO2 pigment was prepared by dispersing 1.0 g of PAL/MB in 0.278 mol of distilled water and modified by the addition of 0.002 mol of TEOS and 0.004 mol of HAc for 1 h, and then centrifugated and dried. These samples were also used for BET, zeta potentials, FTIR, XRD and TGA analyses as well as stability tests. The rod-like crystals of PAL develop very well and tightly pack
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Fig. 5. SEM and TEM images of (a, d, g) PAL, (b, e, h) PAL/MB and (c, f, i) PAL/MB@SiO2 pigments. mMB/mPAL is 12% in the PAL/MB and PAL/MB@SiO2 pigments.
together due to the Van der Waals' force and hydrogen bonding (Fig. 5a, d, g). The crystals of PAL are about 1 mm in length and 20e30 nm in diameter. For the PAL/MB pigment (Fig. 5b, e, h), some rod-like crystals of PAL become shorter due to the ball milling procedure to enhance the interaction between PAL and MB. In addition, the surface of the PAL crystals in the PAL/MB pigment becomes rough compared with that of the pristine PAL, which indicates the successful binding of MB onto PAL. No noticeable change can be observed via SEM after further modification with TEOS
(Fig. 5c, f, i), whereas a thin layer of amorphous SiO2 can be seen via TEM (Fig. 5i). The binding sites of PAL locate at the surface of the fibrous clay mineral according to Shariatmadar et al. [49]. So the surface area is an important parameter influencing the adsorption of PAL for guest molecules. The effects of MB adsorption and subsequent TEOS modification on surface area and total pore volume (Vtotal) of PAL are shown in Table 1. The specific surface area (SBET) is calculated by the BET method. The surface area of micropores (Smicro) and external surface area (Sext) are estimated by the t-plot method. Vtotal is obtained from the volume of N2 held at the relative pressure
Table 1 BET data of PAL, PAL/MB and PAL/MB@SiO2 pigments. Samples
SBET (m2/g)
Smicro (m2/g)
Sext (m2/g)
Vtotal (cm3/g)
PAL PAL/MB PAL/MB@SiO2
262.09 121.80 129.28
92.60 1.05 2.73
169.49 120.75 126.56
0.41 0.26 0.28
Table 2 Zeta potentials of PAL, PAL/MB and PAL/MB@SiO2 pigments. Samples
PAL
MB
PAL/MB
PAL/MB@SiO2
Zeta potentials (mV)
17.73
0.624
17.43
21.80
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Fig. 6. FTIR spectra of PAL, MB, MB treated at 120 C for 12 h, PAL/MB and PAL/ MB@SiO2 pigments.
P/P0 ¼ 0.95. As can be seen in Table 1, the SBET of pure PAL is 262.09 m2/g, which decreases evidently to 121.80 m2/g for the PAL/ MB pigment. The Smicro drastically decreases from 92.60 m2/g to 1.05 m2/g after adsorption of MB, which means 98.87% decrease of the Smicro. Meanwhile the Sext also decreases from 169.49 m2/g to 120.75 m2/g. Since the molecule dimension of MB (16.32 Å 5.64 Å 5.41 Å) is larger than that of the nano-pore of PAL (3.7 Å 6.4 Å) [5,14], it is impossible for MB molecules to enter the channels of PAL. Thus, the MB molecules can only be adsorbed onto the external surface, the grooves and the openings of the channels of PAL. The adsorption of MB at the openings blocks the channels of PAL, which is the main reason for the drastic decrease in the Smicro. The adsorption of MB on the external surface of PAL results in partly decrease of Sext to 120.75 m2/g. The loading of MB onto PAL also decreases the Vtotal from 0.41 cm3/g to 0.26 cm3/g. The further modification with TEOS slightly increases SBET, Smicro, Sext
Fig. 7. TGA curves of PAL, PAL/MB and PAL/MB@SiO2 pigments in the air atmosphere. The insert is the TGA curve of MB in the air atmosphere. mMB/mPAL is 12% in the PAL/MB and PAL/MB@SiO2 pigments.
and Vtotal, which is attributed to the amorphous SiO2 layer on the surface of PAL/MB. The zeta potentials of PAL, MB, PAL/MB and PAL/MB@SiO2 are shown in Table 2. PAL displays a zeta potential of 17.73 mV, which matches well with the published data [50]. The PAL particles can be considered as anions with large size and high charge density [51], which attract ions of opposite charge and repel ions of the same charge [52]. The zeta potential of cationic MB is 0.624 mV. Therefore, the MB molecules can be adsorbed onto PAL via electrostatic interaction when PAL is dispersed in the MB solution. Consequently, the surface negative charge of PAL was partly neutralized and the PAL/MB pigment shows a slight increase in zeta potential compared with PAL. The zeta potential decreases significantly to 21.8 mV after introduction of the amorphous SiO2 layer. This proves again successful surface modification of the PAL/MB pigment via polycondensation of TEOS. The results of zeta potentials match very well with the morphology and BET analyses. FTIR spectra of PAL, MB, MB treated at 120 C for 12 h, PAL/MB and PAL/MB@SiO2 pigments are shown in Fig. 6. In the spectrum of MB, the band at 1599 cm1 is attributed to the vibration of the aromatic ring and the bands in the region of 1300 cm1 to 1700 cm1 are consistent with previous studies [53,54]. The peaks at 1490 cm1 and 1392 cm1 are assigned to the CeN stretching vibrations [55]. An obvious bathochromic shift from 1490 cm1 to 1482 cm1 can be seen in the spectrum of MB after heated at 120 C for 12 h, which indicates the structure changes of MB. The characteristic absorption bands of MB in the range of 1300 cm1 to 1600 cm1 also can be seen in the spectra of PAL/ MB and PAL/MB@SiO2, which confirms formation of the PAL/MB and PAL/MB@SiO2 complexes. No bathochromic shifts for the characteristic bands of MB can be seen in the spectrum of PAL/ MB indicating no structure changes of MB and the thermal stability of MB is improved once formed complex with PAL. Oppositely, hypochromatic shifts (from 1599 cm1 to 1604 cm1, from 1482 cm1 to 1494 cm1, from 1331 cm1 to 1336 cm1) can be observed for the characteristic bands of MB after immobilized onto PAL. The changes in the FTIR spectra should be attributed to the electrostatic interaction and hydrogen bonding between PAL and MB. In the FTIR spectrum of PAL, the absorption band at 3615 cm1 is attributed to the AleAleOH stretching vibration [56]. Also, three strong bands attributed to HeOeH (coordinated and zeolitic water) of PAL appear at 3551, 3414 and 1651 cm1. The absorption bands at 1028 cm1 and 983 cm1 are attributed to the stretching vibration of SieO [57]. In the spectrum of PAL/MB, the intensity of the bands at 1651 cm1 and 3551 cm1 attributed to HeOeH decreases evidently, which is due to the loss of zeolitic water and adsorbed water during adsorption of MB and heat treatment at 120 C. The polycondensation of TEOS on the surface of PAL/MB enhances the intensity of the bands at 3551 cm1 (silanol) and 1028 cm1 (SieO). Fig. S1 shows the XRD patterns of PAL, PAL/MB and PAL/ MB@SiO2 pigments. The diffraction peaks at 2q of 8.38 , 13.74 , 16.34 , 19.83 , 27.54 and 34.38 are the characteristic peaks of PAL [58]. The strong peak of PAL at 2q ¼ 8.38 (d ¼ 10.6 Å) can be seen for all the samples, indicating high order-disorder structural degree of PAL [58]. The peaks at 2q of 26.63 and 30.97 are ascribed to the characteristic peaks of quartz and carbonate impurity, respectively [58]. No change of the characteristic peaks of PAL in the XRD patterns can be seen after forming a complex with MB and polycondensation of TEOS, which reveal that the crystal structure of PAL is well preserved. The XRD result is consistent with the BET analyses that the MB molecules can only be adsorbed onto the external surface, the grooves and the openings of the channels of PAL, but cannot enter the channels. Amorphous SiO2 from the
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Fig. 8. Variations of UVeVis spectra and images of the supernatants of PAL/MB and PAL/MB@SiO2 pigments after chemical attacks for 3 d using (a) 1 M HCl, (b) 1 M NaOH and (c) ethanol, digital images of (d) PAL/MB and (e) PAL/MB@SiO2 pigments before and after attack of 1 M HCl, 1 M NaOH and ethanol.
polycondensation of TEOS often gives a broad peak in the 2q range of 20 e25 [59]. Whereas the SiO2 peak is not observable for the PAL/MB@SiO2 pigment, which may be caused by the overlap with the peaks of PAL in this region. 3.5. Stability of PAL/MB@SiO2 pigment The thermal stability of PAL, MB, PAL/MB and PAL/MB@SiO2 pigments were evaluated by means of thermogravimetric analysis (TGA) and the results are shown in Fig. 7. The slight weight loss of MB before 200 C (2.11%) is mainly attributed to the removal of the adsorbed water. MB exhibits a gradual weight loss starting from 200 C. The weight of MB decreases linearly after 400 C and only 2.27% of the initial weight is reserved with further increasing the temperature to 600 C. The final weight of the dye is 0.12% indicating the complete decomposition of MB at 900 C. The TGA curve of PAL is consistent with previous studies [13,58] and can be
divided in four sections: (i) in the low temperature region (<150 C), a 1.85% weight loss can be observed and is attributed to the loss of both superficially adsorbed and loosely bound zeolitic OH2; (ii) the second region (150e300 C), a further 2.07% weight loss is related to the loss of residual zeolitic OH2 and the first fraction of structural OH2; (iii) the third region (300e580 C), residual first half of structural OH2 and the second half of the structural OH2 are removed from PAL; (iv) the last region (580e800 C), a further progressive weight loss corresponds to the decomposition of structural OH2 and hydroxyl groups. During this process, PAL decomposes into an amorphous phase and 90.18% of the initial weight is reserved. The weight loss of PAL/MB and PAL/MB@SiO2 is obviously less than that of PAL in the range of 100e327 C. This means the thermal stability of PAL/MB and PAL/MB@SiO2 is higher than that of PAL. This is probably because the openings of the channels of PAL are blocked by the dye molecules which partly mitigates the loss of
Fig. 9. Digital images of (a) PAL/MB and (b) PAL/MB@SiO2 after UV irradiation at 60 C for 24 d.
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zeolitic OH2. This result is consistent with the BET analyses. The temperatures of maximum weight loss rate of MB are 400 C, 415 C and 434 C, respectively, for MB, PAL/MB and PAL/MB@SiO2. MB is completely decomposed and the weight loss of PAL is 8.74% at 600 C. Meanwhile the weight loss of PAL/MB and PAL/MB@SiO2 are 13.69% and 12.72%, respectively, which is 4e5% more than that of PAL. This means there are some residual partly decomposed MB in PAL/MB and PAL/MB@SiO2 since the MB content in PAL/MB and PAL/MB@SiO2 is about 12%. All the data indicate that the thermal stability of MB is obviously improved after forming complex with PAL and surface modification with SiO2. This is because of the hosteguest interaction between MB and PAL, and the shielding effect of PAL and the SiO2 layer. In order to evaluate the effect of the SiO2 layer on stability of the pigments, the pigments were immersed in 1 M HCl, 1 M NaOH and ethanol at room temperature for 72 h. The supernatants after centrifugation were measured by UVeVis spectrophotometer and photographed every 24 h (Fig. 8). It can be observed from Fig. 8a that the supernatants of PAL/MB@SiO2 after acidic attack show distinctively lower absorbance (the maximum absorbances are 0.86 (1 d), 1.30 (2 d) and 1.56 (3 d), respectively) than those of the PAL/ MB pigment (the maximum absorbances are 1.11 (1 d), 1.54 (2 d) and 1.77 (3 d)). In Fig. 8b, the same trends were observed for the supernatants attacked by alkaline solution. Also, the supernatants show color (in the web version) change from light yellow to light purple, which probably due to the structure change of MB molecules in the supernatants after attack by alkaline solution. In ethanol, the supernatants of PAL/MB@SiO2 pigment can be measured directly, whereas the supernatants of PAL/MB pigment must be diluted to proper concentration in order to record the UVeVis spectra. Thus, it is obvious that the stability of the pigment against chemical elution was improved after TEOS modification, especially in ethanol. The color differences between PAL/MB and PAL/MB@SiO2 after chemical attack are also shown in Fig. 8d, e. The PAL/MB@SiO2 pigment shows higher stability against chemical attack compared with the PAL/MB pigment. The simulated aging process of the PAL/MB and PAL/MB@SiO2 pigments were also studied in a UV accelerated weathering tester. The representative images of the pigments after the simulated aging tests are shown in Fig. 9. Slight fading of the PAL/MB pigment was observed after 24 d of UV irradiation at 60 C. Whereas the PAL/ MB@SiO2 pigment exhibits excellent stability to UV aging, and no fading of the pigment was observed in 24 d of UV irradiation. 4. Conclusions Stable organic/inorganic hybrid PAL/MB@SiO2 Maya Blue-like pigment has been prepared by adsorption of MB on PAL and polycondensation of TEOS. The weight ratio of MB to PAL in the pigment can be up to 12%. Proper ball milling time and heating temperature could enhance the interaction between PAL and MB, and then improve stability of the pigment. The polycondensation of TEOS on the surface of PAL/MB could further improve stability of the pigment. The MB molecules are adsorbed onto the external surface, the grooves and the openings of the channels of PAL, but cannot enter the channels. The PAL/MB@SiO2 pigment shows excellent stability against chemical elution, thermal aging and UV irradiation. The adsorption-shield of dye molecules reported herein by PAL and the SiO2 layer may pave the way for preparing stable Maya Blue-like pigments of various colors to meet practical applications. Acknowledgments The authors are grateful for financial support of the “Hundred Talents Program” of the Chinese Academy of Sciences, the open
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