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The polyelectrolyte-MoO3 hybrids: Bottom up building of a layered anionic exchanger
Accepted Manuscript Title: The polyelectrolyte-MoO3 hybrids: bottom up building of a layered anionic exchanger Author: Fernando J. Quites Chiara Bisio Leonardo Marchese Heloise O. Pastore PII: DOI: Reference:
S0025-5408(13)00389-9 http://dx.doi.org/doi:10.1016/j.materresbull.2013.05.024 MRB 6709
To appear in:
MRB
Received date: Revised date: Accepted date:
8-2-2013 3-5-2013 7-5-2013
Please cite this article as: F.J. Quites, C. Bisio, L. Marchese, H.O. Pastore, The polyelectrolyte-MoO3 hybrids: bottom up building of a layered anionic exchanger, Materials Research Bulletin (2013), http://dx.doi.org/10.1016/j.materresbull.2013.05.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The polyelectrolyte-MoO3 hybrids: bottom up building of a layered anionic exchanger
a
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Fernando J. Quitesa, Chiara Bisiob,c, Leonardo Marcheseb, Heloise O. Pastorea,* Institute of Chemistry, University of Campinas, R. Monteiro Lobato, 270, 13084-971,
Cidade Universitária Zeferino Vaz, Campinas, SP, Brasil. [email protected] b
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tel.:0055-19-35213095, FAX: +55-19-35213023
Dipartimento di Scienze e Tecnologie Avanzate, Universitá del Piemonte Orientale, A.
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Avogadro, and Centro Interdisciplinare Nano-SiSTeMI, Viale Teresa Michel, 11, 15100, Alessandria, Italy
ISTM-CNR Istituto di Scienze e Tecnologie Molecolari, Via G. Venezian, 21 Milano
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c
(Italy)
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Keywords: intercalation; molybdenum (VI) oxide, lamellar materials, anion exchanger,
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metal oxide reduction
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Highlights A novel method, a hydrothermal treatment, intercalates PDDACl and PAHCl in MoO3
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Intercalation expands the interlayer space of MoO3 to ca the double of the initial one Intercalation causes the reduction of Mo6+ to Mo5+ as shown by UV-Vis spectroscopy
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A new anion exchanger was created.
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Abstract One-pot intercalation of poly(diallymethylammonium chloride) (PDDACl) and poly(allylamine hydrochloride) (PAHCl) between the layers of crystalline MoO3 was
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achieved for the first time using hydrothermal treatment. An experimental study monitoring the time of reaction and temperature of hydrothermal treatment showed that
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at 96 h and 150 ºC the organic-inorganic [PAHCl]0.18[PAH]0.38MoO3 and [PDDACl]0.26[PDDA]0.24MoO3 hybrid materials were produced. In these conditions the
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MoO3 host was able to expand its interlayer spacing from 0.64 to ca. 1.1 nm upon the intercalation of PDDACl and to approximately 2.2 nm with the use of PAHCl. The materials prepared with PAHCl exhibit a larger interlayer space and have a larger
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amount of intercalated polymer than those with PDDACl because of the PAHCl smaller charge density. The characterization of these hybrid materials using powder XRD,
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Raman, FTIR, fluorescence and UV-vis DRS spectroscopy, SEM and TG/DTG analysis supported the intercalation of the polymer between MoO3 layers. UV-Vis DRS analysis clearly shows the presence of the mixed-valence Mo5+/Mo6+ couple in the MoO3
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framework after the polyelectrolyte intercalation which indicates charge transfer from
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the polymer to the MoO3 host. FTIR study also revealed short distances structural disorder related to presence of Mo5+ centers after the intercalation. The hybrid materials
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produced, [PDDACl]0.26[PDDA]0.24MoO3 and [PAHCl]0.18[PAH]0.38MoO3 showed that approximately 53.0% and 32.0% of chloride ions are available for anionic exchange, respectively. These solids were employed as anion exchangers using cyanine dye; the emission band of the dye was monitored after the exchange as an indication of intercalation success. The cyanine dye containing hybrids presented an emission band centered at 602 nm even after extensive extraction with methanol.
Introduction In recent years, the intercalated organic–inorganic hybrid materials have attracted much attention because of their unique micro and nanostructure, and potentially useful properties in the areas of chemical sensors1, field-effect transistors2, batteries3 and catalysts4. For instance, interesting electrical and electrochemical properties of polymer-metal oxide have been extensively reported5. The opportunity to 3 Page 3 of 27
combine metal oxide as host and polymers as guest molecules at the nanometric level also appears as an attractive way to develop new organic-inorganic hybrids provided with properties that are inherent to both types of compounds,2,5. Inorganic layered structures play an important role in the formation of these materials, because they permit the intercalation of different chemical species into the host two-dimensional
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interlayer regions without disrupting the original structural organization. This characteristic of the layered hosts offers a possibility of designing functional materials
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by the direct connection among organic, biologic and inorganic worlds.
In this work, preparative routines were developed to confine polyelectrolyte
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species such as poly (diallylmethylammonium chloride) (PDDACl) (Figure 1a) and poly (allylamine hydrochloride) (PAHCl) (Figure 1b) between the layers of molybdenum trioxide (MoO3) that behaves as host. Molybdenum trioxide is especially
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attractive to be examined in intercalation reactions. It presents a lamellar structure built up by stacked neutral charged layers and it possesses a n-type semiconducting
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behavior6. The structure of MoO3 consists of vertex-sharing chains of distorted MoO6 octahedra, which share edges with two similar chains to form layers (Figure 1c). The two-dimensionally bonded double-octahedra oxide sheets are stacked in a layered
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arrangement and are held together by weak van der Waals forces6,7.
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n
l C n H3 +N
-
l C
+N
(b)
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(a)
(c)
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an
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(d)
d
(e)
Figure 1: Chemical structures of the monomer units of poly(diallyldimethylammonium
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chloride) (PDDACl) (a), poly(allylamine hydrochloride) (PAHCl) (b), representation of
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layered structure of MoO3(c), and (d) and (e) view of the 010 plane of MoO3 . Recently, it was shown that many organic compounds can be intercalated into
the interlayer spaces of MoO38,9,10. Itoh et al.1,11 reported the preparation and characterization of a poly(o-anisidine)–intercalated MoO3 hybrid powder and thin films, and the adsorption capacities of the thin film were determined using several volatile organic compounds (VOCs). Murugan12 also intercalated a conductor poly(3,4ethylenedioxythiophene) into MoO3 using in situ intercalative polymerization to produce a material which presented enhancement of electrochemical supercapacitor properties. In relation to these reactions, the direct intercalation of bulky species cannot be easily achieved on molybdenum trioxide, hence an alternative route is through methods based on the initial reduction of the layered structure by using Li or Na as reducing agents. In fact, the intercalation of polymers into MoO3 usually occurs by the initial step 5 Page 5 of 27
of LixMoO3 or NaxMoO3 bronze formation, constructed by partially reduced MoO3 and ion-exchangeable cations in the MoO3 interlayers. In a second step, an ion exchange procedure places various organic cations, monomers or even polymers in the interlayer spaces13. In order to overcome the need for two steps in the production of the polymer-
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MoO3 hybrid, in the first part of this study an organic-inorganic material is prepared
through the direct in situ intercalation of PDDACl e PAHCl polyelectrolytes into the
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van der Waals gap of MoO3 to produce the new organic-inorganic hybrid using only hydrothermal treatment. The intercalation simultaneously creates anion exchange
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centers (R4N+Cl-) in the interlamellar space of the oxide, i.e., the ion pairs of the polyelectrolytes chains. PDDACl and PAHCl polycations were chosen due to their ability to shield the low charge density of layers14 by coiling. In the second part of this
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work, the materials produced were employed as anionic host by reactions with the anionic cyanine dye.
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The assembled materials were characterized by a set of physical and chemical techniques; powder X-ray diffraction (XRD), chemical and thermal analyses, vibrational (FTIR and Raman) and electronic (UV-Vis DRS and fluorescence)
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spectroscopy, and scanning electron microscopy (SEM). Considering the structural,
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chemical, thermal and optical properties of the synthesized hybrid materials, they are
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good candidates to be investigated for several optical or optoelectronic applications.
2. Experimental Section
2.1 Materials
All chemicals were used as received. Molybdenum trioxide (MoO3), the host
material, was purchased from Merck, a 20.0 wt % aqueous solution of high molecular weight
(Mw
100.000-200.000)
poly(diallyldimethylammonium
chloride)
polyelectrolyte (PDDACl) was used while poly(allylamine hydrochloride) (PAHCl) polyelectrolyte (Mw 15.000) was supplied in the form of a powder obtained from Sigma-Aldrich, the anionic cyanine dye (Iris 3.5b) was supplied by Cyanine Technologies (Turin, Italy). The intercalation of PDDACl and PAHCl polyelectrolytes in the interlayer spaces of molybdenum trioxide (MoO3) to produce [PDDACl]y[PDDA]xMoO3 and 6 Page 6 of 27
[PAHCl]y[PAH]xMoO3 materials, was performed as follows: MoO3 (2.08 mmol) was added to an aqueous solution of PDDACl (10.5 mmol) or PAHCl (5.2 mmol) and stirred until a homogeneous suspension of the solid was obtained. The mixture was heated under hydrothermal conditions in a Teflon-lined stainless steel autoclave for different times and at different temperatures. These parameters were varied in order to produce
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the most extensive intercalation of polycations into lamella of MoO3. The powders obtained were filtered, washed with copious amounts of distilled water and ethanol and
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air dried at room temperature.
The encapsulation of cyanine dye was carried out by the following method: a
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methanol solution of the anionic cyanine dye (0.52 molL-1) was prepared and was added to 100 mg of [PAHCl]y[PAH]xMoO3 hybrid material. The resulting suspension
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was vigorously stirred for 48 h at room temperature, followed by filtration and extensively washing with methanol. After the exchange, Soxleth extraction was performed using methanol as the solvent for 6 h. This step is very important to wash
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away dye molecules that do not effectively interact with the hybrid material. The product obtained was dried at room temperature and, based in the results of CHN elemental analysis before and after the encapsulation of the dye, named In
this
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[PAHCy]z[PAHCl]y[PAH]xMoO3.
formula,
[PAHCy]
means
PAH
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polyelectrolyte interacting with cyanine dye. The same procedure was performed for the encapsulation of the anionic dye into [PDDACl]y[PDDA]xMoO3.
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In order to determine the amount of exchangeable chloride ions in [PDDACl]y
[PDDA]xMoO3 and [PAHCl]y[PAH]xMoO3 that is, the value of y in the formulas, Morh’s method was used 15. Briefly, a solution of KNO3 (5.2 mmolL-1) was stirred with [PDDACl]y[PDDA]xMoO3 and [PAHCl]y[PAH]xMoO3 hybrid materials for 12 h at 25 °C, and the supernatant was isolated and titrated with standard AgNO3 (5.1 mmolL-1).
2.2 Characterization
The obtained materials were characterized by X-ray diffraction (XDR) using a Shimadzu XRD7000 diffractometer (monochromated Cu Kα1, 40 mA, 30 kV) at room temperature over the range 1.5º ≤ 2 ≤ 50º. Fourier-transformed infrared spectra (FTIR) were recorded from 4000 to 400 cm-1 at 4 cm-1 resolution on a Nicolet 6700 spectrometer in pellets of samples dispersed in KBr at a concentration of approximately 0.5 wt. %. Raman scattering spectra were obtained at room temperature using a 7 Page 7 of 27
HORIBA Jobin Yvon confocal micro-Raman spectrometer with a 532 nm laser. The laser power is ~1mW and the spectra resolution was better than 2 cm-1. Carbon, nitrogen and hydrogen analyses were performed on solid samples in a Perkin-Elmer, model PE 2400, microelemental analyzer. Thermogravimetry and derivative thermogravimetry (TG/DTG) measurements were performed on Thermal Analyses equipment model
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5100-TA Instruments. Samples were heated from ambient temperature to 1000ºC at a
heating rate of 10ºC/min under oxygen (50 ml min-1). Diffuse reflectance UV-Vis-NIR
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spectra were measured with Perkin Elmer Lambda spectrophotometer of powder samples dispersed in BaSO4 at a concentration of 10.0 wt. %; the spectra were collected
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in the range of 200-1600 nm. Scanning electron microscopy (SEM) was performed in a Jeol 6360-LV, operating at 20 kV with the sample coated with carbon. Fluorescence spectra were collected on a Horada Spectrofluorometer using Xe lamp with source
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excitation at room temperature.
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3. Results and Discussion
Table 1 presents the composition and structural characteristics of hybrid
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materials produced using different intervals of intercalation time. The composition of
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the organic-inorganic material was determined by elemental analysis. In the molecular structure of PDDACl the polymeric repetitive unit is C8NH16Cl which affords a C/N
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molar ratio of 8 while for PAHCl the repeated unit is C3NH8Cl with a C/N molar ratio of 3 (values near can be seen in the Table 1 to C/N molar radio of the hybrid materials). It also can be seen in the Table 1, increasing the intercalation times the total amount of carbon increases.
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Table 1: Interlayer space, d(020), and elemental analyses of the hybrid materials prepared using hydrothermal treatment (150°C) at different intervals of intercalation
Samples
t/h
d(020)a/nm
C (%)
N(%)
H(%)
C/N
[PDDA]0.14MoO3
24
1.12
8.12
1.22
2.24
7.76
[PDDA]0.22MoO3
48
1.12
12.42
1.95
[PDDA]0.38MoO3
72
1.12
18.54
2.76
[PDDA]0.50MoO3
96
1.12
23.02
3.41
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time.
[PAH]0.22MoO3
24
-
5.08
[PAH]0.41MoO3
48
2.24
[PAH]0.45MoO3
72
[PAH]056MoO3
96
3.95
7.82
4.09
7.88
1.81
1.62
3.27
8.80
3.14
1.77
3.26
2.24
9.56
3.39
1.94
3.29
2.24
11.56
4.07
2.29
3.30
MoO3 d(020) = 0.64 nm.
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7.41
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a
3.36
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Figures 2 and 3 present the X-rays diffractograms of polyelectrolytes-MoO3 hybrids and of pristine layered MoO3 (curves a in both figures). The position of the 020
d
diffraction peak indicates an interlayer spacing (d020) of 0.64 nm, which is consistent with the reported values7,16. X-ray diffraction patterns of polymers-based PDDA-MoO3
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and PAH-MoO3 (see chemical composition in Table 1) synthesized by the direct
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intercalation of PDDACl and PAHCl polycations respectively, into MoO3 crystalline using hydrothermal treatment at increasing reaction times are shown in Figures 2 and 3 (b-e).
The intercalation of these particular polyelectrolytes in MoO3 is not found in the
literature, thus the indexation of hybrids XRD peaks is proposed here. In the present work, it is supposed that no modification of unit cell symmetry is caused by polyelectrolytes intercalation, therefore the (020) diffraction of hybrid is proposed to appear right below 10º 2θ along with the (020) of the pristine MoO3 until 72h of reaction (Figure 2, b-d). The final hybrid, prepared with 96h of reaction, Figure 2e, displays the characteristic broad (020) peak at around 8.0° 2θ for PDDA-MoO3.
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(110) (040) (021) (130) (101) (111) (041)
(060) (150)
40
(200) (210) (002)
(020)
30
(020)
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b c d e 10
20
50
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2 Degree
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Intensity [a.u.]
a
100 cps
Figure 2: Powder XRD pattern of (a) pristine crystalline MoO3 and of the PDDA-
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MoO3 (see Table 1 for the composition) hybrid synthesized at 150 ºC with different hydrothermal treatment times (b) 24 h, (c) 48 h, (d) 72 h, and (e) 96 h. The peaks of
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orthorhombic MoO3 were specified according to JCPDS database number 05-0508. For the product of PAHCl intercalation into MoO3, other aspects have to be
d
taken into account (Figure 3). Again one can suppose that the unit cell symmetry is the same for the MoO3 and for the hybrids as explained above. The result is again that the
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(020) diffraction is displaced to larger distances and the peaks corresponding to the crystalline layered MoO3 are seen in the diffractograms of the products up to a reaction
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time of 72h. After 48h of intercalation another feature appears: two signals are observed below 10º 2θ, the first one with lower intensity in relation to the second one. This could indicate that another layered phase is forming together with the desired hybrid. Other techniques did not show any extra phase besides the target PAH-MoO3 compound. Literature shows that this is not a unique behavior. In fact, potassium niobate displays the same type of diffractograms17 which is due in that case, to the presence of two interlamellar regions, differently spaced. The possibility of having the same type of lamella disposition is not ruled out for the materials prepared here, however no other indication of such organization is found in the present work.
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(002)
(210)
(200)
(060) (150)
(101) (111) (041)
(021) (130)
(110) (040)
(020)
c
20
30
2 Degree
40
50
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10
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d e
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(040)
b (020)
Intensity [a.u.]
a
100 cps
Figure 3: Powder XRD pattern of (a) crystalline MoO3 and of the PAH-MoO3 (see
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Table 1 for the composition) hybrid synthesized at 150 ºC with different hydrothermal treatment times (b) 24 h, (c) 48 h, (d) 72 h, and (e) 96 h. The peaks of orthorhombic
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MoO3 were specified according to JCPDS database number 05-0508. In the diffractogram of PAH-MoO3, Figure 3 curves b to e, a series of diffraction peaks at 4.12, 8.24 and 12.1° 2 can be observed. These peaks are evenly spaced by
d
approximately 4.0° and can be indexed to the 0l0 reflections (l = 2, 4, and 8) for a
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layered structure in excellent agreement with the X-ray diffraction of polymers-MoO3 nanocomposites reported by Murugan et al.12,18 In curves b, c and d of Figure 3, sharp
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peaks attributed to crystalline MoO3 also appear at wider angles in the diffractograms. The (020) reflection is diffuse in materials obtained at longer reaction times. For example, when the hydrothermal treatment time is short (24 h, Figures 2b and 3b) two phases are clear in the XRD patterns: the hybrid and unreacted crystalline MoO3; the color of the products is light blue during the course of reaction, which is characteristic of MoO3 containing hexavalent and pentavalent molybdenum (Mo6+ and Mo5+)19. After a longer hydrothermal treatment, 72 h, the intensity of the (020) line increases to a maximum corresponding to the organic-inorganic hybrids formation, and interestingly, the color of the product changes completely to dark blue. The d(020) interlayer spacing of MoO3 increases from 0.64 nm to ca. 1.1 nm with intercalation of PDDACl and to approximately 2.2 nm with intercalation of PAHCl polyelectrolytes. This result can be difficult to understand since the largest polyelectrolyte gives a hybrid where the interlayer space increased in a much larger extension than the one where the lower molecular weight polyelectrolyte was 11 Page 11 of 27
intercalated. One has to bear in mind that the chemical characteristics of each chain is different. The PDDACl has its nitrogen atoms completely quaternized, independent of the pH. Contrarily, in the PAHCl chain, all N atoms are in NH3+ groups, that can be deprotonated depending on the pH. In the FTIR study (see below), NH3+ and NH2 groups were observed, indicating that H-bonds for instance, may be occurring in the
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PAHCl chain, inside the interlayer space. In that case, it might be energetically more favorable to keep chains elongated in order to expose as much NH groups as possible.
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In the case of PDDACl, such interactions are not observed and thus coiled
conformations are possible. That may be the difference responsible for such a variation
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of interlayer spaces.
Figures 4 and 5 (curves b-d) present the X-rays diffraction patterns of the compounds intercalated with PDDACl and PAHCl polyelectrolytes, respectively, by
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using different temperatures of hydrothermal treatment (in the range of 100-190 ºC). It can be seen that when the hybrid materials were produced using a temperature of 100 ºC
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(Figure 4b and 5b) bulk MoO3 was observed in the final material, indicating that the reaction did not occur in a large extent. Higher temperatures, above 150 ºC, cause the peaks related to crystalline MoO3 to disappear suggesting that the whole material was
d
intercalated. Therefore, the optimum temperature of hydrothermal treatment to produce
b c d
(021)
(200)
(210) (002)
(150)
(060)
(111) (041)
(110) (040)
(020)
a
50 cps
(020)
Intensity [a.u.]
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be 150 ºC.
(130) (101)
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the organic-inorganic hybrid using PDDACl and PAHCl polycations was determined to
e
10
20
30
40
50
2 Degree
Figure 4: Powder XRD pattern of MoO3 pristine (a) and to the PDDA-MoO3 hybrid synthesized by a direct hydrothermal treatment at different temperatures (b) 100 ºC, (c) 150 ºC , (d) 170 ºC and (e) 190oC. The time of intercalation used was of 96 h. The 12 Page 12 of 27
peaks of orthorhombic MoO3 were specified according to JCPDS database number 05-
b
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c
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(002)
(200)
(210)
(060) (150)
(130) (101) (111) (041)
(110) (040)
(020)
(040)
a
100 cps
(020)
Intensity [a.u.]
(021)
0508.
10
20
30
40
50
an
2 Degree
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d e
Figure 5: Powder XRD pattern of MoO3 pristine (a) and of the PAH-MoO3 hybrid synthesized by a direct hydrothermal treatment at different temperature such as (b) 100
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ºC, (c) 150 ºC , (d) 170 ºC and (e) 190oC. The time of intercalation used was of 96 h. The peaks of orthorhombic MoO3 were specified according to JCPDS database number
d
05-0508.
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Thus, with the use of hydrothermal treatment at 150 ºC for 96 h and in the presence of PDDACl or PAHCl the hybrids [PDDA]0.50MoO3 and [PAH]0.56MoO3 were
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produced. Some cationic nitrogen atom in the polyelectrolytes in these solids are counterbalancing the charge in the structure of the oxide and serving as ion pair with the chloride ion. The amount of exchangeable Cl- ions was measured by using Mohr’s method15. To [PDDA]0.50MoO3 and [PAH]0.56MoO3 compounds there are approximately 53.0 and 32.0 % of exchangeable chloride ions, respectively, thus the correct chemical formula
for
the
hybrids
are
[PDDACl]0.26[PDDA]0.24MoO3
and
[PAHCl]0.18[PAH]0.38MoO3, clearly describing the amount of exchangeable anionic ions. The PAH-based compound contains less exchangeable sites in the interlayer space due to the presence of neutral (-NH2) amine groups and Cl- ions are associated to positively charged (-NH3+) groups and generate –NH3+Cl- ion pair. Several materials were prepared under the determined optimum conditions, and their characterization follows.
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Thermal analyses The thermal stability of these materials in oxidizing atmosphere was examined by thermal analyses experiments. The thermogravimetry and derivate thermogravimetry curves (TG/DTG) of the pristine MoO3 (Figure 6) display only one stage of weight loss
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at the temperature range of 750-800 ºC, which is related with MoO3 sublimation20. For the hybrid materials three distinct main stages were observed in as shown in Figure 6.
The first step, up to 120 ºC, corresponds to the removal of the reversibly bound water
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molecules, whereas the second step until approximately 400 ºC corresponds to the loss
of more strongly bound water between the layers and the decomposition/oxidation of continuous
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polyelectrolyte molecules housed into the MoO3 interlayer space. This is followed by a weight loss up to 600 ºC which can be attributed to the
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combustion/oxidation of the organic polymer component and its fragments, in agreement with the signals in the DTG curves. The decomposition/oxidation of polyelectrolytes is at a substantially lower decomposition temperature than the one of
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the polycations themselves, i. e., around 550 °C, suggesting that the close proximity of MoO3 favors catalytic oxidation process of the hybrids that may involve oxygen from
d
the lattice21. Another mass loss in the region of 700-800 ºC can be attributed to the sublimation of orthorhombic MoO3 in the matrix of the inorganic-organic compounds.
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Sublimation of this phase was evidenced also from the DTG curve which showed a relatively sharp intense peak at 750-800 ºC.
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The thermal results suggest that the polycations are intercalated into interlayer
space of the inorganic oxide.
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100
(A)
60
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Mass [%]
80
40 MoO3 20
cr
[PDDACl]0.26[PDDA]0.24MoO3
[PAHCl]0.18[PAH]0.38MoO3 200
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0 400
600
800
Temperature [ºC] 2.5
2.0
(B)
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[PAHCl]0.18[PAH]0.38MoO3 1.5
1.0
d
DTG [a.u.]
an
MoO3
[PDDACl]0.26[PDDA]0.24MoO3
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0.5
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0.0
200
400
600
800
Temperature [ºC]
Figure 6: Thermogravimetry and its derivative curves (TG/DTG) for MoO3 pristine (full
line)
and
for
[PAHCl]0.18[PAH]0.38MoO3
(dotted
line)
and
[PDDACl]0.26[PDDA]0.24MoO3 (dashed line) hybrids materials under oxidizing atmosphere.
SEM
images
of
the
[PAHCl]0.18[PAH]0.38MoO3
and
[PDDACl]0.26[PDDA]0.24MoO3 hybrid materials synthesized by the hydrothermal treatment and of crystalline MoO3 are in Figure 7(a-c). It is apparent that the hybrid materials (Figure 7b-c) form a continuous and relatively homogeneous matrix with a stratified morphology22. The incorporation of the polycations into the MoO3 host is 15 Page 15 of 27
accompanied by morphological changes that are supported by the results from the
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cr
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diffraction studies.
d
Figure 7: Scanning electron micrographs of the molybdenum trioxide matrix (scale bar:
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1.0 μm) (a), [PAHCl]0.18[PAH]0.38MoO3 hybrid material (scale bar: 1.0 μm), (b) and
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[PDDACl]0.26[PDDA]0.24MoO3 hybrid material (scale bar: 0.5 μm) (c).
Raman Spectroscopy
Figure 8a shows a typical Raman spectrum of the MoO3 powders where fourteen
peaks can be observed. The sharpness of the peaks indicates that the corresponding vibrational modes are due to a highly ordered structure, where the position and relative intensity of these Raman peaks are in good agreement with that reported in the literature for MoO323,24 . Table 2 summarizes these frequencies. In particular, the Raman peak at 996 cm−1 is assigned to the stretching mode of the Mo=O group bearing an unshared oxygen. The peak at 819 cm−1 is assigned to the Mo-O-Mo stretching mode with a bridging oxygen atom in corner-sharing between two MoO6 octahedra. The peak at 666 cm−1 is assigned to the Mo-O3 stretching mode bearing triply coordinated oxygen which results from edge-shared oxygen atoms in common to three octahedra24.
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The Raman spectrum of polyelectrolyte-intercalated materials (Figure 8b-c) presents peaks that are wider and red-shifted in relation to pure MoO3. These two factors can be associated with the intercalation of polycations in the MoO3 matrix and are in line with a decreased crystallinity of the hybrids, as verified by XRD and SEM analyses. The downward shifts of Raman peaks can be due to the interactions of
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PDDACl or PAHCl chains with the MoO3 matrix.
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Intensity [a.u.]
500 cps
c b
M
a
1800 1600 1400 1200 1000 800 600 400 200
d
-1 Wavenumber [cm ]
te
Figure 8: Raman spectra of MoO3 (a), organic-inorganic [PAHCl]0.18[PAH]0.38MoO3
Ac ce p
(b) and [PDDACl]0.26[PDDA]0.24MoO3 (c) hybrids in the low and high energy regions.
17 Page 17 of 27
Table 2: Experimental frequencies (cm-1) and assignments of the peaks in Raman spectra of MoO3 bulk and organic-inorganic hybrids. Wavenumber (cm-1)
Vibrations
PDDACl]0.26[PDDA]0.24MoO3
[PAHCl]0.18[PAH]0.38MoO3
(Mo=O)
996
991
993
(O-Mo2)
819
816
818
(O-Mo3)
666
661
663
(O-Mo3)
476
465
δ(Mo=O)
378
374
δ(O-Mo3)
337
335
δ(Mo=O)
284
281
246
236
δ(O-Mo2)
198
193
Lattice
159
vibrations
128
cr us
282
238
146
149
122
125
d
194
113
111
Ac ce p
Infrared spectroscopy
336
te
115
376
M
δ(O-Mo2)
465
an
292
ip t
MoO3
FTIR spectra of MoO3 before and after the intercalation of polyelectrolytes are
shown in Figure 9. The spectrum of pure MoO3 is characterized by the presence of a broad band in the 3800-3200 cm-1 range, attributable to the stretching mode of water molecules physisorbed on MoO3 surface, the bending mode falling at ca. 1632 cm-1 (Fig. 9A, a). At the low frequency side, the spectrum of MoO3 (Fig. 9B, a) exhibits three main absorptions in the 1250-400 cm-1 range with peaks at 993 (intense, sharp and slightly asymmetric towards low frequency side), 875 (very intense and broad), 820 (very weak) and at ca. 605 cm-1 (very intense, broad and with a shoulder at ca. 650 cm-1). In addition, two weak bands are observed at 512 and 488 cm-1.
18 Page 18 of 27
3050
993 1468
820
2925
a 1612
945 912 845
b
c 1505
3500
3000
512 488
2500
2000
1500
893
c
935
1200
1000
800
600
400
Wavenumber [cm-1]
us
Wavenumber [cm-1]
ip t
b
B
0.2
1632
2920 2960 2860
605
875
A
0.4
cr
Absorbance [a.u.]
a
Figure 9: FTIR of the standard MoO3 matrix (a), [PDDACl]0.26[PDDA]0.24MoO3 (b)
an
and [PAHCl]0.18[PAH]0.38MoO3 hybrid compounds (c).
There are several works in the literature reporting the IR bands assignments for
M
MoO325,26. Among them, several discrepancies can be noticed especially in relation to the longitudinal optical and transverse optical (LO-TO) splitting effects which depend on the size and particle morphology of the crystallite of samples in powder forms27,28.
d
In the present study, we are based on the work of Knozinger et al.29 and Eda30,
te
where the band at 993 cm-1 is assigned to the stretching mode of Mo=O groups (B2u or B3u symmetry, Longitudinal Optical component, LO). The broad band centered at 875
Ac ce p
cm-1 is originated by stretching vibrations of Mo-O-Mo groups of MoO3; in particular, the main component at 875 cm-1 is due to the B3u stretching mode (Transversal Optical component, TO), whereas the weak band at 820 cm-1 is due to the Mo-O-Mo B2u stretching mode. Finally, the complex band at 605 cm-1 should be associated to the B1u stretching mode of Mo-O-Mo species, whereas the shoulder at ca. 488 cm-1 is generated by the B2u stretching mode of the same oscillators. The IR spectra of hybrid materials (Figure 9, b-c) showed that the intercalation
leads to significant modifications of the IR spectrum of MoO3 sample. The IR spectrum of PDDACl intercalated material is dominated in the high frequency region (Fig. 9 A, b) by a broad band in the 3800-3000 cm-1, accompanied by absorptions at ca. 2960, 2920 and 2855 cm-1. In addition, two bands at 1632 and 1468 cm-1 (asymmetric towards the low frequency side) are also observed. The sample contains an additional amount of adsorbed water with respect to MoO3 parent sample and it is characterized by adsorptions typical of PDDACl species, thus confirming that the intercalation process 19 Page 19 of 27
was successfully attained. In fact, bands at 2960 and 2855 cm-1 are associated to the asymmetric and symmetric modes of CH3 species, respectively, whereas the peak at 2920 cm-1 is due to the asymmetric stretching mode of methylene groups of PDDACl species, the symmetric vibration falling at ca. 2850 cm-1, masked by the CH3 symmetric stretching. The band at 1468 cm-1 is due to deformation vibrations of CH2 species of
ip t
PDDA chains.
The intercalation procedure significantly affects the phonon vibrations of MoO3
cr
lattice (Fig. 9B, b). The PDDA-exchanged materials indeed show weak absorptions in
the 1000-500 cm-1 range, in which peaks at 945, 912 and 845 cm-1 due to inorganic
us
MoO3 matrix are found. The observed differences should be taken as an indication of the fact that intercalation procedure modifies the particle size and organization of MoO3 layers, thus resulting in a deep modification of LO and TO vibrational modes of the
an
MoO3 matrix.
The spectrum of PAH-exchanged material at high frequency (Fig. 9A, c) is
M
characterized by a broad absorption in the 3650-2750 cm-1 range, with maxima at ca. 3430, 3050, 2925 and 2860 (very weak and partially masked by the band at 2925 cm-1) cm-1 and sharp bands at 1612 and 1505 cm-1 (with shoulder at 1468 cm-1). The bands at
d
2925 and 2860 cm-1 are due to asymmetric and symmetric modes, respectively, of CH2
te
groups of PAHCl, whereas the broad absorption in the 3650-2750 cm-1 range is due to polyelectrolyte in interaction with the MoO3 structure. The presence of protonated NH3+
Ac ce p
groups in PAHCl species is witnessed by the bands due to the asymmetric and symmetric bending modes of these species, that can be observed at 1612 and 1505 cm-1, respectively. The stretching vibrations of NH3+/NH2 groups are probably hidden in the broad component present in the 3650-2750 cm-1 frequency range. The comparison with the spectrum in Figure 9A curve b shows the absence of these vibrations and confirms the assignment.
At the low frequency range (Fig. 9B, c) the spectrum is characterized by the
presence of two broad bands in the 1000-800 (with evident maxima at 935 and 893 cm-1) and 750-500 cm-1 frequency ranges. The first absorption is due to MoO3 inorganic matrix, whereas bands typical of PAHCl polyelectrolyte31 are present in the 750-500 cm-1 range. Also in this case, FTIR spectroscopy suggests that PAH intercalation has a clear effect on layer organization of MoO3 matrix.
20 Page 20 of 27
Diffuse reflectance spectroscopy The UV-Vis diffuse reflectance spectra of [PDDACl]0.26[PDDA]0.24MoO3, [PAHCl]0.18[PAH]0.38MoO3 and MoO3 parent material are shown in Figure 10. The DRUV-Vis spectrum of MoO3 is characterized by the presence of a band in the 200-360
ip t
nm range with a maximum at 250 nm and an evident shoulder at ca. 290 nm which can
be associated to charge-transfer transitions of polymolybdate-like structure containing
cr
Mo6+ ions in octahedral configuration32,33. In addition, the band in the 300-400 nm range should be due to the presence of Mo6+(Oh) ions in crystalline arrangement34.
us
The DR-UV-Vis spectra of both hybrid samples (Fig. 10, curves b-c) are characterized by similar absorptions: a band at 250 nm with a clear shoulder at 330 nm (associated to the presence of polymolybdate-like species) and complex absorption
an
pattern in the 600-1600 nm range in which bands at ca. 600, 800, 930, 1140, 1270 and 1500 nm are visible. Absorption bands at 600 and 930 nm are associated to
M
Mo5+Mo6+ intervalence charge transfer (polaronic) transitions35,36 whereas the bands at 800, 1140, 1270 and 1500 nm should be due to d-d transitions of Mo5+ ions in distorted polyhedron sites of MoO3 solid. The presence of Mo5+ ions in hybrid materials ions.
Kubelka-Munk
Ac ce p
1,0
te
d
suggests that the intercalation procedure leads to a partial reduction of Mo6+ to Mo5+
0,8
250
c
0,6 0,4
b
0,2
a 0,0 200
400
600
800
1000
1200
1400
1600
Wavelength [nm] Figure 10: UV-Vis diffuse reflectance spectra of MoO3 (a), organic-inorganic [PAHCl]0.18[PAH]0.38MoO3 (b) and [PDDACl]0.26[PDDA]0.24MoO3(c) hybrid materials. 21 Page 21 of 27
The organic-inorganic hybrids as anionic exchangers In this section, the results obtained after the occlusion of the cyanine anionic dye in
the
interlayer
space
of
hybrid
[PAHCl]0.18[PAH]0.38MoO3
and
[PDDACl]0.26[PDDA]0.24MoO3 compounds will be presented. The materials exchanged
ip t
with dye were characterized by elemental analysis (CHN), UV-Vis diffuse reflectance and fluorescence spectroscopies.
cr
Table 3 shows the quantities of dye ion exchanged in the hybrids after Soxhlet
us
extraction.
Table 3: Chemical formulas of hybrid materials after the occlusion of anionic dye.
an
[PDDACy]0.06[PDDACl]0.21[PDDA]0.24MoO3 [PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3
M
The optical properties of the dye were studied by diffuse reflectance spectroscopy technique in the UV-Vis region. The spectrum of cyanine dye dissolved in methanol (not shown) presented absorption at 519 and 552 nm, related with the dye
d
chromophor37,38. The same bands are red-shifted in the spectra of hybrid materials (not
te
shown) due the formation of J-type aggregated of dye occluded in the organic-inorganic hybrids or due an environment new that the dye is confined38. This red-shift is in good
Ac ce p
agreement with results obtained by Ogawa39 which intercalated cationic dyes in the interlamelar space of clays.
Figure 11 shows the results of luminescence properties of dye confined in the
[PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3 hybrid material, compared with the cyanine in methanol solution. The spectrum of the cyanine dye in solution (full line) shows an emission at 596 nm after excitation at 560 nm and another one at ca. 650 nm (sh) that can be related at J-type aggregates of cyanine dye. The spectrum of the cyanine-hybrid material before the extraction with methanol using excitation wavelength of 560 nm is on Figure 11 (dotted line). An emission band at 614 nm can be observed in this spectra; it is red-shifted by 18 nm in comparison to cyanine in methanol solution. This red-shift can be associated with the formation of the J-type dye aggregates occluded in the hybrid40, or to the confinement of the dye into the oxide matrix, with different polarity with
respect
to
the
solution.
After
the
extraction
with
methanol,
the
[PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3 hybrid (Figure 11, dashed line) showed an 22 Page 22 of 27
emission band at 602 nm that is red-shifted by 6 nm of pure dye and blue-shifted by 12 nm in relation to the hybrid before the extraction of methanol. Therefore, the red-shift (from 596 to 602 nm) can be associated with the change in polarity of media due to the new environment of dye confined between the lamella of MoO3. After the extraction with methanol is observed also that the relative intensity of emission band at ca. 650 nm
ip t
of hybrid (dashed line, Figure 11) decrease in relation at cyanine dye solution (full line,
Figure 11) probably associated with the disappearance of some aggregated dye. This discussed
above
was
also
for
the
M
te
d
614
Intensity [a.u.]
an
602
596
[PDDACy]0.06[PDDACl]0.21[PDDA]0.24MoO3 material.
verified
us
behavior
cr
behavior is in excellent agreement with results published recently38,38. The same
Ac ce p
580 600 620 640 660 680 700 720 740
Wavelength [nm]
Figure 11: Emission spectra at room temperature (excitation wavelength of 560 nm) of cyanine dye in methanol solution (5x10-5 mol L-1) (full line); and intercalated in [PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3 hybrid material before (dotted line) and after the extraction (dashed line).
Conclusions This work showed a novel method of introduction of PDDACl and PAHCl polymers between the layers of MoO3 by using only hydrothermal treatment as the intercalation technique. The analysis of hydrothermal treatment duration and of temperature of intercalation with both polyelectrolytes revealed that the PDDACl and 23 Page 23 of 27
PAHCl chains are inserted in the interlayer spacing of molybdenum trioxide at the optimum temperature of 150oC for 96 h of reaction. Upon intercalation, the interlayer space of original MoO3 expanded from 0.64 nm to ca. 1.1 nm with PDDACl and approximately 2.2 nm with PAHCl intercalation. The hybrid based on interlayer PAHCl contains neutral (-NH2) amine groups and positively charged (-NH3+) groups that are
ip t
associated with Cl- (to generate –NH3+Cl- ion pair) and thus, when intercalated in MoO3, it created a lower amount of exchangeable ion pairs in the interlayer spaces than
cr
the PDDA-based hybrid since the last polyelectrolyte contains only quaternary ammonium groups (-NR3+Cl-).
us
The intercalation process occurs with the reduction of some Mo6+ centers to Mo5+ as shown by UV-Vis spectroscopy. In fact, the hydrothermal process is advantageous in relation to the methods reported in the literature, since it does not need
an
a reducing initial step before polyelectrolytes ion exchange. The hybrids display clear anion exchange capacities, shown in this work by the insertion of an anionic cyanine
M
dye in the interlamellar spaces, the monitoring of absorption and emission bands of the
Acknowledgements:
d
dye confirmed its presence in the interlayer space of the hybrids.
te
The authors acknowledge Prof. M. Cossi and Dr. A. Fraccarollo for fruitful discussions. The authors acknowledge the Fundação de Amparo à Pesquisa no Estado
Ac ce p
de São Paulo (FAPESP), the Piedmont Region (Italy) [NANOLED project (Novel Nanostructured Materials for Light Emitting Devices and Application to Automotive Displays), CIPE 2006] and the European Community [INNOVASOL project (Innovative Materials for Future Generation Excitonic Solar Cells), of the FP7] for the financial support to this work and to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the fellowships.
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ip t
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A. V. Murugan, J. Power Sources 2006, 159, 312-318.
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G. D. Christian, Analytical Chemistry 2003, Sixth Edition, John Wiley and Sons.
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(a) X. M. Wei, H. C. Zeng, J. Phys. Chem. B 2003, 107, 2619-2624. (b) S. Deki, A. B. Béléké, Y. Kotani, M. Mizuhata, J. Solid State Chem. 2009, 182,
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29
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34
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19, 1727-1734. 35
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Ghiotti, J. Solid State Chem., 2009, 182, 3342-3352. 36
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37
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38
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M. Ogawa, Chem. Mater. 1996, 8, 1347-1349.
40
S. Kirstein, S. Daehne, Int. J. Photoenergy 2006, 2006, 1-21.
26 Page 26 of 27
i cr 596
602 614
ed ce pt
Intensity [a.u.]
(020)
hν
MoO3
(040)
[PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3 hybrid
10
20
30
2 Degree
Ac
(020)
Intensity [a.u.]
100 cps
M an
us
Graphical abstract
40
50
580 600 620 640 660 680 700 720 740
Wavelength [nm]
Page 27 of 27
S0025-5408(13)00389-9 http://dx.doi.org/doi:10.1016/j.materresbull.2013.05.024 MRB 6709
To appear in:
MRB
Received date: Revised date: Accepted date:
8-2-2013 3-5-2013 7-5-2013
Please cite this article as: F.J. Quites, C. Bisio, L. Marchese, H.O. Pastore, The polyelectrolyte-MoO3 hybrids: bottom up building of a layered anionic exchanger, Materials Research Bulletin (2013), http://dx.doi.org/10.1016/j.materresbull.2013.05.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The polyelectrolyte-MoO3 hybrids: bottom up building of a layered anionic exchanger
a
ip t
Fernando J. Quitesa, Chiara Bisiob,c, Leonardo Marcheseb, Heloise O. Pastorea,* Institute of Chemistry, University of Campinas, R. Monteiro Lobato, 270, 13084-971,
Cidade Universitária Zeferino Vaz, Campinas, SP, Brasil. [email protected] b
cr
tel.:0055-19-35213095, FAX: +55-19-35213023
Dipartimento di Scienze e Tecnologie Avanzate, Universitá del Piemonte Orientale, A.
us
Avogadro, and Centro Interdisciplinare Nano-SiSTeMI, Viale Teresa Michel, 11, 15100, Alessandria, Italy
ISTM-CNR Istituto di Scienze e Tecnologie Molecolari, Via G. Venezian, 21 Milano
an
c
(Italy)
M
Keywords: intercalation; molybdenum (VI) oxide, lamellar materials, anion exchanger,
Ac ce p
te
d
metal oxide reduction
1 Page 1 of 27
Highlights A novel method, a hydrothermal treatment, intercalates PDDACl and PAHCl in MoO3
ip t
Intercalation expands the interlayer space of MoO3 to ca the double of the initial one Intercalation causes the reduction of Mo6+ to Mo5+ as shown by UV-Vis spectroscopy
Ac ce p
te
d
M
an
us
cr
A new anion exchanger was created.
2 Page 2 of 27
Abstract One-pot intercalation of poly(diallymethylammonium chloride) (PDDACl) and poly(allylamine hydrochloride) (PAHCl) between the layers of crystalline MoO3 was
ip t
achieved for the first time using hydrothermal treatment. An experimental study monitoring the time of reaction and temperature of hydrothermal treatment showed that
cr
at 96 h and 150 ºC the organic-inorganic [PAHCl]0.18[PAH]0.38MoO3 and [PDDACl]0.26[PDDA]0.24MoO3 hybrid materials were produced. In these conditions the
us
MoO3 host was able to expand its interlayer spacing from 0.64 to ca. 1.1 nm upon the intercalation of PDDACl and to approximately 2.2 nm with the use of PAHCl. The materials prepared with PAHCl exhibit a larger interlayer space and have a larger
an
amount of intercalated polymer than those with PDDACl because of the PAHCl smaller charge density. The characterization of these hybrid materials using powder XRD,
M
Raman, FTIR, fluorescence and UV-vis DRS spectroscopy, SEM and TG/DTG analysis supported the intercalation of the polymer between MoO3 layers. UV-Vis DRS analysis clearly shows the presence of the mixed-valence Mo5+/Mo6+ couple in the MoO3
d
framework after the polyelectrolyte intercalation which indicates charge transfer from
te
the polymer to the MoO3 host. FTIR study also revealed short distances structural disorder related to presence of Mo5+ centers after the intercalation. The hybrid materials
Ac ce p
produced, [PDDACl]0.26[PDDA]0.24MoO3 and [PAHCl]0.18[PAH]0.38MoO3 showed that approximately 53.0% and 32.0% of chloride ions are available for anionic exchange, respectively. These solids were employed as anion exchangers using cyanine dye; the emission band of the dye was monitored after the exchange as an indication of intercalation success. The cyanine dye containing hybrids presented an emission band centered at 602 nm even after extensive extraction with methanol.
Introduction In recent years, the intercalated organic–inorganic hybrid materials have attracted much attention because of their unique micro and nanostructure, and potentially useful properties in the areas of chemical sensors1, field-effect transistors2, batteries3 and catalysts4. For instance, interesting electrical and electrochemical properties of polymer-metal oxide have been extensively reported5. The opportunity to 3 Page 3 of 27
combine metal oxide as host and polymers as guest molecules at the nanometric level also appears as an attractive way to develop new organic-inorganic hybrids provided with properties that are inherent to both types of compounds,2,5. Inorganic layered structures play an important role in the formation of these materials, because they permit the intercalation of different chemical species into the host two-dimensional
ip t
interlayer regions without disrupting the original structural organization. This characteristic of the layered hosts offers a possibility of designing functional materials
cr
by the direct connection among organic, biologic and inorganic worlds.
In this work, preparative routines were developed to confine polyelectrolyte
us
species such as poly (diallylmethylammonium chloride) (PDDACl) (Figure 1a) and poly (allylamine hydrochloride) (PAHCl) (Figure 1b) between the layers of molybdenum trioxide (MoO3) that behaves as host. Molybdenum trioxide is especially
an
attractive to be examined in intercalation reactions. It presents a lamellar structure built up by stacked neutral charged layers and it possesses a n-type semiconducting
M
behavior6. The structure of MoO3 consists of vertex-sharing chains of distorted MoO6 octahedra, which share edges with two similar chains to form layers (Figure 1c). The two-dimensionally bonded double-octahedra oxide sheets are stacked in a layered
Ac ce p
te
d
arrangement and are held together by weak van der Waals forces6,7.
4 Page 4 of 27
n
l C n H3 +N
-
l C
+N
(b)
cr
ip t
(a)
(c)
M
an
us
(d)
d
(e)
Figure 1: Chemical structures of the monomer units of poly(diallyldimethylammonium
te
chloride) (PDDACl) (a), poly(allylamine hydrochloride) (PAHCl) (b), representation of
Ac ce p
layered structure of MoO3(c), and (d) and (e) view of the 010 plane of MoO3 . Recently, it was shown that many organic compounds can be intercalated into
the interlayer spaces of MoO38,9,10. Itoh et al.1,11 reported the preparation and characterization of a poly(o-anisidine)–intercalated MoO3 hybrid powder and thin films, and the adsorption capacities of the thin film were determined using several volatile organic compounds (VOCs). Murugan12 also intercalated a conductor poly(3,4ethylenedioxythiophene) into MoO3 using in situ intercalative polymerization to produce a material which presented enhancement of electrochemical supercapacitor properties. In relation to these reactions, the direct intercalation of bulky species cannot be easily achieved on molybdenum trioxide, hence an alternative route is through methods based on the initial reduction of the layered structure by using Li or Na as reducing agents. In fact, the intercalation of polymers into MoO3 usually occurs by the initial step 5 Page 5 of 27
of LixMoO3 or NaxMoO3 bronze formation, constructed by partially reduced MoO3 and ion-exchangeable cations in the MoO3 interlayers. In a second step, an ion exchange procedure places various organic cations, monomers or even polymers in the interlayer spaces13. In order to overcome the need for two steps in the production of the polymer-
ip t
MoO3 hybrid, in the first part of this study an organic-inorganic material is prepared
through the direct in situ intercalation of PDDACl e PAHCl polyelectrolytes into the
cr
van der Waals gap of MoO3 to produce the new organic-inorganic hybrid using only hydrothermal treatment. The intercalation simultaneously creates anion exchange
us
centers (R4N+Cl-) in the interlamellar space of the oxide, i.e., the ion pairs of the polyelectrolytes chains. PDDACl and PAHCl polycations were chosen due to their ability to shield the low charge density of layers14 by coiling. In the second part of this
an
work, the materials produced were employed as anionic host by reactions with the anionic cyanine dye.
M
The assembled materials were characterized by a set of physical and chemical techniques; powder X-ray diffraction (XRD), chemical and thermal analyses, vibrational (FTIR and Raman) and electronic (UV-Vis DRS and fluorescence)
d
spectroscopy, and scanning electron microscopy (SEM). Considering the structural,
te
chemical, thermal and optical properties of the synthesized hybrid materials, they are
Ac ce p
good candidates to be investigated for several optical or optoelectronic applications.
2. Experimental Section
2.1 Materials
All chemicals were used as received. Molybdenum trioxide (MoO3), the host
material, was purchased from Merck, a 20.0 wt % aqueous solution of high molecular weight
(Mw
100.000-200.000)
poly(diallyldimethylammonium
chloride)
polyelectrolyte (PDDACl) was used while poly(allylamine hydrochloride) (PAHCl) polyelectrolyte (Mw 15.000) was supplied in the form of a powder obtained from Sigma-Aldrich, the anionic cyanine dye (Iris 3.5b) was supplied by Cyanine Technologies (Turin, Italy). The intercalation of PDDACl and PAHCl polyelectrolytes in the interlayer spaces of molybdenum trioxide (MoO3) to produce [PDDACl]y[PDDA]xMoO3 and 6 Page 6 of 27
[PAHCl]y[PAH]xMoO3 materials, was performed as follows: MoO3 (2.08 mmol) was added to an aqueous solution of PDDACl (10.5 mmol) or PAHCl (5.2 mmol) and stirred until a homogeneous suspension of the solid was obtained. The mixture was heated under hydrothermal conditions in a Teflon-lined stainless steel autoclave for different times and at different temperatures. These parameters were varied in order to produce
ip t
the most extensive intercalation of polycations into lamella of MoO3. The powders obtained were filtered, washed with copious amounts of distilled water and ethanol and
cr
air dried at room temperature.
The encapsulation of cyanine dye was carried out by the following method: a
us
methanol solution of the anionic cyanine dye (0.52 molL-1) was prepared and was added to 100 mg of [PAHCl]y[PAH]xMoO3 hybrid material. The resulting suspension
an
was vigorously stirred for 48 h at room temperature, followed by filtration and extensively washing with methanol. After the exchange, Soxleth extraction was performed using methanol as the solvent for 6 h. This step is very important to wash
M
away dye molecules that do not effectively interact with the hybrid material. The product obtained was dried at room temperature and, based in the results of CHN elemental analysis before and after the encapsulation of the dye, named In
this
d
[PAHCy]z[PAHCl]y[PAH]xMoO3.
formula,
[PAHCy]
means
PAH
te
polyelectrolyte interacting with cyanine dye. The same procedure was performed for the encapsulation of the anionic dye into [PDDACl]y[PDDA]xMoO3.
Ac ce p
In order to determine the amount of exchangeable chloride ions in [PDDACl]y
[PDDA]xMoO3 and [PAHCl]y[PAH]xMoO3 that is, the value of y in the formulas, Morh’s method was used 15. Briefly, a solution of KNO3 (5.2 mmolL-1) was stirred with [PDDACl]y[PDDA]xMoO3 and [PAHCl]y[PAH]xMoO3 hybrid materials for 12 h at 25 °C, and the supernatant was isolated and titrated with standard AgNO3 (5.1 mmolL-1).
2.2 Characterization
The obtained materials were characterized by X-ray diffraction (XDR) using a Shimadzu XRD7000 diffractometer (monochromated Cu Kα1, 40 mA, 30 kV) at room temperature over the range 1.5º ≤ 2 ≤ 50º. Fourier-transformed infrared spectra (FTIR) were recorded from 4000 to 400 cm-1 at 4 cm-1 resolution on a Nicolet 6700 spectrometer in pellets of samples dispersed in KBr at a concentration of approximately 0.5 wt. %. Raman scattering spectra were obtained at room temperature using a 7 Page 7 of 27
HORIBA Jobin Yvon confocal micro-Raman spectrometer with a 532 nm laser. The laser power is ~1mW and the spectra resolution was better than 2 cm-1. Carbon, nitrogen and hydrogen analyses were performed on solid samples in a Perkin-Elmer, model PE 2400, microelemental analyzer. Thermogravimetry and derivative thermogravimetry (TG/DTG) measurements were performed on Thermal Analyses equipment model
ip t
5100-TA Instruments. Samples were heated from ambient temperature to 1000ºC at a
heating rate of 10ºC/min under oxygen (50 ml min-1). Diffuse reflectance UV-Vis-NIR
cr
spectra were measured with Perkin Elmer Lambda spectrophotometer of powder samples dispersed in BaSO4 at a concentration of 10.0 wt. %; the spectra were collected
us
in the range of 200-1600 nm. Scanning electron microscopy (SEM) was performed in a Jeol 6360-LV, operating at 20 kV with the sample coated with carbon. Fluorescence spectra were collected on a Horada Spectrofluorometer using Xe lamp with source
an
excitation at room temperature.
M
3. Results and Discussion
Table 1 presents the composition and structural characteristics of hybrid
d
materials produced using different intervals of intercalation time. The composition of
te
the organic-inorganic material was determined by elemental analysis. In the molecular structure of PDDACl the polymeric repetitive unit is C8NH16Cl which affords a C/N
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molar ratio of 8 while for PAHCl the repeated unit is C3NH8Cl with a C/N molar ratio of 3 (values near can be seen in the Table 1 to C/N molar radio of the hybrid materials). It also can be seen in the Table 1, increasing the intercalation times the total amount of carbon increases.
8 Page 8 of 27
Table 1: Interlayer space, d(020), and elemental analyses of the hybrid materials prepared using hydrothermal treatment (150°C) at different intervals of intercalation
Samples
t/h
d(020)a/nm
C (%)
N(%)
H(%)
C/N
[PDDA]0.14MoO3
24
1.12
8.12
1.22
2.24
7.76
[PDDA]0.22MoO3
48
1.12
12.42
1.95
[PDDA]0.38MoO3
72
1.12
18.54
2.76
[PDDA]0.50MoO3
96
1.12
23.02
3.41
ip t
time.
[PAH]0.22MoO3
24
-
5.08
[PAH]0.41MoO3
48
2.24
[PAH]0.45MoO3
72
[PAH]056MoO3
96
3.95
7.82
4.09
7.88
1.81
1.62
3.27
8.80
3.14
1.77
3.26
2.24
9.56
3.39
1.94
3.29
2.24
11.56
4.07
2.29
3.30
MoO3 d(020) = 0.64 nm.
us
cr
7.41
an
a
3.36
M
Figures 2 and 3 present the X-rays diffractograms of polyelectrolytes-MoO3 hybrids and of pristine layered MoO3 (curves a in both figures). The position of the 020
d
diffraction peak indicates an interlayer spacing (d020) of 0.64 nm, which is consistent with the reported values7,16. X-ray diffraction patterns of polymers-based PDDA-MoO3
te
and PAH-MoO3 (see chemical composition in Table 1) synthesized by the direct
Ac ce p
intercalation of PDDACl and PAHCl polycations respectively, into MoO3 crystalline using hydrothermal treatment at increasing reaction times are shown in Figures 2 and 3 (b-e).
The intercalation of these particular polyelectrolytes in MoO3 is not found in the
literature, thus the indexation of hybrids XRD peaks is proposed here. In the present work, it is supposed that no modification of unit cell symmetry is caused by polyelectrolytes intercalation, therefore the (020) diffraction of hybrid is proposed to appear right below 10º 2θ along with the (020) of the pristine MoO3 until 72h of reaction (Figure 2, b-d). The final hybrid, prepared with 96h of reaction, Figure 2e, displays the characteristic broad (020) peak at around 8.0° 2θ for PDDA-MoO3.
9 Page 9 of 27
(110) (040) (021) (130) (101) (111) (041)
(060) (150)
40
(200) (210) (002)
(020)
30
(020)
ip t
b c d e 10
20
50
us
2 Degree
cr
Intensity [a.u.]
a
100 cps
Figure 2: Powder XRD pattern of (a) pristine crystalline MoO3 and of the PDDA-
an
MoO3 (see Table 1 for the composition) hybrid synthesized at 150 ºC with different hydrothermal treatment times (b) 24 h, (c) 48 h, (d) 72 h, and (e) 96 h. The peaks of
M
orthorhombic MoO3 were specified according to JCPDS database number 05-0508. For the product of PAHCl intercalation into MoO3, other aspects have to be
d
taken into account (Figure 3). Again one can suppose that the unit cell symmetry is the same for the MoO3 and for the hybrids as explained above. The result is again that the
te
(020) diffraction is displaced to larger distances and the peaks corresponding to the crystalline layered MoO3 are seen in the diffractograms of the products up to a reaction
Ac ce p
time of 72h. After 48h of intercalation another feature appears: two signals are observed below 10º 2θ, the first one with lower intensity in relation to the second one. This could indicate that another layered phase is forming together with the desired hybrid. Other techniques did not show any extra phase besides the target PAH-MoO3 compound. Literature shows that this is not a unique behavior. In fact, potassium niobate displays the same type of diffractograms17 which is due in that case, to the presence of two interlamellar regions, differently spaced. The possibility of having the same type of lamella disposition is not ruled out for the materials prepared here, however no other indication of such organization is found in the present work.
10 Page 10 of 27
(002)
(210)
(200)
(060) (150)
(101) (111) (041)
(021) (130)
(110) (040)
(020)
c
20
30
2 Degree
40
50
us
10
cr
d e
ip t
(040)
b (020)
Intensity [a.u.]
a
100 cps
Figure 3: Powder XRD pattern of (a) crystalline MoO3 and of the PAH-MoO3 (see
an
Table 1 for the composition) hybrid synthesized at 150 ºC with different hydrothermal treatment times (b) 24 h, (c) 48 h, (d) 72 h, and (e) 96 h. The peaks of orthorhombic
M
MoO3 were specified according to JCPDS database number 05-0508. In the diffractogram of PAH-MoO3, Figure 3 curves b to e, a series of diffraction peaks at 4.12, 8.24 and 12.1° 2 can be observed. These peaks are evenly spaced by
d
approximately 4.0° and can be indexed to the 0l0 reflections (l = 2, 4, and 8) for a
te
layered structure in excellent agreement with the X-ray diffraction of polymers-MoO3 nanocomposites reported by Murugan et al.12,18 In curves b, c and d of Figure 3, sharp
Ac ce p
peaks attributed to crystalline MoO3 also appear at wider angles in the diffractograms. The (020) reflection is diffuse in materials obtained at longer reaction times. For example, when the hydrothermal treatment time is short (24 h, Figures 2b and 3b) two phases are clear in the XRD patterns: the hybrid and unreacted crystalline MoO3; the color of the products is light blue during the course of reaction, which is characteristic of MoO3 containing hexavalent and pentavalent molybdenum (Mo6+ and Mo5+)19. After a longer hydrothermal treatment, 72 h, the intensity of the (020) line increases to a maximum corresponding to the organic-inorganic hybrids formation, and interestingly, the color of the product changes completely to dark blue. The d(020) interlayer spacing of MoO3 increases from 0.64 nm to ca. 1.1 nm with intercalation of PDDACl and to approximately 2.2 nm with intercalation of PAHCl polyelectrolytes. This result can be difficult to understand since the largest polyelectrolyte gives a hybrid where the interlayer space increased in a much larger extension than the one where the lower molecular weight polyelectrolyte was 11 Page 11 of 27
intercalated. One has to bear in mind that the chemical characteristics of each chain is different. The PDDACl has its nitrogen atoms completely quaternized, independent of the pH. Contrarily, in the PAHCl chain, all N atoms are in NH3+ groups, that can be deprotonated depending on the pH. In the FTIR study (see below), NH3+ and NH2 groups were observed, indicating that H-bonds for instance, may be occurring in the
ip t
PAHCl chain, inside the interlayer space. In that case, it might be energetically more favorable to keep chains elongated in order to expose as much NH groups as possible.
cr
In the case of PDDACl, such interactions are not observed and thus coiled
conformations are possible. That may be the difference responsible for such a variation
us
of interlayer spaces.
Figures 4 and 5 (curves b-d) present the X-rays diffraction patterns of the compounds intercalated with PDDACl and PAHCl polyelectrolytes, respectively, by
an
using different temperatures of hydrothermal treatment (in the range of 100-190 ºC). It can be seen that when the hybrid materials were produced using a temperature of 100 ºC
M
(Figure 4b and 5b) bulk MoO3 was observed in the final material, indicating that the reaction did not occur in a large extent. Higher temperatures, above 150 ºC, cause the peaks related to crystalline MoO3 to disappear suggesting that the whole material was
d
intercalated. Therefore, the optimum temperature of hydrothermal treatment to produce
b c d
(021)
(200)
(210) (002)
(150)
(060)
(111) (041)
(110) (040)
(020)
a
50 cps
(020)
Intensity [a.u.]
Ac ce p
be 150 ºC.
(130) (101)
te
the organic-inorganic hybrid using PDDACl and PAHCl polycations was determined to
e
10
20
30
40
50
2 Degree
Figure 4: Powder XRD pattern of MoO3 pristine (a) and to the PDDA-MoO3 hybrid synthesized by a direct hydrothermal treatment at different temperatures (b) 100 ºC, (c) 150 ºC , (d) 170 ºC and (e) 190oC. The time of intercalation used was of 96 h. The 12 Page 12 of 27
peaks of orthorhombic MoO3 were specified according to JCPDS database number 05-
b
cr
c
ip t
(002)
(200)
(210)
(060) (150)
(130) (101) (111) (041)
(110) (040)
(020)
(040)
a
100 cps
(020)
Intensity [a.u.]
(021)
0508.
10
20
30
40
50
an
2 Degree
us
d e
Figure 5: Powder XRD pattern of MoO3 pristine (a) and of the PAH-MoO3 hybrid synthesized by a direct hydrothermal treatment at different temperature such as (b) 100
M
ºC, (c) 150 ºC , (d) 170 ºC and (e) 190oC. The time of intercalation used was of 96 h. The peaks of orthorhombic MoO3 were specified according to JCPDS database number
d
05-0508.
te
Thus, with the use of hydrothermal treatment at 150 ºC for 96 h and in the presence of PDDACl or PAHCl the hybrids [PDDA]0.50MoO3 and [PAH]0.56MoO3 were
Ac ce p
produced. Some cationic nitrogen atom in the polyelectrolytes in these solids are counterbalancing the charge in the structure of the oxide and serving as ion pair with the chloride ion. The amount of exchangeable Cl- ions was measured by using Mohr’s method15. To [PDDA]0.50MoO3 and [PAH]0.56MoO3 compounds there are approximately 53.0 and 32.0 % of exchangeable chloride ions, respectively, thus the correct chemical formula
for
the
hybrids
are
[PDDACl]0.26[PDDA]0.24MoO3
and
[PAHCl]0.18[PAH]0.38MoO3, clearly describing the amount of exchangeable anionic ions. The PAH-based compound contains less exchangeable sites in the interlayer space due to the presence of neutral (-NH2) amine groups and Cl- ions are associated to positively charged (-NH3+) groups and generate –NH3+Cl- ion pair. Several materials were prepared under the determined optimum conditions, and their characterization follows.
13 Page 13 of 27
Thermal analyses The thermal stability of these materials in oxidizing atmosphere was examined by thermal analyses experiments. The thermogravimetry and derivate thermogravimetry curves (TG/DTG) of the pristine MoO3 (Figure 6) display only one stage of weight loss
ip t
at the temperature range of 750-800 ºC, which is related with MoO3 sublimation20. For the hybrid materials three distinct main stages were observed in as shown in Figure 6.
The first step, up to 120 ºC, corresponds to the removal of the reversibly bound water
cr
molecules, whereas the second step until approximately 400 ºC corresponds to the loss
of more strongly bound water between the layers and the decomposition/oxidation of continuous
us
polyelectrolyte molecules housed into the MoO3 interlayer space. This is followed by a weight loss up to 600 ºC which can be attributed to the
an
combustion/oxidation of the organic polymer component and its fragments, in agreement with the signals in the DTG curves. The decomposition/oxidation of polyelectrolytes is at a substantially lower decomposition temperature than the one of
M
the polycations themselves, i. e., around 550 °C, suggesting that the close proximity of MoO3 favors catalytic oxidation process of the hybrids that may involve oxygen from
d
the lattice21. Another mass loss in the region of 700-800 ºC can be attributed to the sublimation of orthorhombic MoO3 in the matrix of the inorganic-organic compounds.
te
Sublimation of this phase was evidenced also from the DTG curve which showed a relatively sharp intense peak at 750-800 ºC.
Ac ce p
The thermal results suggest that the polycations are intercalated into interlayer
space of the inorganic oxide.
14 Page 14 of 27
100
(A)
60
ip t
Mass [%]
80
40 MoO3 20
cr
[PDDACl]0.26[PDDA]0.24MoO3
[PAHCl]0.18[PAH]0.38MoO3 200
us
0 400
600
800
Temperature [ºC] 2.5
2.0
(B)
M
[PAHCl]0.18[PAH]0.38MoO3 1.5
1.0
d
DTG [a.u.]
an
MoO3
[PDDACl]0.26[PDDA]0.24MoO3
te
0.5
Ac ce p
0.0
200
400
600
800
Temperature [ºC]
Figure 6: Thermogravimetry and its derivative curves (TG/DTG) for MoO3 pristine (full
line)
and
for
[PAHCl]0.18[PAH]0.38MoO3
(dotted
line)
and
[PDDACl]0.26[PDDA]0.24MoO3 (dashed line) hybrids materials under oxidizing atmosphere.
SEM
images
of
the
[PAHCl]0.18[PAH]0.38MoO3
and
[PDDACl]0.26[PDDA]0.24MoO3 hybrid materials synthesized by the hydrothermal treatment and of crystalline MoO3 are in Figure 7(a-c). It is apparent that the hybrid materials (Figure 7b-c) form a continuous and relatively homogeneous matrix with a stratified morphology22. The incorporation of the polycations into the MoO3 host is 15 Page 15 of 27
accompanied by morphological changes that are supported by the results from the
M
an
us
cr
ip t
diffraction studies.
d
Figure 7: Scanning electron micrographs of the molybdenum trioxide matrix (scale bar:
te
1.0 μm) (a), [PAHCl]0.18[PAH]0.38MoO3 hybrid material (scale bar: 1.0 μm), (b) and
Ac ce p
[PDDACl]0.26[PDDA]0.24MoO3 hybrid material (scale bar: 0.5 μm) (c).
Raman Spectroscopy
Figure 8a shows a typical Raman spectrum of the MoO3 powders where fourteen
peaks can be observed. The sharpness of the peaks indicates that the corresponding vibrational modes are due to a highly ordered structure, where the position and relative intensity of these Raman peaks are in good agreement with that reported in the literature for MoO323,24 . Table 2 summarizes these frequencies. In particular, the Raman peak at 996 cm−1 is assigned to the stretching mode of the Mo=O group bearing an unshared oxygen. The peak at 819 cm−1 is assigned to the Mo-O-Mo stretching mode with a bridging oxygen atom in corner-sharing between two MoO6 octahedra. The peak at 666 cm−1 is assigned to the Mo-O3 stretching mode bearing triply coordinated oxygen which results from edge-shared oxygen atoms in common to three octahedra24.
16 Page 16 of 27
The Raman spectrum of polyelectrolyte-intercalated materials (Figure 8b-c) presents peaks that are wider and red-shifted in relation to pure MoO3. These two factors can be associated with the intercalation of polycations in the MoO3 matrix and are in line with a decreased crystallinity of the hybrids, as verified by XRD and SEM analyses. The downward shifts of Raman peaks can be due to the interactions of
cr
ip t
PDDACl or PAHCl chains with the MoO3 matrix.
us an
Intensity [a.u.]
500 cps
c b
M
a
1800 1600 1400 1200 1000 800 600 400 200
d
-1 Wavenumber [cm ]
te
Figure 8: Raman spectra of MoO3 (a), organic-inorganic [PAHCl]0.18[PAH]0.38MoO3
Ac ce p
(b) and [PDDACl]0.26[PDDA]0.24MoO3 (c) hybrids in the low and high energy regions.
17 Page 17 of 27
Table 2: Experimental frequencies (cm-1) and assignments of the peaks in Raman spectra of MoO3 bulk and organic-inorganic hybrids. Wavenumber (cm-1)
Vibrations
PDDACl]0.26[PDDA]0.24MoO3
[PAHCl]0.18[PAH]0.38MoO3
(Mo=O)
996
991
993
(O-Mo2)
819
816
818
(O-Mo3)
666
661
663
(O-Mo3)
476
465
δ(Mo=O)
378
374
δ(O-Mo3)
337
335
δ(Mo=O)
284
281
246
236
δ(O-Mo2)
198
193
Lattice
159
vibrations
128
cr us
282
238
146
149
122
125
d
194
113
111
Ac ce p
Infrared spectroscopy
336
te
115
376
M
δ(O-Mo2)
465
an
292
ip t
MoO3
FTIR spectra of MoO3 before and after the intercalation of polyelectrolytes are
shown in Figure 9. The spectrum of pure MoO3 is characterized by the presence of a broad band in the 3800-3200 cm-1 range, attributable to the stretching mode of water molecules physisorbed on MoO3 surface, the bending mode falling at ca. 1632 cm-1 (Fig. 9A, a). At the low frequency side, the spectrum of MoO3 (Fig. 9B, a) exhibits three main absorptions in the 1250-400 cm-1 range with peaks at 993 (intense, sharp and slightly asymmetric towards low frequency side), 875 (very intense and broad), 820 (very weak) and at ca. 605 cm-1 (very intense, broad and with a shoulder at ca. 650 cm-1). In addition, two weak bands are observed at 512 and 488 cm-1.
18 Page 18 of 27
3050
993 1468
820
2925
a 1612
945 912 845
b
c 1505
3500
3000
512 488
2500
2000
1500
893
c
935
1200
1000
800
600
400
Wavenumber [cm-1]
us
Wavenumber [cm-1]
ip t
b
B
0.2
1632
2920 2960 2860
605
875
A
0.4
cr
Absorbance [a.u.]
a
Figure 9: FTIR of the standard MoO3 matrix (a), [PDDACl]0.26[PDDA]0.24MoO3 (b)
an
and [PAHCl]0.18[PAH]0.38MoO3 hybrid compounds (c).
There are several works in the literature reporting the IR bands assignments for
M
MoO325,26. Among them, several discrepancies can be noticed especially in relation to the longitudinal optical and transverse optical (LO-TO) splitting effects which depend on the size and particle morphology of the crystallite of samples in powder forms27,28.
d
In the present study, we are based on the work of Knozinger et al.29 and Eda30,
te
where the band at 993 cm-1 is assigned to the stretching mode of Mo=O groups (B2u or B3u symmetry, Longitudinal Optical component, LO). The broad band centered at 875
Ac ce p
cm-1 is originated by stretching vibrations of Mo-O-Mo groups of MoO3; in particular, the main component at 875 cm-1 is due to the B3u stretching mode (Transversal Optical component, TO), whereas the weak band at 820 cm-1 is due to the Mo-O-Mo B2u stretching mode. Finally, the complex band at 605 cm-1 should be associated to the B1u stretching mode of Mo-O-Mo species, whereas the shoulder at ca. 488 cm-1 is generated by the B2u stretching mode of the same oscillators. The IR spectra of hybrid materials (Figure 9, b-c) showed that the intercalation
leads to significant modifications of the IR spectrum of MoO3 sample. The IR spectrum of PDDACl intercalated material is dominated in the high frequency region (Fig. 9 A, b) by a broad band in the 3800-3000 cm-1, accompanied by absorptions at ca. 2960, 2920 and 2855 cm-1. In addition, two bands at 1632 and 1468 cm-1 (asymmetric towards the low frequency side) are also observed. The sample contains an additional amount of adsorbed water with respect to MoO3 parent sample and it is characterized by adsorptions typical of PDDACl species, thus confirming that the intercalation process 19 Page 19 of 27
was successfully attained. In fact, bands at 2960 and 2855 cm-1 are associated to the asymmetric and symmetric modes of CH3 species, respectively, whereas the peak at 2920 cm-1 is due to the asymmetric stretching mode of methylene groups of PDDACl species, the symmetric vibration falling at ca. 2850 cm-1, masked by the CH3 symmetric stretching. The band at 1468 cm-1 is due to deformation vibrations of CH2 species of
ip t
PDDA chains.
The intercalation procedure significantly affects the phonon vibrations of MoO3
cr
lattice (Fig. 9B, b). The PDDA-exchanged materials indeed show weak absorptions in
the 1000-500 cm-1 range, in which peaks at 945, 912 and 845 cm-1 due to inorganic
us
MoO3 matrix are found. The observed differences should be taken as an indication of the fact that intercalation procedure modifies the particle size and organization of MoO3 layers, thus resulting in a deep modification of LO and TO vibrational modes of the
an
MoO3 matrix.
The spectrum of PAH-exchanged material at high frequency (Fig. 9A, c) is
M
characterized by a broad absorption in the 3650-2750 cm-1 range, with maxima at ca. 3430, 3050, 2925 and 2860 (very weak and partially masked by the band at 2925 cm-1) cm-1 and sharp bands at 1612 and 1505 cm-1 (with shoulder at 1468 cm-1). The bands at
d
2925 and 2860 cm-1 are due to asymmetric and symmetric modes, respectively, of CH2
te
groups of PAHCl, whereas the broad absorption in the 3650-2750 cm-1 range is due to polyelectrolyte in interaction with the MoO3 structure. The presence of protonated NH3+
Ac ce p
groups in PAHCl species is witnessed by the bands due to the asymmetric and symmetric bending modes of these species, that can be observed at 1612 and 1505 cm-1, respectively. The stretching vibrations of NH3+/NH2 groups are probably hidden in the broad component present in the 3650-2750 cm-1 frequency range. The comparison with the spectrum in Figure 9A curve b shows the absence of these vibrations and confirms the assignment.
At the low frequency range (Fig. 9B, c) the spectrum is characterized by the
presence of two broad bands in the 1000-800 (with evident maxima at 935 and 893 cm-1) and 750-500 cm-1 frequency ranges. The first absorption is due to MoO3 inorganic matrix, whereas bands typical of PAHCl polyelectrolyte31 are present in the 750-500 cm-1 range. Also in this case, FTIR spectroscopy suggests that PAH intercalation has a clear effect on layer organization of MoO3 matrix.
20 Page 20 of 27
Diffuse reflectance spectroscopy The UV-Vis diffuse reflectance spectra of [PDDACl]0.26[PDDA]0.24MoO3, [PAHCl]0.18[PAH]0.38MoO3 and MoO3 parent material are shown in Figure 10. The DRUV-Vis spectrum of MoO3 is characterized by the presence of a band in the 200-360
ip t
nm range with a maximum at 250 nm and an evident shoulder at ca. 290 nm which can
be associated to charge-transfer transitions of polymolybdate-like structure containing
cr
Mo6+ ions in octahedral configuration32,33. In addition, the band in the 300-400 nm range should be due to the presence of Mo6+(Oh) ions in crystalline arrangement34.
us
The DR-UV-Vis spectra of both hybrid samples (Fig. 10, curves b-c) are characterized by similar absorptions: a band at 250 nm with a clear shoulder at 330 nm (associated to the presence of polymolybdate-like species) and complex absorption
an
pattern in the 600-1600 nm range in which bands at ca. 600, 800, 930, 1140, 1270 and 1500 nm are visible. Absorption bands at 600 and 930 nm are associated to
M
Mo5+Mo6+ intervalence charge transfer (polaronic) transitions35,36 whereas the bands at 800, 1140, 1270 and 1500 nm should be due to d-d transitions of Mo5+ ions in distorted polyhedron sites of MoO3 solid. The presence of Mo5+ ions in hybrid materials ions.
Kubelka-Munk
Ac ce p
1,0
te
d
suggests that the intercalation procedure leads to a partial reduction of Mo6+ to Mo5+
0,8
250
c
0,6 0,4
b
0,2
a 0,0 200
400
600
800
1000
1200
1400
1600
Wavelength [nm] Figure 10: UV-Vis diffuse reflectance spectra of MoO3 (a), organic-inorganic [PAHCl]0.18[PAH]0.38MoO3 (b) and [PDDACl]0.26[PDDA]0.24MoO3(c) hybrid materials. 21 Page 21 of 27
The organic-inorganic hybrids as anionic exchangers In this section, the results obtained after the occlusion of the cyanine anionic dye in
the
interlayer
space
of
hybrid
[PAHCl]0.18[PAH]0.38MoO3
and
[PDDACl]0.26[PDDA]0.24MoO3 compounds will be presented. The materials exchanged
ip t
with dye were characterized by elemental analysis (CHN), UV-Vis diffuse reflectance and fluorescence spectroscopies.
cr
Table 3 shows the quantities of dye ion exchanged in the hybrids after Soxhlet
us
extraction.
Table 3: Chemical formulas of hybrid materials after the occlusion of anionic dye.
an
[PDDACy]0.06[PDDACl]0.21[PDDA]0.24MoO3 [PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3
M
The optical properties of the dye were studied by diffuse reflectance spectroscopy technique in the UV-Vis region. The spectrum of cyanine dye dissolved in methanol (not shown) presented absorption at 519 and 552 nm, related with the dye
d
chromophor37,38. The same bands are red-shifted in the spectra of hybrid materials (not
te
shown) due the formation of J-type aggregated of dye occluded in the organic-inorganic hybrids or due an environment new that the dye is confined38. This red-shift is in good
Ac ce p
agreement with results obtained by Ogawa39 which intercalated cationic dyes in the interlamelar space of clays.
Figure 11 shows the results of luminescence properties of dye confined in the
[PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3 hybrid material, compared with the cyanine in methanol solution. The spectrum of the cyanine dye in solution (full line) shows an emission at 596 nm after excitation at 560 nm and another one at ca. 650 nm (sh) that can be related at J-type aggregates of cyanine dye. The spectrum of the cyanine-hybrid material before the extraction with methanol using excitation wavelength of 560 nm is on Figure 11 (dotted line). An emission band at 614 nm can be observed in this spectra; it is red-shifted by 18 nm in comparison to cyanine in methanol solution. This red-shift can be associated with the formation of the J-type dye aggregates occluded in the hybrid40, or to the confinement of the dye into the oxide matrix, with different polarity with
respect
to
the
solution.
After
the
extraction
with
methanol,
the
[PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3 hybrid (Figure 11, dashed line) showed an 22 Page 22 of 27
emission band at 602 nm that is red-shifted by 6 nm of pure dye and blue-shifted by 12 nm in relation to the hybrid before the extraction of methanol. Therefore, the red-shift (from 596 to 602 nm) can be associated with the change in polarity of media due to the new environment of dye confined between the lamella of MoO3. After the extraction with methanol is observed also that the relative intensity of emission band at ca. 650 nm
ip t
of hybrid (dashed line, Figure 11) decrease in relation at cyanine dye solution (full line,
Figure 11) probably associated with the disappearance of some aggregated dye. This discussed
above
was
also
for
the
M
te
d
614
Intensity [a.u.]
an
602
596
[PDDACy]0.06[PDDACl]0.21[PDDA]0.24MoO3 material.
verified
us
behavior
cr
behavior is in excellent agreement with results published recently38,38. The same
Ac ce p
580 600 620 640 660 680 700 720 740
Wavelength [nm]
Figure 11: Emission spectra at room temperature (excitation wavelength of 560 nm) of cyanine dye in methanol solution (5x10-5 mol L-1) (full line); and intercalated in [PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3 hybrid material before (dotted line) and after the extraction (dashed line).
Conclusions This work showed a novel method of introduction of PDDACl and PAHCl polymers between the layers of MoO3 by using only hydrothermal treatment as the intercalation technique. The analysis of hydrothermal treatment duration and of temperature of intercalation with both polyelectrolytes revealed that the PDDACl and 23 Page 23 of 27
PAHCl chains are inserted in the interlayer spacing of molybdenum trioxide at the optimum temperature of 150oC for 96 h of reaction. Upon intercalation, the interlayer space of original MoO3 expanded from 0.64 nm to ca. 1.1 nm with PDDACl and approximately 2.2 nm with PAHCl intercalation. The hybrid based on interlayer PAHCl contains neutral (-NH2) amine groups and positively charged (-NH3+) groups that are
ip t
associated with Cl- (to generate –NH3+Cl- ion pair) and thus, when intercalated in MoO3, it created a lower amount of exchangeable ion pairs in the interlayer spaces than
cr
the PDDA-based hybrid since the last polyelectrolyte contains only quaternary ammonium groups (-NR3+Cl-).
us
The intercalation process occurs with the reduction of some Mo6+ centers to Mo5+ as shown by UV-Vis spectroscopy. In fact, the hydrothermal process is advantageous in relation to the methods reported in the literature, since it does not need
an
a reducing initial step before polyelectrolytes ion exchange. The hybrids display clear anion exchange capacities, shown in this work by the insertion of an anionic cyanine
M
dye in the interlamellar spaces, the monitoring of absorption and emission bands of the
Acknowledgements:
d
dye confirmed its presence in the interlayer space of the hybrids.
te
The authors acknowledge Prof. M. Cossi and Dr. A. Fraccarollo for fruitful discussions. The authors acknowledge the Fundação de Amparo à Pesquisa no Estado
Ac ce p
de São Paulo (FAPESP), the Piedmont Region (Italy) [NANOLED project (Novel Nanostructured Materials for Light Emitting Devices and Application to Automotive Displays), CIPE 2006] and the European Community [INNOVASOL project (Innovative Materials for Future Generation Excitonic Solar Cells), of the FP7] for the financial support to this work and to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the fellowships.
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26 Page 26 of 27
i cr 596
602 614
ed ce pt
Intensity [a.u.]
(020)
hν
MoO3
(040)
[PAHCy]0.08[PAHCl]0.12[PAH]0.38MoO3 hybrid
10
20
30
2 Degree
Ac
(020)
Intensity [a.u.]
100 cps
M an
us
Graphical abstract
40
50
580 600 620 640 660 680 700 720 740
Wavelength [nm]
Page 27 of 27