ECASIA special issue paper Received: 1 September 2015
Revised: 27 January 2016
Accepted: 28 January 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/sia.5973
Surface spectroscopy and structural analysis of nanostructured multifunctional (Zn, Al) layered double hydroxides M. Richetta,a* L. Digiamberardino,a A. Mattoccia,a P. G. Medaglia,a R. Montanari,a R. Pizzoferrato,a D. Scarpellini,b A. Varone,a S. Kaciulis,c A. Mezzi,c P. Soltania,c and A. Orsinid Two types of (Zn, Al) layered double hydroxide were prepared by a hydrothermal process at room temperature using Zn salt precursors on Al foils. The examined LDHs differ for the hosted anions in the interlamellar space, namely Cl and NO 3 . Scanning electron microscopy, X-ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy have been used to characterize four types of the samples, representative of the two hosted anions (Cl and NO 3 ) and two times of growth (6 and 24 h). X-ray photoelectron spectroscopy results permitted to describe the interactions between inorganic anions hosted in the interlamellar space and the metallic cations on the brucite layer. They also allowed giving a tentative explanation of the different morphologies observed by scanning electron microscopy. Copyright © 2016 The Authors Surface and Interface Analysis Published by John Wiley & Sons Ltd. Additional supporting information may be found in the online version of the publisher’s web-site. Keywords: Zn; Al; layered double hydroxide; XPS; UPS
Introduction Layered double hydroxides (LDHs) are ionic lamellar materials belonging to the group of anionic clays, whose structure (Fig. 1) is based on brucite-like layers containing a divalent M2+ cation coordinated with six OH- hydroxyl groups.[1,2] The substitution of the M2+ metal (for instance Ca2+, Zn2+, Mg2+ and Ni2+), with a trivalent M3+ cation, gives rise to the infinite repetition of positively charged sheets (lamellas) alternating with An ions (Fig. 1). The layered structure has attracted increasing interest, because it can host even complex organic molecules, such as drugs and biomolecules, intercalated in the large interlamellar space, with wide flexibility in composition and functionalization. Engineered LDHs can find a large variety of applications. For instance, because of the capacity of exchanging ions and hosting anions in inter-lamellar template, LDHs have been investigated as additives in organic anticorrosion coatings,[3,4] in flame retardants,[5] for water treatment and purification[6,7] or for biomedical applications like drugs delivery and biosensors.[8–10] A wide range of synthesis techniques can prepare (Zn, Al) LDH crystallites, because of their typical shape of nanosheets. Among these, we can mention the simple one-step hydrothermal process at room temperature, in which one can use Zn salt precursors on Al sputtered thin substrates, or Al foils.[11–14] In this case, the metal substrate (Al) acts as a support and reactant (forming a reservoir for the trivalent M3+ cation). Changing the characteristics of the Al coating can vary the morphology, and the dimensions of the nanostructures. Furthermore, both the compositions of brucite-like layer and of the hosted anions can be easily changed. Surface spectroscopic techniques are quite important in understanding how the mechanisms of doping and the metals–oxygen
Surf. Interface Anal. 2016, 1–5
coordination could be correlated to the functional properties of the investigated material. For instance, Parida et al.[15] found that the incorporation of Fe3+ ions into (Mg, Al) LDH changes the photocatalytic activity for H2 generation of these systems. In this case, the X-ray photoelectron spectroscopy (XPS) studies had the capability to unambiguously show the presence of Fe3+ in the hydroxides layers and the coordination of Mg2+ and Al3+ ions, as well as the presence of anions (carbonates) in the interlayer space. Recently, XPS measurements have been performed to investigate the valence state of Vanadium in (Zn, Al) LDH samples with VO3 vanadate anions intercalated in the interlamellar space, used as corrosion inhibitor.[16] Wen et al.[17] used XPS spectroscopy to clarify the surface state of LDH and graphene oxide nanocomposites, before and after the Arsenic anions removals from aqueous solutions. XPS and UPS
* Correspondence to: M. Richetta, Department of Industrial Engineering, University of Rome – Tor Vergata, 00133 Rome, Italy E-mail:
[email protected] This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. a Department of Industrial Engineering, University of Rome – Tor Vergata, 00133 Rome, Italy b L-NESS and Department of Materials Science, University of Milan Bicocca, Milan, Italy c ISMN – CNR, P.O. Box 10, 00015 Monterotondo Stazione, Rome, Italy d Department of Electronic Engineering, University of Rome – Tor Vergata, 00133 Rome, Italy
Copyright © 2016 The Authors Surface and Interface Analysis Published by John Wiley & Sons Ltd.
M. Richetta et al.
Figure 1. Schematic view of the general structure of (Zn, Al) LDH, with Cl and NO3 anions intercalated in the brucite-like structure. Other possible chemical species eventually present in the interlamellar space are shown. The basal spacing, d, is also indicated.
also provided valuable information on C bonds of nanocomposites with graphene oxide.[18] In order to investigate intercalated anions in interlamellar space, and metallic cations in the brucite layer, the present work was carried out on (Zn, Al) LDH samples, grown on Al foils, hosting two different anions, namely Cl and NO 3 , in the inter-lamellar space.
Experimental – samples preparation (Zn, Al) LDH nanostructures were grown on aluminium foils. A nutrient solution composed of a 1 : 1 ratio of zinc chloride ZnCl2 and hexamethylenetetramine (C6H12 N4) at 5 mM concentration was employed for the hydrothermal growth of LDH. The same concentration of Zn (NO3)2 was used for samples intercalated with NO 3 anions. Hexamethylenetetramine acted as a pH regulator for the solution basicity, through the hydrolyzation and release of ammonia. During the growth, the samples were anchored at a 45° tilted sample holder and fixed in the middle of the solution bottle. The growth temperature was fixed at 80 °C, while the growth time was varied from 6 to 24 h. The samples were cooled down in ambient atmosphere and then washed with ethanol at room temperature, to remove the residuals on the top of the LDH surface. Four types of the samples, representative of two hosted anions and two growth times, have been investigated: Samples 1 and 2 – intercalated with Cl, growth time 6 and 24 h respectively; Samples 3 and 4 – intercalated with NO 3 , growth time 6 and 24 h respectively.
Experimental – samples analysis The properties of LDH nanoplatelets were analysed by using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) techniques.
X-ray photoelectron spectroscopy experiments were performed by using a spectrometer Escalab 250Xi (Thermo Fisher Scientific, UK) equipped with monochromatized Al Kα source, electromagnetic lenses, electron and ion flood guns for charge neutralization, 6-channeltron detection system for spectroscopy and multichannel plate for chemical imaging. An area of 1 mm in diameter was analysed at constant analyser pass energy of 40 keV, standard electromagnetic lens mode and large spot charge xompensation. The binding energy (BE) scale of acquired spectra was not corrected for the sample charging, which was prevented by the charge compensation. During XPS experiments, the survey scan was acquired for each sample at the pass energy of 150 eV, followed by measurements of the elemental regions. It should be noted that all the spectra were stable during the acquisition; i.e. there was not observed any sample modification because of the possible outgassing of water and/or hydroxyl groups. A He lamp emitting He I (21.2 eV) or He II (40.8 eV) photons was used to carry out the UPS at constant pass energy of 5 eV. Obtained XPS were processed by the Avantage v.5 software (Thermo Fisher Scientific, UK). Shirley background subtraction and standard set of Scofield sensitivity factors were used for XPS quantification.
Results and discussion All the samples have been preliminarily observed by SEM in order to examine the structure and possible defects. Fig. 2 shows the micrographs of (Zn, Al) LDHs hosting Cl (samples 1 and 2) and NO 3 (samples 3 and 4) in the interlamellar space, at the same magnification. The samples exhibit a petal-like morphology, where each petal is either a single crystal or consists of few crystalline grains. The growth of LDH grains takes place mainly along two directions giving rise to their peculiar shape. The surface of all samples is homogeneous, and macroscopic defects were not detected. The petal-like morphology of the two types of samples is similar but, after the same growth time, petals
wileyonlinelibrary.com/journal/siaCopyright © 2016 The Authors Surface and Interface Analysis Published by John Wiley & Sons Ltd.Surf. Interface Anal. 2016, 1–5
XPS and UPS investigation of nanostructured multifunctional LDH
Figure 2. SEM images (at the same magnification) of (Zn, Al) LDHs hosting Cl (samples 1 and 2) and NO3 (samples 3 and 4) in the interlamellar space. The growth time is 6 h for the samples 1 and 3, 24 h for the samples 2 and 4.
are of larger size in the samples with the intercalated NO 3 anions. Furthermore, comparing the morphology of the samples 1 and 2, hosting Cl anions, it appears that a longer time of growth gives rise to a coarser structure. The same occurs for the samples 3 and 4 hosting NO 3 anions. XRD spectra provide the confirmation of hexagonal (Zn, Al) LDH crystal structure. Both two basal reflections, (003) and (006), giving evidence of parallel-oriented LDH nanoplatelets, and some ( kl) non-basal reflections (i.e. having h, k ≠0), from almost perpendicular-oriented nanoplatelets, are present in the spectra. Not appreciable differences are observed on the ‘a’ lattice parameter, corresponding to the distance between two metal cations, evaluated from XRD spectra. Namely the average values are close to 3.065 Å for all samples, containing Cl and NO 3 anions. Furthermore, even the basal spacing ‘d’, obtained from the ‘c’ lattice parameter, lies within the range (7.55 ÷ 7.57) Å, in a good agreement with values reported in literature.[1,2,11] The results of elemental identification and quantification from XPS spectra are reported in Table 1. The BE values of Al 2p and Zn 2p peaks indicated their main oxidation states of Al3+ and Zn2+. To identify exactly the chemical state of Zn, the modified Auger parameter ‘ α′’ was also evaluated from the relationship: α′ ¼ 1021:2ðBEÞ þ 987:5ðKEÞ ¼ 2008:7 eV
(1)
The obtained values of Auger parameter (Table 1) correspond to the Zn2+ oxidation state.[19,20] The peak fitting analysis of Al 2p revealed that Al was present as oxide and hydroxide. The atomic ratio of Zn/Al changed from 0.7 in the sample 1 to 2.0 in the sample 2. The Zn/Al atomic ratio for the
samples 3 and 4 was 1.9 and 2.0 respectively. A significant change of this ratio was observed between the samples 1 and 2; i.e. it noticeably increased changing the growth time from 6 to 24 h. The peaks of Cl 2p and Mg 1 s were positioned at BE = 198.6 eV and 1304.4 eV and assigned to Cl and Mg2+ respectively. The presence of Mg2+, detected only in the samples 1 and 2, is probably caused by a low surface contamination. Changing the time of growth from 6 to 24 h, the amount of Mg and Cl was increased. A small amount of adventitious carbon, less than 10%, was detected on the samples surface. From these results, the relative amount of Al seems to be correlated to the LDH morphology, i.e. the larger the Al amount (as in the sample 1), the smaller the petal size. This size depends on nucleation and growth rates: high nucleation rate and slow growth lead to grains of smaller size and vice versa. In this case, a higher Al content is always associated to the grains (petals) of smaller size; therefore, Al favours the nucleation of LDH grains by increasing the number of nucleation sites. As the thickness of deposited layer increases, the Al content and consequently the nucleation rate decrease. If the growth rate is constant, the lower nucleation rate leads to the larger grains (samples 2–4). This mechanism could also explain the variations in the Zn/Al ratio values revealed by XPS. From Fig. 2, it appears that this phenomenon is also affected by the presence of different anions interacting with Al. UPS spectra, acquired by using He I and He II photons, are shown in Fig. 3. The work function calculated from the cut-off of the He I spectra was of 4.4 and 4.7 eV for the samples intercalated with Cl and NO 3 respectively. The peak at about BE = 10 eV, which is present in He II spectra of the samples 3 and 4, is attributed to Zn 3d. The second band at about 15 eV can be because of the
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M. Richetta et al. Table 1. XPS quantitative results for the examined samples Peak Sample 1 Al2p A Al2p B C1s A C1s B C1s C Cl2p Mg1s O1s A O1s B Zn2p3/2 Sample 2 Al2p A Al2p B C1s A C1s B Cl2p Mg1s O1s A Zn2p3/2 Sample 3 Al2p A Al2p B C1s A C1s B C1s C N1s A N1s B O1s A O1s B Zn2p3/2 Sample 4 Al2p C1s A C1s B N1s O1s A O1s B Zn2p3/2
BE (eV) FWHM Atomic Chemical state (eV) (%)
Auger parameter (eV)
73.6 75.9 284.6 287.2 290.6 198.6 1304.4 530.8 532.5 1021.4
2.3 2.3 3.0 3.0 3.0 3.9 2.9 2.3 2.3 2.2
10.0 14.2 3.6 2.6 2.0 2.2 1.6 34.9 11.8 17.1
AlO(OH) boehmite Al2O3, Al(OH)3 C–C O-C carboxyl O–C=O chlorides MgO oxides adsorbed H2O Zn(+2)
2009.0
73.6 75.2 284.6 289.1 198.2 1304.3 530.9 1021.1
1.9 1.9 2.8 2.8 2.6 5.2 2.1 1.9
4.2 6.6 6.7 2.7 6.9 3.9 47.0 22.2
AlO(OH) boehmite Al2O3, Al(OH)3 C–C carboxyl O–C=O chlorides MgO oxides Zn(+2)
2008.7
73.9 75.1 284.4 285.9 289.0 406.1 407.2 529.3 531.4 1021.2
1.7 1.7 2.0 2.0 2.0 1.3 1.3 2.0 2.0 1.9
4.5 6.0 4.2 1.7 1.3 3.0 1.5 2.3 55.9 19.7
AlO(OH) boehmite Al2O3, Al(OH)3 C–C C–O carboxyl O–C=O NO2 nitrate oxides OH Zn(+2)
2008.7
74.0 284.2 288.7 406.2 529.5 531.2 1021.2
1.9 2.0 2.0 1.4 1.8 1.8 1.9
10.8 6.8 2.2 3.2 7.5 48.5 21.2
Al(OH)3, Al2O3 C–C carboxyl O–C=O nitrate oxides OH Zn(+2)
2008.9
N 2 s signal from NO2 and nitrates. In the samples 1 and 2, the signal of Zn 3d is absent, indicating that the surface is heavily contaminated. The band centred at about 13 eV is probably because of the convolution of C 2 s-2p states together with Cl 3 s. This difference of the valence band spectra between the samples intercalated with Cl and NO 3 is not reflected in the XPS results, because the UPS with He II at KE = 20–40 eV is probing only the topmost surface layer, whereas XPS information depth is much higher. A simple evaluation by using a model of Seah and Dench[21] for the fotoelectrons with KE = 20–40 eV in Al matrix gives the value of information depth about 0.4–0.5 nm, whereas the recent study[22] indicated even lower values of about 0.2–0.3 nm. At the same time, the XPS information depth for the Zn 2p peak (KE ≅ 460 eV) is about 1.1 nm.[21,22] Therefore, a thin layer of surface contamination can suppress the signal of Zn 3d in UPS, whereas the peak of Zn 2p is clearly visible in XPS.
Figure 3. UPS spectra acquired with He I and He II sources.
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XPS and UPS investigation of nanostructured multifunctional LDH
Conclusions In this work, the properties of the LDH nanoplatelets were analysed by different techniques. Obtained XRD and SEM results revealed the typical hexagonal structure and the petal-like morphology. The UPS results highlighted the differences between the valence band structure of Cl and NO 3 intercalated samples. XPS and UPS results demonstrated a correlation between the Al content and the LDH morphology. In particular, a ‘high available’ content of Al is connected to a small dimension of petals, as it could favour the nucleation by increasing the number of nucleation sites. Even the values of Zn/Al ratio in different samples are supporting this hypothesis.
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