An Electrochemical Approach - American Chemical Society

J. Phys. Chem. C 2007, 111, 4585-4595

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Indigo/Dehydroindigo/Palygorskite Complex in Maya Blue: An Electrochemical Approach Antonio Dome´ nech,*,† Marıá Teresa Dome´ nech-Carbo´ ,‡ and Marıá Luisa Va´ zquez de Agredos Pascual‡,§ Departament de Quı´mica Analı´tica, UniVersitat de Vale` ncia, Dr. Moliner, 50, 46100 Burjassot, Vale` ncia, Spain, Institut de ConserVacio´ del Patrimoni/Departament de ConserVacio´ i Restauracio´ de Bens Culturals, UniVersitat Polite´ cnica de Vale` ncia, Camı´ de Vera 14, 46022, Vale` ncia, Spain, and Departament de Historia del Arte, UniVersitat de Vale` ncia, Passeig al Mar, Vale` ncia, Spain ReceiVed: NoVember 8, 2006; In Final Form: February 2, 2007

The voltammetry of microparticles methodology is applied to describe the electrochemistry of indigo and Maya Blue, a nanostructured organic-inorganic material, in contact with different MeCN electrolytes. The voltammetric response of synthetic specimens and genuine Maya Blue samples differ significantly from that of indigo microparticles, all being conditioned by the size-dependent insertion of electrolyte ions into the solid. Electrochemical data indicate that formation of Maya Blue involves a significant reorganization of zeolitic and/or structural water of the clay and confirms the presence of dehydroindigo accompanying indigo in the palygorskite matrix.

1. Introduction Maya Blue is a nanostructured material used as a pigment by the ancient Mayas. The peculiar palette of Maya Blue, ranging from a bright turquoise to a dark greenish blue, and its enormous stability has claimed considerable attention during the years.1 The use of Maya Blue in Central America, mostly in Mexico, is documented from the VIII to the XVI centuries,2 being extended even to recent times.3 However, there is no detailed knowledge of the procedure of preparation of Maya Blue by Pre-Culombian cultures, and, in fact, the composition and structure of the pigment was in the past controversial.4-10 Currently, it is well-known that Maya Blue consists of an hybrid organic-inorganic material resulting from the association of indigo, a natural dye obtained from several plants, mainly Indigofera suffruticosa Miller in Mesoamerica, and palygorskite, a fibrous phyllosilicate extracted almost exclusively from one village in the northern Yucatan: Sacalum (a Spanish corruption of Sak lu’um).11,12 Palygorskite, of ideal composition (Mg,Al)4Si8(O,OH,H2O)24‚ nH2O, can be described as a continuous set of layers formed by two-dimensional tetrahedral and octahedral sheets. Palygorskite exhibits two different polytypes: one monoclinic and one orthorrombic.13,14 Both polytytpes show discontinuous 2:1 tetrahedral:octahedral layers framework with one 2:1 unit joined to the next by inversion of the SiO4 tetrahedra along Si-O-Si bonds. The tetrahedral and octahedral mesh gives rise to a series of rectangular tunnels of dimensions 6.4 × 3.7 Å. Such clays are therefore crossed by zeolite-like channels and permeated by weakly bound, nonstructural (zeolitic) water. Magnesium and aluminum cations complete their coordination with tightly bound water molecules (structural water). Indigo (in the following, H2IN) is a blue dye formed by indigotin (3H-indol-3-one,2-(1,3-dihydro-3-oxo-2H-indol-2* Corresponding author. E-mail: [email protected]. † Departament de Quı´mica Analı´tica. ‡ Institut de Conservacio ´ del Patrimoni/Departament de Conservacio´ i Restauracio´ de Bens Culturals. § Departament de Historia del Arte.

ylidene)-1,2-dihydro), a quasiplanar molecule of approximate dimensions 4.8 × 12 Å. The indigo molecule has a slightly elongated central CdC bond and two elongated CdO bonds. Hydrogen bonds between adjacent molecules cause solid-state aggregation in which one indigo molecule is linked to four others.15 The nature of the indigo-palygorskite association and the reasons for the color and durability of Maya Blue have been discussed recently on the basis of optical spectrometry,16 Raman spectroscopy,17,18 synchroton techniques,19 and X-ray fluorescence and particle induced X-ray emission20 data. A few years ago, Jose´-Yacamań et al.21 discovered that most palygorskite particles exhibit a superstructure along the a axis about 14 Å in length, which roughly corresponds to three times the original lattice constant a and might well be explained by the presence of indigo in the channels. Such authors reported the presence of iron and iron oxide nanoparticles in Maya Blue, suggesting that these may at least partly account for the color.21 The presence of iron oxide and an amorphous phase of FeO(OH) and iron nanoparticles outside the lattice of the crystallites of palygorskite as well as inside the channels has been more recently reported by the same group.22 In contrast, Sańchez del Rio et al.23 measured at the ESRF ID26 beamline the Fe K-edge XANES spectra of the blue pigment in Maya Blue samples and synthetic ones, concluding that the iron found in Maya Blue pigment is related to the Fe exchanged in the palygorskite clay. These authors did not find iron in metallic form or goethite in archaeological Maya Blue.17,23 In this context, different models for the interaction between indigo and palygorskite have been proposed.24-27 Using thermal analysis and multinuclear magnetic resonance, Hubbard et al.24 proposed a model based on the formation of hydrogen bonds between the carbonyl and amino groups of indigo with edge silanol units of the clay. Chiari et al.25 and Giustetto et al.27 combined XRD, thermal, FTIR, and Raman data with molecular modeling concluding that hydrogen bonds between CdO and N-H groups of indigo molecules and structural water molecules occurs. Fois et al.,26 using classical molecular dynamics simula-

10.1021/jp067369g CCC: $37.00 © 2007 American Chemical Society Published on Web 03/07/2007

4586 J. Phys. Chem. C, Vol. 111, No. 12, 2007 tions, concluded that hydrogen bonds between indigo carbonyl and structural water are seemingly the most important interaction but emphasized that van der Waals interactions and a direct interaction between indigo and clay octahedral cations, not mediated by structural water, could also play an important role in anchoring indigo molecules. Remarkably, computational methods applied to the refinement of the indigo-palygorskite structure indicate that structural water lies only with carbonyl groups of indigo,25,27 whereas molecular modeling taking a H2IN‚2H2O complex and spectroscopic data suggest that both Cd O and N-H indigo groups are involved in hydrogen bond formation with structural water.27 In a previous work,28 we studied a set of Maya Blue samples from different archaeological sites of Yucatań and Campeche using a novel technique, the voltammetry of microparticles (VMP). This technique, developed by Scholz et al.,29,30 provides information on the redox reactivity of sparingly soluble solids. Combining VMP with spectroscopic (vis, FTIR/ATR), atomic force microscopy (AFM), and transmission electron microscopy (TEM) provided evidence for the presence of dehydroindigo, the oxidized form of indigo, in the palygorskite system, contributing significantly to the greenish color of Maya Blue. In this context, some unsolved questions arise: (i) the detailed nature of the indigo-palygorskite interaction, including an account of participation of CdO and N-H indigo groups in hydrogen bond formation; (ii) the stoichiometry of the H2Oindigo complex proposed for describing such an interaction; (iii) the nature of the dehydroindigo-palygorskite interaction and the structural influence of the formation of dehydroindigo within the palygorskite framework, in particular with regard to the aforementioned 3-cells superlattice structure.21 All of these matters deal with the possible preparation procedure of Maya Blue, in particular, with the characteristics of the presumed thermal treatment accompanying indigo plus palygorskite crushing. In order to study problems i and ii, VMP has been applied to indigo microparticles, synthetic Maya Blue specimens prepared by crushing palygorskite and indigo under different thermal treatments, and genuine Maya Blue samples in contact with nonaqueous electrolytes. The use of nonaqueous electrolytes allows us to study the water interactions in terms of interchange of water molecules between the palygorskite support and the solvent. In this study, samples from archaeological sites of Chicheń Itza´, Mulchic, and El Tabasquenõ (Yucatań, Early Classical Maya period) were used. Cyclic and square wave voltammetries (CVs and SQWVs, respectively) at paraffinimpregnated graphite electrodes (PIGEs) and polymer film electrodes (PFEs) were used as detection modes. Different MeCN and DMSO electrolytes (Bu4NPF6, Et4NClO4, and LiClO4) to account size-exclusion effects associated with the ingress/issue of charge-balancing counterions to/from the aluminosilicate channel system have been used in order to gain information on the attachment of dye molecules to the inorganic support. Point iii was also addressed by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) combined with electrochemical experiments. Analysis of data derives from models on the electrochemistry of insertion solids due to Lovric, Scholz, Oldham, and coworkers.31-34 Within this theoretical frame, Maya Blue electrochemistry is compared with that reported for indigo microparticles35-37 and electroactive species attached to microand mesoporous aluminosilicates.38-41 Additionally, an electrochemical fractal analysis of Maya Blue deposited onto glassy

Domeńech et al. carbon electrodes is described. From the seminal work of Pajkossy and co-workers,42-46 this type of analysis has been revealed as a powerful tool for describing roughness surfaces.47-49 Theoretical approaches for describing electron transfer through a modified electrode with a fractal structure have been provided by Dassas and Duby,50 Stromme et al.,51 and Andrieux and Audebert.52 2. Experimental Section 2.1. Sampling and Reference Materials. Micro- (ca. 1 µg) and eventually nanosamples (ca. 20-50 ng) of Maya Blue wall paintings were taken with the help of a microscalpel during routine examination and restoration. A light microscope Leica DMR (25×-400×) was used for selecting the samples to be analyzed and for morphological examination of them. Palygorskite was collected from the Sacalum classical deposits in Yucatan. Synthetic indigo (Fluka) was used as a reference material. To study the attachment of indigo to different inorganic hosts, a series of synthetic specimens (S-1 to S-6) were prepared by crushing indigo (3% w/w) with palygorskite or silica (95% w/w) and heating at temperatures of 40, 100, 160, 200, 300, and 400 °C for periods of 12 h. A second series of synthetic specimens (ST-1 to ST-6) was submitted to a thermal treatment at 200 °C for 1, 2, 4, 6, 12, and 24 h. With exception of sample S-6, which acquired a brown hue, all Maya Blue synthetic specimens became greenish-blue, the greenish hue increasing on increasing temperature along the S-1 to S-5 series. 2.2. Instrumentation and Procedures. Voltammetry of microparticles experiments were performed using paraffinimpregnated graphite electrodes (PIGEs, preparation described in refs 29 and 30) immersed into dry MeCN and DMSO using Bu4NPF6 (Fluka), Et4NClO4 (Acros), and LiClO4 (Aldrich) as supporting electrolytes. Additional experiments were performed on deposits of Maya Blue samples over glassy carbon electrodes (BAS MF7012, geometrical area 0.071 cm2) from suspensions (10 mg/mL) in acetone solutions (3% w/w) of Paraloid B72 acrylic polymer. Preparation details for analogue zeolitemodified electrodes are described in refs 39 and 40. Measurements were performed in a thermostated threeelectrode cell under argon atmosphere using a AgCl (3M NaCl)/ Ag reference electrode separated from the bulk solution with a salt bridge and a platinum-wire auxiliary electrode. Cyclic and square wave voltammograms (CVs and SQWVs, respectively) and chronoamperograms (CAs) were obtained with a CH I420 equipment. For modified electrode preparation, the samples were powdered in an agate mortar and pestle and extended forming a spot of finely distributed material. The lower end of the graphite electrode was pressed over that spot of sample to obtain a sample-modified surface. Commercially available Paraloid B72, an ethyl acrylate (70%)-methyl acrylate (30%) copolymer (P[EMA/MA]) was selected for polymer-film electrode preparation because of its mechanical stability and ability to form porous films able to adhere the zeolite microparticles to the electrode surface. Polymer-film zeolite-modified electrodes were prepared, as previously described,39,40 by transferring a few microlitres (typically 50 µL) of a dispersion of the zeolite (10 mg) in acetone (5 mL) to the surface of a freshly polished glassy carbon electrode and allowing the coating to dry in air. After the electrode surface was air-dried, one drop of a solution of the acrylic resin (1%) in acetone was added, and the modified electrode was again air-dried. Attenuated total reflectance/Fourier transform infrared (ATR/ FTIR) spectra of indigo-modified electrodes were obtained with

An Electrochemical Approach to Maya Blue

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Figure 2. SQWVs of indigo-modified PIGEs immersed into (a) 0.10 M Bu4NPF6/MeCN, (b) 0.10 M Et4NClO4/MeCN, and (c) 0.10 M LiClO4/MeCN. Potential scan initiated at +1850 mV in the negative direction. Potential step increment 4 mV; square wave amplitude 25 mV; frequency 15 Hz.

in contact mode. An oxide-sharpened silicon nitride probe Olympus, VEECO Methodology Group, model NP-S has been used with a V-shaped cantilever configuration. The spring constant is 0.06 N/m and the tip radius of curvature is 5-40 nm. For electrochemical measurements, the AFM was coupled to a Digital Instruments Universal Bipotentiostat (VEECO Methodology group, USA). All measurements were performed at room temperature in 0.50 M aqueous acetate buffer (pH 4.85) previously deaerated with argon for 15 min. 3. Results and Discussion

Figure 1. CVs of indigo microparticles attached to PIGE in contact with (a) 0.10 M Bu4NPF6/DMSO (semi-derivative convolution), (b) Bu4NPF6/MeCN potential scan initiated at 0.0 V in the negative direction, (c) Id., potential scan initiated at 0.0 V in the positive direction. Potential scan rate 50 mV/s.

a Vertex 70 Fourier transform infrared spectrometer with a FRDTGS (fast recovery deuterated triglycine sulfate) temperaturestabilized coated detector. Number of co-added scans: 32; resolution: 4 cm-1. Scanning electron microscopic (SEM) examination of coatings was performed with a Jeol JSM 6300 scanning electron microscope operating with a Link-Oxford-Isis X-ray microanalysis system. The analytical conditions were as follows: accelerating voltage, 20 kV; beam current, 2 × 10-9 A; and working distance, 15 mm. Samples were carbon coated to eliminate charging effects. A transmission electron microscope Philips CM10 with Keen view camera soft imaging system was used operating at 100 kV. Maya blue pigment from Dzibilnocac and palygorskite from Sakalum sites were prepared by grinding a few micrograms of the samples in an agate mortar and then dispersing them by the help of an ultrasons bath in dichloroethane. A drop of the dispersions was poured on TEM grids pretreated with a polymer film layer with holes in order to improve the images obtained. A Multimode AFM (Digital Instruments VEECO Methodology Group, USA) with a NanoScope IIIa controller and equipped with a J-type scanner (max. scan size of 150 × 150 × 6 µm) was used. The topography of the samples was studied

3.1. Voltammetry of Indigo. Cyclic voltammetry of indigo microparticles in contact with 0.10 M Bu4NPF6/DMSO is shown in Figure 1a. During the experiment the electrolyte solution acquires a blue color, denoting that a significant dissolution of indigo microparticles occurs. The voltammogram exhibits illdefined peaks which can be more clearly seen upon semiderivative convolution. CVs initiated at 0.0 V in the negative direction show two main almost reversible waves at -835 (I) and ca. -1300 mV (II), followed, in the subsequent scan by anodic coupled waves at -1250 and -720 mV. These are accompanied by an additional oxidation peak at +730 mV (III) without cathodic counterpart. In contact with 0.10 M Bu4NPF6/MeCN, the dissolution of indigo particles is negligible, as denoted by the absence of a blue color in the electrolyte solution. As depicted in Figure 1b, CVs initiated at 0.0 V in the negative direction show a welldefined couple I at peak potentials of -835 (cathodic) and -590 (anodic), accompanied by ill-defined signals. In contrast, CVs initiated at 0.0 V in the positive direction (Figure 1c) exhibit two prominent anodic peaks at +1175 (IV) and +1590 mV (V) while the couple I becomes considerably lowered. This electrochemistry varies significantly in contact with Et4NClO4- or LiClO4-containing MeCN and DMSO electrolytes. This can be seen in Figure 2, where SQWVs of indigo-modified PIGEs immersed into (a) 0.10 M Bu4NPF6/MeCN, (b) 0.10 M Et4NClO4/MeCN, and (c) 0.10 M LiClO4/MeCN are shown. In contact with 0.10 M Bu4NPF6/MeCN, a main peak at -1020 mV (II) appears; however, in contact with 0.10 M Et4NClO4/ MeCN and 0.10 M LiClO4/MeCN, overlapping peaks at ca. +1400 (V), +1150 (IV), -400 (VI), -750 (I), and -1300 mV (II) are recorded.

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Figure 3. SQWVs of (a) synthetic sample S-2 and (b) Maya Blue sample from El Tabasquenõ (Yucatań, Early classical period) in contact with 0.10 M Et4NClO4/MeCN. Potential scan initiated at -1850 mV in the positive direction. Potential step increment 4 mV; square wave amplitude 25 mV; frequency 5 Hz.

As can be seen in Figures 1 and 2, in the second and subsequent scans, voltammetric peaks almost entirely vanish. This feature can be interpreted as a result of the peculiar nature of solid-state electrochemistry (vide infra). 3.2. Voltammetry of Maya Blue Specimens. CVs and SQWVs of Maya Blue specimens in contact with DMSO electrolytes produced no well-defined responses, close to those previously described for indigo microparticles. Similar voltammetric behavior was observed upon immersion into 0.10 M Bu4NPF6/MeCN. In contact with Et4NClO4/MeCN and LiClO4/ MeCN electrolytes, however, Maya Blue specimens exhibit some significant features. This can be seen in Figure 3, where SQWVs of (a) synthetic sample S-2 and (c) Maya Blue sample from El Tabasquenõ (Yucatań, Early classical period) in contact with 0.10 M Et4NClO4/MeCN are shown. In these voltammograms, peaks IV and V are accompanied by a peak at +800 mV (VII) preceded by a shoulder near +400 mV. A similar voltammetry was observed for specimens S-1 to S-5 and ST-1 to ST-6. In contact with 0.10 M LiClO4/MeCN, again synthetic Maya Blue specimens and genuine Maya Blue samples produce similar profiles. As can be seen in Figure 4a, prominent reduction peaks at -120 (IX) and -400 mV (X) accompany peaks I and II (this ill-defined) in SQWVs. As can be seen on comparing in this figure with Figure 4b, a significant peak splitting appears on lowering the square wave frequency. Interestingly, different variations of peak potentials on the square wave frequency, f, were obtained for the different electrochemical processes. For data in contact with 0.10 M Et4NClO4/MeCN, peaks I, II, IV, and V varied slightly with f for indigo microparticles, whereas peak III varied significantly with f. In contrast, peaks I, II, VI, VII, VIII, IX, and X were found to be essentially f-independent for Maya Blue specimens, suggesting an essentially reversible behavior.

Figure 4. SQWVs of synthetic sample S-4 immersed into 0.10 M LiClO4/MeCN. Potential scan initiated at +1850 mV in the negative direction. Potential step increment 4 mV; square wave amplitude 25 mV; frequency: (a) 15 and (b) 5 Hz.

3.3. Characterization of Maya Blue Specimens. The oxidative voltammetry of Maya Blue specimens is significantly simplified using PFEs. As can be seen in Figure 5a, for specimen S-1, prepared by crushing indigo and palygorskite and heating at 40 °C, two consecutive oxidation peaks at +750 (VII) and +1250 mV (VIII) appear. On increasing the square wave frequency, the peak VIII is decreased with respect to the peak VII. For the series ST-1 to ST-6, prepared by crushing indigo and palygorskite and heating at 200 °C during different times, the peak VIII increased slowly with respect to the peak VII on prolonging the duration of the thermal treatment. Along the series S-1 to S-6, on increasing temperature, peak VIII increases at the expense of peak VII, as can be seen in Figures 5b for sample S-4. (temperature of thermal treatment 200 °C). For sample S-6 (Figure 5c), treated at 400 °C, the peak VIII replaces entirely the peak VII, thus denoting that the species resulting from the electrochemical oxidation VII is the same that is chemically generated upon heating the palygorskiteindigo complex. Changes in the voltammetric response in the series S-1 to S-6 were parallel to hue changes in the samples, from blue (S1), to increasingly greenish (S-2 to S-5) and, finally, to brown in sample S-6. Examination of synthetic specimens by SEM, shown in Figure 6, revealed a similar morphology for Maya Blue specimens S-1 to S-5. As shown in Figure 6a, fibrous palygorskite crystals are accompanied by irregular particles of



Figure 5. SQWVs for synthetic Maya Blue specimens: (a) S-1, (b) S-3, and (c) S-6, immersed into 0.10 M LiClO4/DMSO. Potential step increment 4 mV; square wave amplitude 25 mV; frequency 5 Hz.

silica and other materials including, eventually, elongated crystals of indigo (Figure 6b) remaining from the crushing process. In sharp contrast, sample S-6 showed no fibrous palygorskite crystals, replaced here by irregular grains, as can be seen in Figure 6c. Changes in the composition of specimens can also be monitored by ATR/FTIR spectroscopy and compared with data reported for indigo, palygorskite, and Maya Blue samples.27,28,54 As shown in Figure 7, specimen S-1 provides a complex spectrum with a sharp band at 1625 cm-1 overlapped with a band ranging from 1650 to 1750 cm-1. These bands can be attributed to the superposition of the δ(H2O) mode of a particular kind of structural water associated to indigo, described by Giustetto et al.,27 and υ(CdO) antisymmetric stretching mode of palygorskite-associated indigo and dehydroindigo molecules28 whose frequencies, following the theoretical and experimental study of Klessinger and Luettke,54 are 1629 and 1735 cm-1, respectively. On increasing crushing temperature, the specimens showed a decrease of the sharp band at 1625 cm-1, as can be seen in Figure 7 for sample S-4. This feature is accompanied by a decrease in bands at 1475, 1455, 1405, and 1324 cm-1, all associated with indigo molecules, with a concomitant band shift to higher wavenumbers. Synthetic specimens showed welldefined bands at 3611 and 3541 cm-1, attributable to zeolitic water. The bands are shifted toward higher wavenumbers along the series S-1 to S-4, the second being enhanced at the expense of the first. For specimens S-5 and S-6, the aforementioned peak vanishes, as a result of the release of zeolitic water at temperatures above 110-120 °C. These features suggest that both zeolitic and, especially, structural water experience some interaction with indigo molecules, thus resulting in the abovedescribed spectral shifts. This is consistent with data provided by thermal analysis: Following Hubbard et al.24 and Giustetto et al.,27 between 110 and 120 °C desorption of loosely bound (physisorbed) water and some zeolitic water occurs, whereas the loss of the residual fraction of zeolite water and perhaps some of weakly bound structural water takes place in the 220230 °C temperature range. Between 450 and 480 °C, the release

Figure 6. SEM images of (a) sample S-2, (b) S-2, shown detail of indigo crystals, and (c) sample S-6.

of structural water occurs, palygorskite transforming into palygorskite anhydride. Finally, at 700 °C, dehydroxylation and phase transformation to clino-estatite takes place.27 Thermal decomposition of indigo molecules occurs at 360 °C.24 AFM images of palygorskite consists of aggregates of elongated crystals 0.5-1 µm sized having fibber structures with thicknesses from 30 to 60 nm as shown in Figure 8a. In contrast, AFM images of Maya Blue samples show narrow fibbers with a corrugated structure as can be seen in Figure 8b. Palygorskite crystals in Maya Blue samples present elongated crystals divided into almost square sections of 125 ( 5 nm size, attributable to the presence of domains of regular distribution of indigo on the surface of palygorskite rods or to extensive dislocations of the palygorskite associated to the distribution of indigo mol-

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Figure 7. ATR/FTIR spectra of (A) sample S-1 and (B) sample S-4.

ecules. Several palygorskite fibers in Maya Blue samples exhibit a peculiar texture as can be seen in Figure 9, corresponding to a TEM image of a Maya Blue sample from Mulchic. This may be associated with water evacuation and the distortion of the indigo-based superlattice resulting from the partial conversion of indigo into dehydroindigo which accompanies the formation of the pigment. Electrochemical experiments performed at the AFM cell were consistent with that idea. In order to test possible damages in the palygorskite crystals induced by the indigo/dehydroindigo interconversion, an oxidative potential cycle was applied to synthetic specimens and the Maya Blue sample from Mulchic in contact with aqueous acetate buffer (pH 4.85). This media was preferred to nonaqueous electrolytes for obtaining a “clean” electrochemical response and avoid solvent perturbations in the AFM cell. In agreement with prior results,28 an oxidation wave was recorded at a potential of ca. +0.45 V vs AgCl/Ag (see the Supporting Information). That oxidation process was accompanied by the increase in the corrugated structure of palygorskite crystals. The effect was increased by applying a constant potential step of +0.50 V during times between 5 and 15 min. As can be seen in Figure 10, the initial corrugated palygorskite crystals (Figure 10a) experienced a significant morphological change, shown an irregular, apparently disaggregated surface after application of the potential step (Figure 10b). 3.4. Characterizing Solid-State Electrochemistry. In order to assign the electrochemical response of Maya Blue specimens in contact with nonaqueous electrolytes to palygorskite-associated indigo forms, chronoamperometric data were taken. Shorttime chronoamperometric experiments recorded for Maya Blue specimens in LiClO4/MeCN by application of a constant potential 200 mV past that of peaks VI, VII, and VIII provided results in agreement with those theoretically predicted for

Figure 8. AFM images of (a) palygorskite crystals and (b) Maya Blue sample from Mulchic.

Figure 9. TEM image of a Maya Blue sample from Mulchic, Late Classical period. Image size 200 nm.

electroactive species attached to microporous solids,41 derived from the model of Lovric, Scholz, Oldham, and co-workers for ion insertion solids.31-34 In that model, electron-transfer pro-


J. Phys. Chem. C, Vol. 111, No. 12, 2007 4591 sample from Mulchic) a current-time variation close to that expected for ion-insertion solids (vide infra), characterized by a well-defined maximum in the current.34 In the time interval between 0.01 and 103 s, however, chronoamperometric measurements provided linear log i vs log t plots (see the Supporting Information). This response can be rationalized by considering that indigo molecules are distributed within the palygorskite voids so that, once the semi-infinite diffusion conditions are reached, the situation parallels that for nonplanar electrodes. Fractal diffusion has been used for describing nonplanar electrode surfaces leading to deviations from classical planar diffusion on electrochemical timescales. For semi-infinite diffusion conditions, the Cottrell equation becomes42-50

i)

Figure 10. AFM images recorded (a) before and (b) after application of a potential step of +0.5 V during 15 min to a Maya Blue sample from Mulchic, immersed into aqueous acetate buffer in the AFM cell. Size of the images 2000 nm.

cesses involving immobile redox centers attached to nonconducting solids are described in terms of the transport of electrons and ions through solid particles deposited on inert electrodes. The electrochemical process begins at the electrode/electrolyte/ particle interface and expands through the particle via electron transfer across the electrode/particle interface and ion transfer across the electrolyte/particle interface. The redox conductivity is then ensured by electron hopping between immobile redox centers in the solid coupled by the diffusion of charge-balancing electrolyte ions through the solid, a process allowed for microand mesoporous solids.38-41 This model explains the significant weakening of voltammetric peaks in repetitive CV, previously noted. For indigo particles, there is no easy ion transport that occurs across the solid, so that the electroactive region is confined to a shallow layer in the more external region of the grains. Then, electrochemically available indigo molecules are rapidly exhausted during the CV experiment, as described for the electrochemistry of indigo in contact with aqueous electrolytes.37 Upon application of a potential of +1400 mV, corresponding to the sum of electrode processes VII and VIII, experimental data for Maya Blue specimens at times shorter than 0.01 s exhibit (see the Supporting Information for a genuine Maya Blue

nFAcxD xπ tR

(1)

where D is the diffusion coefficient of the electroactive species, c is its concentration in the bulk media, and the exponent R is related with the fractal dimension, dF, of the surface through the relationship R ) (dF - 1)/2.50-52 Fractal-scaling behavior is representative of systems whose topology is the same regardless of the length scale used for their examination. In the case of rough electrode surfaces, the fractal dimension can be viewed as a measure of the surface roughness, dF varying from 2, corresponding to a plane surface, to 3, representative of an ideal 3-dimensions porous electrode. In the studied systems, dF values calculated from log i vs log t plots for samples S-1 to S-6 increased monotonically from 2.20 to 2.28 (see the Supporting Information). Since indigo molecules are immobilized in an inorganic support, the fractal dimension of the indigo-centered electrochemical process can be viewed as representing the indigo distribution in the palygorskite framework. Accordingly, on increasing the crushing temperature, indigo molecules should move from the surface of palygorskite crystals (limiting case with dF ) 2) to the channels of such crystals, with dF increasing progressively and approaching a 3-dimensions porous electrode. 3.5. Electrochemical Pathways. In contact with DMSO, the voltammetric response of indigo microparticles is dominated by solution-phase electrochemistry, so that, in agreement with Bond et al.,35 processes I and II in solution phase can be ascribed to successive one-electron reductions of indigo, H2IN, to an anion radical, H2INΑ-, and a dianion, H2IN2-, respectively +e-

+e-

H2IN (solv) 98 H2IN•- (solv) 98 H2IN2- (solv) (2) where (solv) denotes solvated species in solution phase. Process III is attributable to the two-electron oxidation of indigo to dehydroindigo, IN, accompanied by the loss of two protons.35 This can be represented as

H2IN (solv) f IN (solv) + 2H+ (solv) + 2e-

(3)

Such electrochemical processes are structurally depicted in Scheme 1. In contact with MeCN electrolytes, solid-state electrochemistry can be more clearly recorded. Here, peaks I and II are lowered, being attributable to the reduction of indigo molecules

4592 J. Phys. Chem. C, Vol. 111, No. 12, 2007 SCHEME 1: Representation of Redox Processes Involving Indigo in Aqueous (Upper Part) and Nonaqueous Environments (Lower Part)

Domeńech et al. In this scheme, the weak reduction peak VI observed for indigo microparticles in contact with MeCN electrolytes is probably induced by protons resulting from oxidation process III. Accordingly, indigo is reduced to leucoindigo, H4IN, following a pattern similar to that displayed by indigo immersed into aqueous electrolytes35-37

{H2IN} + 2H+ (solv) + 2e- f {H4IN}

(6)

The electrochemical oxidation of indigo microparticles can be described in terms of the formation of dehydroindigo accompanied by proton loss

H2IN (c) f IN (c) + 2H+ (solv) + 2e-

(7)

A representation of redox processes is depicted in Scheme 1. Reduction processes IX and X recorded for Maya Blue samples appear at potentials intermediate between those recorded for indigo in aqueous media and those obtained for indigo in contact with nonaqueous electrolytes. This suggests that within the palyygorskite matrix, where zeolitic and structural water molecules exist, leucoindigo can be formed via internal proton transfer, accompanied by the ingress of electrolyte chargebalancing cations in the palygorskite matrix. Representing the indigo-palygorskite complex forming Maya Blue as {H2IN.xH2O}, reduction processes IX and X can be represented as +M+ (MeCN) + e-

{H2IN‚xH2O} 98 +M+ (MeCN) + e-

{H3IN‚(x - 1)H2O‚OH-‚M+} 98 passing to the solution phase. For indigo microparticles, reduction steps I and II can be formulated as +M+ (solv) + e-

H2IN (c) 98 H2IN•-...M+ +M+ (solv) + e-

(c) 98 H2IN2-...2M+ (c) (4) Here, (c) denotes solid phases and M+ ()Bu4N+, Et4N+, Li+) is an electrolyte countercation. These reduction steps are accompanied by electroinsertion of electrolyte cations. Such processes are probably hindered for Bu4N+/MeCN but are allowed in contact with Et4N+/MeCN or Li+/MeCN, thus providing the significant electrolyte-dependent voltammetry illustrated in Figure 2. Oxidation process III is absent for indigo- and Maya Bluemodified PIGEs in contact with MeCN electrolytes. Here, that process is replaced by processes IV and V. These processes are significantly electrolyte-dependent, as can be seen in Figure 3. The potentials for such processes are clearly separated from that for process III in solution phase, suggesting that a significantly different mechanism is involved. In agreement with prior considerations on solid-state elctrochemistry, processes IV and V can be described in terms of two successive one-electron oxidation steps involving the ingress of electrolyte counteranions in the solid phase +X- (solv), -e-

+X- (solv), -e-

{H2IN} 98 {H2IN.+...X-} 98 {H2IN.+...X-} (5) where {} denotes the palygorskite-associated species and X) PF6-, ClO4-. Here, insertion of ClO4- is allowed, whereas insertion of PF6- is hindered, thus resulting in large differences in the voltammetric response, as shown in Figure 3.

{H4IN‚(x - 2)H2O‚2OH-‚2M+} (8) Electrochemical oxidation of palygorskite-associated indigo to palygorskite-associated dehydroindigo occurs via two-electrontransfer process VII. Here, two different mechanisms can be proposed. First, an oxidation with proton issue to the electrolyte

{H2IN‚xH2O} f {IN‚xH2O} + 2H+ (MeCN) + 2e-

(9)

or, alternatively, an internal proton transfer to water molecules coupled with the ingress of electrolyte counteranions to the palygorskite system

{H2IN‚xH2O} + 2X- (MeCN) f {HIN‚(x - 2)H2O‚2H3O+‚2X-} + 2e- (10) Since process VII appears for ClO4--containing electrolytes but not for PF6--containing ones, one can conclude that the second of the above mechanisms is operative. These features suggest that indigo/dehydroindigo interconversions within the palygorskite framework occur via reorganization of water molecules; thus modifying the distribution of hydrogen bonds in the palygorskite-indigo-dehydroindigo complex rather than merely via ion exchange with the supporting electrolyte. Additionally, it was observed that (i) the peak VIII increases at the expense of peak VII along the series S-1 to S-6 and (ii) the peak current of VIII was systematically larger than that of peak VIII. Since peak VIII corresponds to the oxidation of dehydroindigo to any unidentified compound, it is concluded that the amount of dehydroindigo increases on increasing the crushing temperature, confirming the expectances from prior thermochemical data.28 3.6. Thermochemical Considerations. Following Chiari et al.,25 the formation of Maya Blue by attachment of indigo forms



to palygorskite involves the loss of zeolitic water. This process can be represented as

H2IN (c) + {} f {H2IN}

indigo (s) + {hydrated palygorskite} (s) f {indigo + palygorskite} (s) + xH2O (11)

and the free energy change for the oxidation of solid indigo to dehydroindigo (process III), described as the inverse of eq 9, ∆G°III, by the relationship

Using electrochemical data in contact with aqueous buffers, the values of the free energy of attachment of indigo, dehydroindigo and leucoindigo to anhydrous palygorskite

∆G°DM ) ∆G°III - ∆G°DP + ∆G°IP + x∆G°iw

Combining the above expression with eq 17, one can arrive at

indigo (s) + {anhydrous palygorskite} (s) f {indigo + palygorskite} (s) (12)

∆G°DW ) ∆G°III - ∆G°DP + ∆G°IP + x∆G°iw + 2∆G°pt + x∆G°wt (22)

were calculated as +5.6, -72.1, and +43.5 kJ/mol, respectively.28 Since indigo molecules are associated with the palygorskite support via hydrogen bonds, formulating electrochemical processes must take into account such interactions. Here, the indigo-palygorskite complex will be represented in terms of a indigo-water complex, H2IN‚xH2O, as used by Giustetto et al.27 for molecular modeling. Accordingly, reduction/oxidation of palygorskite-associated indigo involves the change in the hydrogen-bonding system and water content, as represented by eqs 8-12. Taking into account the foregoing considerations, the electrochemical reduction of dehydroindigo to indigo in Maya Blue immersed into MeCN electrolytes can be represented as

Here, ∆G°DW can be estimated as - 2FE°DW, E°DW being the formal electrode potential for the dehydroindigo/indigo reduction in contact with aqueous buffers. Taking the value of E°DW extrapolated at pH 7.0 (+380 mV vs AgCl/Ag), E°III ) +700 mV and the above-mentioned values of ∆G°DP, ∆G°IP, ∆G°pt, and x∆G°wt, one can obtain ∆G°iw ) -64.0 kJ/mol for x ) 2. This value is close to the average binding energy of zeolitic water calculated in optimized Maya Blue molecular models (-66.5 kJ/mol).25,27 Intra-palygorskite water reactivity suggested, for instance, by eq 10, is in principle consistent with data for the reactivity of water in zeolites.56 This result appears to be consistent with molecular modeling and thermal and spectral literature data.24-27 Thus, position and intensities of δ(H2O) and υ(H2O) modes in FTIR spectroscopy of Maya Blue, as well as micro-Raman spectral details, showed, following Giustetto et al.,27 that a part of structural water molecules interact via hydrogen bond with both the CdO and N-H indigo groups, in agreement with molecular modeling taking a H2IN‚2H2O complex.27 Molecular dynamic calculations for palygorskite and indigo performed by the same authors27 and Fois et al.,26 on indigo-palygorskite lattices, however, provided no significant hydrogen bond formation between structural water and N-H indigo groups. Prior thermochemical data analysis suggests that the formation of the dehydroindigo-palygorskite complex (via the inverse of eqs 13 and 16) should involve the release of two water molecules per dye molecule. Accordingly, the indigo-palygorskite complex should involve a H2IN‚2H2O complex, as denoted by spectral data.27 In this scheme, the dehydroindigo-palygorskite complex should involve hydrogen bond formation uniquely between structural water and CdO groups of the dye. The more favorable free energy of attachment of IN to palygorskite with respect to that of H2IN probably results from hydrogen bond reorganization, direct interaction of indigo with clay octahedral cations, and van der Waals interactions, as suggested by Fois et al.26 for indigo-palygorskite attachment. This last factor should be related with the flexible nature of the dehydroindigo molecule (having a central single C-C bond), which favors its accommodation into the palygorskite channels in contrast with indigo, a more rigid molecule, due to is central CdC bond. Our data suggest that preparation of Maya Blue involves a thermal treatment in which the oxidation of indigo to dehydroindigo involves the loss of two water molecules (i.e., the breaking of the indigo-palygorskite original complex). Remarkably, SEM images (Figure 7) and AFM ones of Maya Blue specimens indicate that when indigo decomposes (above 360 °C) fibrous palygorskite crystals are apparently destroyed, a situation consistent with the occurrence of the indigo-palygorskite super-lattice described by Jose´-Yacamań et al.21 In this scheme, indigo decomposition should give rise to a collapse of that super-lattice with the concomitant alteration of the palygorskite structure. At the expense of a more detailed description

{IN} + 2H+ (MeCN) + xH2O (MeCN) + 2e- f {H2IN‚xH2O} (13) The free energy change for this process, ∆G°DM, can be expressed in terms of the free energy changes for the process VII (described as the inverse of eq 10), ∆G°VII, and the processes of insertion of protons, water, and electrolyte anions into the palygorskite framework, respectively, ∆G°ip, ∆G°iw, ∆G°ix

Z (MeCN) + {} f {Z}

(14)

where Z ) H+, H2O, X-. The combination of the above processes yields

∆G°DM ) ∆G°VII + 2∆G°ip + x∆G°iw + 2∆G°ix

(15)

The free energy change for the palygorskite-associated indigo oxidation process in contact with aqueous electrolytes

{IN} + 2H+ (aq) + xH2O (aq) + 2e- f {H2IN‚xH2O} (16) Labeled as ∆G°DW, can be related with ∆G°DM through the relationship

∆G°DW ) ∆G°DM + 2∆G°pt + x∆G°wt

(17)

where ∆G°pt and ∆G°wt are respectively the free energy changes for proton and water transfer from water to MeCN

Z (aq) f Z (MeCN)

(18)

The values of such parameters can be evaluated to be +44 and +12 kJ/mol, respectively.55 The above thermochemical parameters can be related with the free energy changes for indigo and dehydroindigo attachment to palygorskite, ∆G°DP, ∆G°IP, corresponding to the processes

IN (c) + {} f {IN}

(19)

(20)

(21)

4594 J. Phys. Chem. C, Vol. 111, No. 12, 2007 of the reaction mechanism (via, for instance, XPS, TG-DSC data), one can suggest that the conditions of preparation of the pigment by the ancient Mayas involved a prolonged crushing and probably derived from a compromise between dehydroindigo stabilization (on increasing temperature) and destabilization of the palygorskite lattice (favored by the conversion of indigo into dehydroindigo). 4. Conclusions Upon immersion into different DMSO and MeCN electrolytes, Maya Blue synthetic specimens attached to graphite electrodes provide an indigo-centered voltammetric response essentially identical to that displayed by genuine Maya Blue samples. This response differs from that for indigo microparticles in contact with such electrolytes. Reduction of the indigo-palygorskite complex which constitutes Maya Blue can be described in terms of two successive cation-assisted one-electron-transfer processes involving presumably internal proton transfer from zeolitic and/or structural water molecules existing in the palygorskite matrix. Electrochemical oxidation of the indigo-palygorskite complex proceeds via two successive one-electron transfers coupled with protonation of zeolitic/structural water of the clay and the concomitant ingress of electrolyte charge-balancing anions in the inorganic host. Electrochemical data suggest that (i) both CdO and N-H indigo groups participate in hydrogen bond formation with palygorskite water molecules, (ii) the stoichiometry of the waterindigo complex is of two water molecules per indigo unit, (iii) dehydroindigo formation within the palygorskite framework involves a significant hydrogen bond rearranging, with separation of two water molecules per indigo unit, and produces a notable alteration in the indigo-palygorskite structure, and (iv) the thermodynamically favorable attachment of dehydroindigo to the palygorskite matrix probably results, apart from hydrogen bond rearrangement, from van der Waals interactions and/or interaction with cclay octahedral cations. Preparation of the pigment by the ancient Mayas involved probably a prolonged crushing, devoted to facilitate indigo insertion into the palygorskite lattice, accompanied by a moderate heating, this resulting from a compromise between the stabilization of dehydroindigo, required for color modulation from blue to turquoise and greenish hues, favored on increasing temperature, and stabilization of the palygorskite lattice, disfavored on increasing the conversion of indigo into dehydroindigo. Acknowledgment. The authors thank technical support from Prof. Ramoń Carrasco Vargas, Director of the Calakmul Archaeological Project, and Dr. Jose´ Luis Moya Lo´pez and Mr. Manuel Planes Insausti (Microscopy Service, Polytechnical University of Valencia). Financial support is gratefully acknowledged from the Generalitat Valenciana GVAE06/131, GV04B/197, and GV04B/441 I+D+I Projects and the MEC projects CTQ2005-09339-CO3-01 and 02, which are also supported with FEDER funds. Supporting Information Available: Voltammogram for Maya Blue (Figure S.1). Chronoamperometric curve for Maya Bule (Figure S.2). The fractal behavior for Maya Bule specimens (Figures S.3 and S.4). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Romero, P.; Sańchez, C. New J. Chem. 2005, 29, 57-58.

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