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Organo-layered double hydroxides composite thin films deposited by laser techniques
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Organo-layered double hydroxides composite thin films deposited by laser techniques R. Birjega a , A. Vlad a,∗ , A. Matei a , M. Dumitru a , F. Stokker-Cheregi a , M. Dinescu a , R. Zavoianu b , V. Raditoiu c , M.C. Corobea c a
National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Str., 77125 Bucharest-Magurele, Romania University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, 4-12 Regina Elisabeta Bd., Bucharest 030018, Romania c National R.&D. Institute for Chemistry and Petrochemistry, ICECHIM, 202 Splaiul Independentei Str., CP-35-274, 060021 Bucharest, Romania b
a r t i c l e
i n f o
Article history: Received 13 June 2015 Received in revised form 9 December 2015 Accepted 12 December 2015 Available online xxx
a b s t r a c t We used laser techniques to create hydrophobic thin films of layered double hydroxides (LDHs) and organo-modified LDHs. A LDH based on Zn-Al with Zn2+ /Al3+ ratio of 2.5 was used as host material, while dodecyl sulfate (DS), which is an organic surfactant, acted as guest material. Pulsed laser deposition (PLD) and matrix assisted pulsed laser evaporation (MAPLE) were employed for the growth of the films. The organic anions were intercalated in co-precipitation step. The powders were subsequently used either as materials for MAPLE, or they were pressed and used as targets for PLD. The surface topography of the thin films was investigated by atomic force microscopy (AFM), the crystallographic structure of the powders and films was checked by X-ray diffraction. FTIR spectroscopy was used to evidence DS interlayer intercalation, both for powders and the derived films. Contact angle measurements were performed in order to establish the wettability properties of the as-prepared thin films, in view of functionalization applications as hydrophobic surfaces, owing to the effect of DS intercalation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDHs) are a large class of inorganic x+ M3+ (OH2 )] [An− ]x/n materials, having as general formula, [M2+ 1−x x mH2 O where M2+ and M3+ are divalent and trivalent cations that occupy octahedral positions in the hydroxide layers, An− is an interlayer anion and x is the molar ratio [1]. The flexibility of their chemical composition allows the preparation of LDHs having a wide variety of properties, making them promising materials for applications in different fields such as catalysis, pollutants absorption, additives or precursors [2–5]. The anionic exchange capacity of LDHs is utilized to incorporate surfactant organic anions into the interlayer space in order to obtain organo-LDHs [1,6–8]. The ability to transform the LDH hydrophilic surface to hydrophobic and to access the interlayer region can extend the applicability of LDHs toward different types of functional nanocomposites [9–13]. The capacity of this new emerging class of multifunctional LDH based composite to preserve or to develop new properties as thin films is an open topic. Research on LDH films is expanding, in view of their applications as sensors
∗ Corresponding author. E-mail address: [email protected] (A. Vlad).
[14,15], electrodes [16], corrosion-resistant coatings [11,12,17] etc. Usually, the LDH films are prepared by two methods: physical deposition and in situ growth, i.e. substrate-induced growth. We have previously reported on the ability of pulsed laser deposition (PLD) and matrix assisted pulsed laser evaporation (MAPLE) [18–20] to produce adherent, well oriented LDH films. In particular, hybrid LDH/polymer structures were transferred onto silicon substrates using MAPLE [20]. The targets used in the above study are organoLDH composites accommodating the organic guest molecule in the interlayer space. In this study we used targets of pristine ZnAl based LDHs (Zn/Al molar ratio of 2.5) and intercalated dodecyl sulfateLDHs (DS-ZnAl-LDHs) to obtain composite films. We performed a comparative study of the performance of two laser techniques in producing films which preserve the complex structures of the targets and we discuss the derived wettability properties.
2. Materials and characterization methods 2.1. Powders All the chemicals used for the preparation of the ZnAl-LDHs and organo-modified LDHs were of analytical grade or of the highest purity commercially available.
http://dx.doi.org/10.1016/j.apsusc.2015.12.099 0169-4332/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: R. Birjega, et al., Organo-layered double hydroxides composite thin films deposited by laser techniques, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.12.099
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The pristine ZnAl-LDH powder was prepared via coprecipitation at 40 ◦ C under low supersaturation conditions, pH 9.5–10. Zn(NO3 )2 ·6H2 O, Al(NO3 )3 ·9H2 O anhydrous Na2 CO3 and NaOH were the raw materials. The same coprecipitation method described by Reichle [21] and Carriazo et al. [22] was used. The molar ratio of Zn/Al was 2.5. The steps followed to obtain the final LDH powder were similar to those used for the preparation of MgAl-LDHs [23]. In the following we will refer to this powder as Zn2.5Al. The organo-modified LDHs were prepared using two methods: direct coprecipitation and, reconstruction via the “memory effect”. The coprecipitation method used an adapted protocol reported by Du et al. [24]. The raw materials were Zn(NO3 )2 ·6H2 O, Al(NO3 )3 ·9H2 O (Zn/Al molar ratio of 2.5), and sodium dodecyl sulfate (SDS = NaC12 H25 SO4 , SDS/Al molar ratio of 1.5). Sodium hydroxide was used for pH adjustment (pH = 9.5–10). The final powder, labeled PZn2.5Al-DS, was obtained following the same procedure as for the Zn2.5Al powder. The DS intercalation via reconstruction occurred by immersing the mixed oxide powder derived from the gentle calcination (18 h at 460 ◦ C in air flow) of the Zn2.5Al powder in an aqueous solution of SDS (SDS/Al molar ratio of 1.5). The immersion occurred at room temperature for 24 h. The recovered solid powder was labeled RZn2.5Al-DS. X-ray diffraction – XRD (PANalytical X’Pert MPD system, ˚ and Fourier transform infrared spectroscopy – CuK␣ = 1.5418 A) FTIR (JASCO FTIR 6300 spectrometer equipped with ATR Specac Golden Gate unit) were the characterization techniques used to evidence the crystallographic structure and the intercalation of the dodecyl sulfate anions, respectively. The water contact angles were measured on the dry pressed pellets prepared as targets for PLD depositions. Contact angle (CA) measurements were performed at room temperature using a Contact Angle Tensiometer CAM 200 from KSV Instruments. 2.2. Thin films The targets to be used in PLD experiments were dry pressed pellets obtained from the prepared Zn2.5Al, PZn2.5Al-DS and RZn2.5Al-DS powders. A Nd:YAG laser working at 1064 nm and having a 10 Hz pulse repetition rate was used. The PLD films were deposited at room temperature on silicon substrates following 12,000 pulses at fluences between 1 and 3 J/cm2 . MAPLE thin films of Zn2.5Al, PZn2.5Al-DS and RZn2.5Al-DS were obtained using the fourth harmonic of a Nd:YAG laser (266 nm) having a pulse width of 5 ns and working at a pulse repetition rate of 10 Hz. The laser fluence was 1–2 J/cm2 . The Zn2.5Al, PZn2.5AlDS and RZn2.5Al-DS powders (10% w/w) were mixed in water and ethanol (1:1), then frozen and used as targets. The films were grown on silicon substrates placed at 4 cm in front of the target and parallel to it following ablation by 80,000 pulses. The XRD patterns were collected in grazing incidence geometry (GI angle of 0.25◦ ). The FTIR spectra were recorded in the 400–4000 cm−1 range, with a resolution of 4 cm−1 and averaging upon 1024 scans. The surface morphology of the films was examined by atomic force microscopy (AFM), using a Park XE-100 system with silicon nitride cantilevers in non-contact mode. The water contact angles were measured using a KSV CAM101 optical microscope, with water drops of 0.5–1 L. 3. Results and discussion 3.1. Powders The XRD patterns of the powders prepared to be used as targets are presented in Fig. 1. The pristine ZnAl-LDH (sample Zn2.5Al) exhibits the typical pattern of a carbonate-layered double
Fig. 1. The XRD pattern of the pristine Zn2.5 Al, PZ2.5Al-DS and of the RZn2.5Al-DS powders.
Fig. 2. The FTIR spectra of the Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS powders.
hydroxide material (JCPDS card no. 048-1022) with an R3m rhombohedral symmetry and was Miller indexed in a hexagonal lattice. The XRD pattern of the PZ2.5Al-DS sample reveals a shift of all the basal reflections toward small angles, which, along with the appearance of higher order peaks, is indicative of dodecyl sulfate anion intercalation in the interlayer free space. The XRD pattern of the RZn2.5Al-DS sample displays a mixture between a dominant modified DS-LDH, with basal reflections shifted toward low angles, and a small amount of a carbonate LDH phase, thus marking a partial DS intercalation. The lattice parameters c, calculated as c = 3/2(d003 + 2d006 ), and a, expressed as a = 2d110 are listed in Table 1. Given the 0.48 nm thickness of the brucite-type layer [25], the interlayer free space values included in Table 1 are slighter smaller than the reported value of 2.08 nm of the DS molecule length [26,27], which indicates a tilted arrangement of the DS anions inside the LDH gallery. FTIR further confirms that the DS was successfully intercalated into the LDH, for both modified samples. In the pristine Zn2.5Al LDH structure, carbonate ion absorptions were revealed by the 1358 cm−1 asymmetric stretching mode (3), 829 cm−1 for the out of plane bending mode (2) and 693 cm−1 for the in plane bending mode (Fig. 2). The 3 and 2 bands are shifted to lower wavenumbers in comparison with IR vibrations positions of free CO3 2− anions in solution [26] due to a decrease in symmetry upon intercalation. After DS intercalation, the intensities of these absorption bands were reduced, in particular for the PZn2.5Al-DS powder. The result for the RZn2.5Al-DS sample is consistent with the presence of a small amount of unmodified LDH revealed by XRD measurement. The presence of the organic molecules in the modified LDH structures is further confirmed by the appearance of characteristic peaks at 2956, 2918 and 2850 cm−1 (ascribed to dodecyl sulfate counterions) corresponding to CH2 stretching vibrations, along with
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Table 1 The phase composition and the structural data of Zn2.5Al, PZn2.5Al-DS and RZn2.5Al-DS powders. The contact angle on pressed pellets used as targets in PLD depositions is included. Powder samples
Zn2.5Al PZn2.5Al-DS RZn2.5Al-DS
Phase composition
LDH phase DS-LDH phase Partial DS intercalation
Contact angle (◦ )
Structural data
Main phase: DS-LDH Secondary phase: LDH
a (nm)
c (nm)
IS (nm)
D00l (nm)
D110 (nm)
0.306 0.305 0.305 0.305
2.250 7.440 7.457 2.238
0.27 2.00 2.01 0.27
23 21 15 17
28 28 25 25
11 108 101
Fig. 3. The AFM images of Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS thin films grown by PLD and MAPLE. Contact angle is shown.
the bending vibration at 1468 cm−1 . The SO4 symmetric vibrations could also be evidenced in the 1197–1058 cm−1 region [27]. Some of carbonates are still present in the organo-modified LDS powder, observable for example by the presence of the peaks around 1360 cm−1 , which is the CO3 2− – asymmetric stretching mode (3). The peak is intense for the RZn2.5Al-DS powder due to the existence of the secondary unmodified carbonate-LDH phase. The presence of carbonate in the PZn2.5Al-DS sample in the absence of a carbonateLDH phase is due to the coexistence of DS and carbonate anions in the same interlayer space. The result is to be expected due to affinity of carbonate for LDHs. The water contact angles values measured on pressed pellets are comprised in Table 1, revealing the change from hydrophilic surface for Zn2.5Al (11◦ ) to a hydrophobic one for PZn2.5Al-DS and RZn2.5Al-DS (108◦ and 101◦ , respectively). 3.2. Thin films We compare the films deposited via the two techniques, PLD and MAPLE. The morphological aspect of the films depicted in the AFM images is characterized by rough surfaces, with big grains and large voids between them (Fig. 3). Unmodified Zn2.5Al and RZn2.5Al-DS films deposited via PLD display smaller grains and larger voids which could be associated with smaller contact angles. The other films exhibit a combination of small and large grains. This aspect apparently favored higher contact angles. The film deposited via PLD exhibit, in generally, slight higher thickness in
comparison with films deposited using MAPLE due to the dilution of the MAPLE target (Table 2). However, for these experiments, the numbers of pulses used for the deposition by MAPLE were high enough to obtain films having thicknesses comparable to those obtained by PLD. The roughness of the films, expressed as root mean square (RMS) deviation, shows smaller values for the film deposited from the pristine Zn2.5Al LDH target, irrespective to the technique used, while for organo-LDH composite films the roughness is considerable higher. It should be mentioned that due to these relatively high roughness, AFM analysis encountered difficulties in measuring such as unstable feedback, tip alteration during scanning, etc. The XRD patterns of the films deposited via PLD and MAPLE from the three LDH based targets are presented in Fig. 4a–c. Only the (0 0 l) reflections are visible in the XRD patterns of the films deposited from the pristine Zn2.5Al LDH target. Both techniques, PLD at 1064 nm and MAPLE at 266 nm produced oriented ZnAlLDH films, similar to MgAl-LDHs [18] or NiAl or CoAl-LDHs [28]. The peak intensities, recorded under the same conditions, are smaller in the case of MAPLE deposition due to smaller amount of deposited material/smaller thickness of film. The films deposited from the DS-intercalated LDH target prepared by co-precipitation, PZn2.5AlDS, exhibits only the basal reflections, shifted toward small angles, as a consequence of DS anions intercalation. The lower intensities for the MAPLE deposited film indicate a smaller film thickness. The films deposited from the organo-modified LDH by reconstruction, RZn2.5Al-DS, exhibit, in the case of PLD deposition, the same phase composition as the target, i.e. intercalated DS-LDH phase
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Table 2 The phase composition and the structural data of as deposited Zn2.5Al, PZn2.5Al-DS and RZn2.5Al-DS thin films deposited by laser techniques. The roughness, thickness (from AFM analysis) and the contact angle of the obtained films are presented. Thin films
Zn2.5Al PZn2.5Al-DS RZn2.5Al-DS
Deposition technique
PLD MAPLE PLD MAPLE PLD MAPLE
Phase composition
LDH-phase LDH-phase DS-LDH phase DS-LDH phase DS-LDH phase LDH-phase DS-LDH phase
XRD structural data c (nm)
IS (nm)
D00l (nm)
2.263 2.284 7.801 7.798 7.960 2.285 7.584
0.27 0.28 2.12 2.12 2.17 0.28 2.06
20 22 20 25 20 14 18
and an unmodified LDH phase. Both phases are c-oriented. In the case of MAPLE deposition, the amount of transferred material was not enough to evidence by XRD the eventual presence of a secondary unmodified LDH phase. The structural data are included in
Fig. 4. The XRD pattern of Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS thin films grown by PLD on silicon substrates at ablation wavelength 1064 nm from pellets target of Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS. The thin films deposited by MAPLE from Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS powders (10% w/w) dissolved and then frozen in water and ethanol (1:1). MAPLE deposition were done at wavelength 266 nm.
Roughness RSM (nm)
Thickness (nm)
Contact angle (◦ )
85 87 307 190 218
495 420 667 640 689
105 161 136 147 115
270
657
171
Table 2. The c lattice parameters are slightly higher for organo-LDH films in comparison with their corresponding targets, while for the pristine LDH films they remained almost unchanged. The effect evidenced that the strong electrostatic interaction between the sheets when carbonate anionic species and water molecules are present in the interlayer is weakened by the DS intercalation. The growth of oriented films increases the accessibility in the LDH gallery and
Fig. 5. The FTIR spectra of the Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS films for both PLD and MAPLE depositions.
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probably contributes to the enlargement of interlayer distances. This combination between intercalation and orientation increase the propensity of the films to change their hydrophilic properties or to easily be functionalized by co-intercalation of other organic molecules with appropriate functional groups. The FTIR spectra of the films are presented along with the FTIR spectrum of their corresponding targets. The FTIR spectra of the Zn2.5Al films (Fig. 4a) exhibit similar characteristic as the respective targets. The increased broadening and splitting indicate a slight alteration of the interlayer carbonate anions, probably due to the oriented growth. The effect is stronger for the MAPLE film. The broad band around 3200–3500 cm−1 , assigned to O H stretching, became broader and asymmetric, while the smaller shoulder around 3000 cm−1 , attributed to H-bonded O H stretching vibration [29], almost disappeared. The FTIR spectra of the PZn2.5Al-DS films, for both PLD and MAPLE depositions (Fig. 4b), almost mimic the PZn2.5Al-DS powder. The CH2 stretching bands, as well as symmetric vibration of S O characteristic for SDS, are clearly visible. The peaks assigned to carbonate counterions are almost absent, confirming the formation of an organo-LDH nanocomposite film. The DS intercalation in PZn2.5Al-DS films is nearly complete for both deposition techniques. The same comments could be delivered for the FTIR spectra of the RZn2.5Al-DS films (Fig. 4c). The CH2 stretching bands and symmetric vibration of S O are distinguishable. The CO3 2− asymmetric stretching mode is also visible in both cases meaning that although the carbonate-LDH phase is not detected by XRD in the MAPLE film, lack of a long range order, the secondary phase is present (Fig. 5). The contact angles are also included in Table 2. In general, superhydrophobic materials require a combination of a hierarchical micro/nanostructure and low-surface-energy. The roughness of the surface of all the films increases considerably the film contact angles, in comparison with the respective targets. The values are higher for all the samples obtained via MAPLE, more likely due to a higher homogeneity of film surface topology. During the deposition by PLD at 1064 nm, the wavelength penetration depth is higher and therefore larger cluster are ejected from the target and deposited onto the substrate. This mechanism secured the transfer of the very same material as the target, but could create sometimes non-homogenous surfaces. The DS-LDH films exhibit highly hydrophobic or superhydrophobic properties, except for the RZn2.5Al-DS film deposited by PLD. The lower angle contact for this sample can be explained by its non-homogeneity of the surface, probably as a result of the mixed phase composition of the target. 4. Conclusions Hybrid nanocomposites films could be prepared by laser techniques using both PLD at 1064 nm or MAPLE at 266 nm. Dodecyl sulfate intercalated ZnAl-LDH materials were transferred as thin films by PLD and/or MAPLE with the conservation of the organic guest anions in the interlayer space. The protective role of LDH’s gallery system could be considered for the transfer of other organic or even biological molecules of interest in thin film composition. All the films were well oriented. A combination of appropriate morphologies exhibiting high roughness and targets compositions
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having increased hydrophobicity allowed the obtaining of superhydrophobic films. PLD is a more time-efficient and facile deposition technique, but the eventual occurrence of non-homogeneous surface topographies is more difficult to eliminate through the adjustment of growth parameters. On the other hand, by using MAPLE we obtained films with the same composition and structure as their respective targets, as well as increases hydrophobicity. The organo-layered double hydroxides composite thin films could be used for further functionalizations, for example by anion exchange with proper functional molecules, due to the effect of DS intercalation which weakens the electrostatic force between the layers. Acknowledgments This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PCCA 137/2012. The authors thank Dr. Simona Brajnicov for the AFM analysis. References [1] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173–301. [2] M. Mokhtar, S.N. Basahel, Y.O. Al Angary, J. Alloys Compd. 493 (2010) 376–384. [3] R. Marangoni, M. Bouhent, C. Taviot-Gueho, F. Wypych, F. Leroux, J. Colloid Interface Sci. 333 (2009) 120–127. [4] J.J. Yu, J. Cheng, C.Y. Ma, H.L. Wang, L.D. Li, Z.P. Hao, Z.P. Xu, J. Colloid Interface Sci. 333 (2009) 423–430. [5] M. Halma, K.A.D.F. Castro, V. Prevot, C. Forano, F. Wypycn, S. Nakagaki, J. Mol. Catal. A: Chem. 310 (2009) 42–50. [6] S.P. Newman, W. Jones, New. J. Chem. 22 (1998) 105–155. [7] N. Nhlapo, T. Motumi, E. Landman, S.M.C. Verryn, W.W. Focke, J. Mater. Sci. 43 (2008) 1033–1043. [8] L. Moyo, N. Nhapo, W.W. Focke, J. Mater. Sci. 43 (2008) 6144–6158. [9] F.R. Costa, M. Saphianniakova, U. Wagenknecht, G. Heinrich, Adv. Polym. Sci. 210 (2008) 101–168. [10] U. Constantino, V. Ambrogi, M. Nocchetti, L. Perioli, Microporous Mesoporous Mater. 107 (2008) 149–160. [11] F. Zhang, L. Zhao, H. Chen, S. Xu, D.G. Evans, X. Duan, Angew. Chem. Int. Ed. 47 (2008) 2466–2469. [12] J. Wang, D. Li, X. Yu, X. Jing, M. Zhang, Z. Jiang, J. Alloys Compd. 494 (2010) 271–274. [13] L. Zang, Y. Lin, S. Xu, R. Li, X. Xheng, F. Zhang, Appl. Clay Sci. 48 (2010) 641–645. [14] D. Shan, S. Cosnier, C. Mousty, Anal. Chem. 75 (2003) 3872–3879. [15] X. Li, J. Liu, X. Ji, J. Jiang, R. Ding, Y. Hu, X. Huang, Sens. Actuators B 147 (2010) 241–247. [16] X. He, K. Kobayashi, M. Takahashi, G. Villemure, A. Yamagishi, Thin Solid Films 397 (2001) 255–265. [17] R.G. Buchheit, H. Huanm, J. Coat. Technol. Res. 1 (2004) 277–290. [18] A. Matei, R. Birjega, A. Nedelcea, A. Vlad, D. Colceag, M.D. Ionita, C. Luculescu, M. Dinescu, R. Zavoianu, O.D. Pavel, Appl. Surf. Sci. 257 (2011) 5308–5311. [19] A. Matei, R. Birjega, A. Vlad, C. Luculescu, G. Epurescu, F. Stokker-Cheregi, M. Dinescu, R. Zavoianu, O.D. Pavel, Appl. Phys. A 110 (2013) 841–846. [20] R. Birjega, A. Matei, B. Mitu, M.D. Ionita, M. Filipescu, F. Stokker-Cheregi, C. Luculescu, M. Dinescu, R. Zavoianu, O.D. Pavel, M.C. Corobea, Thin Solid Films 543 (2013) 63–68. [21] W.T. Reichle, Solid State Ionics 22 (1986) 135–141. [22] D. Carriazo, M. del Arco, E. Garcia-Lopez, G. Marci, C. Martin, L. Palmisano, V. Rives, J. Mol. Catal. A: Chem. 342–343 (2011) 83–90. [23] R. Birjega, O.D. Pavel, G. Costentin, M. Che, E. Angelescu, Appl. Catal. A: Gen. 288 (2005) 185–193. [24] B. Du, Z. Guo, Z. Fang, Polym. Degrad. Stab. 94 (2009) 1979–1985. [25] S. Miyata, Clays Clay Miner. 23 (1975) 369–376. [26] D.G. Evans, D.A. Xue, Chem. Commun. (2006) 485–496. [27] C. Yilmazet, U. Unal, H.Y. Acar, J. Solid State Chem. 187 (2012) 295–299. [28] R. Birjega, A. Matei, M. Filipescu, F. Stokker-Cheregi, C. Luculescu, D. Colceag, R. Zavoianu, O.D. Pavel, M. Dinescu, Appl. Surf. Sci. 278 (2013) 122–126. [29] P.S. Braterman, Z.P. Xu, J. Mater. Chem. 13 (2003) 268–273.
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Organo-layered double hydroxides composite thin films deposited by laser techniques R. Birjega a , A. Vlad a,∗ , A. Matei a , M. Dumitru a , F. Stokker-Cheregi a , M. Dinescu a , R. Zavoianu b , V. Raditoiu c , M.C. Corobea c a
National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Str., 77125 Bucharest-Magurele, Romania University of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, 4-12 Regina Elisabeta Bd., Bucharest 030018, Romania c National R.&D. Institute for Chemistry and Petrochemistry, ICECHIM, 202 Splaiul Independentei Str., CP-35-274, 060021 Bucharest, Romania b
a r t i c l e
i n f o
Article history: Received 13 June 2015 Received in revised form 9 December 2015 Accepted 12 December 2015 Available online xxx
a b s t r a c t We used laser techniques to create hydrophobic thin films of layered double hydroxides (LDHs) and organo-modified LDHs. A LDH based on Zn-Al with Zn2+ /Al3+ ratio of 2.5 was used as host material, while dodecyl sulfate (DS), which is an organic surfactant, acted as guest material. Pulsed laser deposition (PLD) and matrix assisted pulsed laser evaporation (MAPLE) were employed for the growth of the films. The organic anions were intercalated in co-precipitation step. The powders were subsequently used either as materials for MAPLE, or they were pressed and used as targets for PLD. The surface topography of the thin films was investigated by atomic force microscopy (AFM), the crystallographic structure of the powders and films was checked by X-ray diffraction. FTIR spectroscopy was used to evidence DS interlayer intercalation, both for powders and the derived films. Contact angle measurements were performed in order to establish the wettability properties of the as-prepared thin films, in view of functionalization applications as hydrophobic surfaces, owing to the effect of DS intercalation. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDHs) are a large class of inorganic x+ M3+ (OH2 )] [An− ]x/n materials, having as general formula, [M2+ 1−x x mH2 O where M2+ and M3+ are divalent and trivalent cations that occupy octahedral positions in the hydroxide layers, An− is an interlayer anion and x is the molar ratio [1]. The flexibility of their chemical composition allows the preparation of LDHs having a wide variety of properties, making them promising materials for applications in different fields such as catalysis, pollutants absorption, additives or precursors [2–5]. The anionic exchange capacity of LDHs is utilized to incorporate surfactant organic anions into the interlayer space in order to obtain organo-LDHs [1,6–8]. The ability to transform the LDH hydrophilic surface to hydrophobic and to access the interlayer region can extend the applicability of LDHs toward different types of functional nanocomposites [9–13]. The capacity of this new emerging class of multifunctional LDH based composite to preserve or to develop new properties as thin films is an open topic. Research on LDH films is expanding, in view of their applications as sensors
∗ Corresponding author. E-mail address: [email protected] (A. Vlad).
[14,15], electrodes [16], corrosion-resistant coatings [11,12,17] etc. Usually, the LDH films are prepared by two methods: physical deposition and in situ growth, i.e. substrate-induced growth. We have previously reported on the ability of pulsed laser deposition (PLD) and matrix assisted pulsed laser evaporation (MAPLE) [18–20] to produce adherent, well oriented LDH films. In particular, hybrid LDH/polymer structures were transferred onto silicon substrates using MAPLE [20]. The targets used in the above study are organoLDH composites accommodating the organic guest molecule in the interlayer space. In this study we used targets of pristine ZnAl based LDHs (Zn/Al molar ratio of 2.5) and intercalated dodecyl sulfateLDHs (DS-ZnAl-LDHs) to obtain composite films. We performed a comparative study of the performance of two laser techniques in producing films which preserve the complex structures of the targets and we discuss the derived wettability properties.
2. Materials and characterization methods 2.1. Powders All the chemicals used for the preparation of the ZnAl-LDHs and organo-modified LDHs were of analytical grade or of the highest purity commercially available.
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The pristine ZnAl-LDH powder was prepared via coprecipitation at 40 ◦ C under low supersaturation conditions, pH 9.5–10. Zn(NO3 )2 ·6H2 O, Al(NO3 )3 ·9H2 O anhydrous Na2 CO3 and NaOH were the raw materials. The same coprecipitation method described by Reichle [21] and Carriazo et al. [22] was used. The molar ratio of Zn/Al was 2.5. The steps followed to obtain the final LDH powder were similar to those used for the preparation of MgAl-LDHs [23]. In the following we will refer to this powder as Zn2.5Al. The organo-modified LDHs were prepared using two methods: direct coprecipitation and, reconstruction via the “memory effect”. The coprecipitation method used an adapted protocol reported by Du et al. [24]. The raw materials were Zn(NO3 )2 ·6H2 O, Al(NO3 )3 ·9H2 O (Zn/Al molar ratio of 2.5), and sodium dodecyl sulfate (SDS = NaC12 H25 SO4 , SDS/Al molar ratio of 1.5). Sodium hydroxide was used for pH adjustment (pH = 9.5–10). The final powder, labeled PZn2.5Al-DS, was obtained following the same procedure as for the Zn2.5Al powder. The DS intercalation via reconstruction occurred by immersing the mixed oxide powder derived from the gentle calcination (18 h at 460 ◦ C in air flow) of the Zn2.5Al powder in an aqueous solution of SDS (SDS/Al molar ratio of 1.5). The immersion occurred at room temperature for 24 h. The recovered solid powder was labeled RZn2.5Al-DS. X-ray diffraction – XRD (PANalytical X’Pert MPD system, ˚ and Fourier transform infrared spectroscopy – CuK␣ = 1.5418 A) FTIR (JASCO FTIR 6300 spectrometer equipped with ATR Specac Golden Gate unit) were the characterization techniques used to evidence the crystallographic structure and the intercalation of the dodecyl sulfate anions, respectively. The water contact angles were measured on the dry pressed pellets prepared as targets for PLD depositions. Contact angle (CA) measurements were performed at room temperature using a Contact Angle Tensiometer CAM 200 from KSV Instruments. 2.2. Thin films The targets to be used in PLD experiments were dry pressed pellets obtained from the prepared Zn2.5Al, PZn2.5Al-DS and RZn2.5Al-DS powders. A Nd:YAG laser working at 1064 nm and having a 10 Hz pulse repetition rate was used. The PLD films were deposited at room temperature on silicon substrates following 12,000 pulses at fluences between 1 and 3 J/cm2 . MAPLE thin films of Zn2.5Al, PZn2.5Al-DS and RZn2.5Al-DS were obtained using the fourth harmonic of a Nd:YAG laser (266 nm) having a pulse width of 5 ns and working at a pulse repetition rate of 10 Hz. The laser fluence was 1–2 J/cm2 . The Zn2.5Al, PZn2.5AlDS and RZn2.5Al-DS powders (10% w/w) were mixed in water and ethanol (1:1), then frozen and used as targets. The films were grown on silicon substrates placed at 4 cm in front of the target and parallel to it following ablation by 80,000 pulses. The XRD patterns were collected in grazing incidence geometry (GI angle of 0.25◦ ). The FTIR spectra were recorded in the 400–4000 cm−1 range, with a resolution of 4 cm−1 and averaging upon 1024 scans. The surface morphology of the films was examined by atomic force microscopy (AFM), using a Park XE-100 system with silicon nitride cantilevers in non-contact mode. The water contact angles were measured using a KSV CAM101 optical microscope, with water drops of 0.5–1 L. 3. Results and discussion 3.1. Powders The XRD patterns of the powders prepared to be used as targets are presented in Fig. 1. The pristine ZnAl-LDH (sample Zn2.5Al) exhibits the typical pattern of a carbonate-layered double
Fig. 1. The XRD pattern of the pristine Zn2.5 Al, PZ2.5Al-DS and of the RZn2.5Al-DS powders.
Fig. 2. The FTIR spectra of the Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS powders.
hydroxide material (JCPDS card no. 048-1022) with an R3m rhombohedral symmetry and was Miller indexed in a hexagonal lattice. The XRD pattern of the PZ2.5Al-DS sample reveals a shift of all the basal reflections toward small angles, which, along with the appearance of higher order peaks, is indicative of dodecyl sulfate anion intercalation in the interlayer free space. The XRD pattern of the RZn2.5Al-DS sample displays a mixture between a dominant modified DS-LDH, with basal reflections shifted toward low angles, and a small amount of a carbonate LDH phase, thus marking a partial DS intercalation. The lattice parameters c, calculated as c = 3/2(d003 + 2d006 ), and a, expressed as a = 2d110 are listed in Table 1. Given the 0.48 nm thickness of the brucite-type layer [25], the interlayer free space values included in Table 1 are slighter smaller than the reported value of 2.08 nm of the DS molecule length [26,27], which indicates a tilted arrangement of the DS anions inside the LDH gallery. FTIR further confirms that the DS was successfully intercalated into the LDH, for both modified samples. In the pristine Zn2.5Al LDH structure, carbonate ion absorptions were revealed by the 1358 cm−1 asymmetric stretching mode (3), 829 cm−1 for the out of plane bending mode (2) and 693 cm−1 for the in plane bending mode (Fig. 2). The 3 and 2 bands are shifted to lower wavenumbers in comparison with IR vibrations positions of free CO3 2− anions in solution [26] due to a decrease in symmetry upon intercalation. After DS intercalation, the intensities of these absorption bands were reduced, in particular for the PZn2.5Al-DS powder. The result for the RZn2.5Al-DS sample is consistent with the presence of a small amount of unmodified LDH revealed by XRD measurement. The presence of the organic molecules in the modified LDH structures is further confirmed by the appearance of characteristic peaks at 2956, 2918 and 2850 cm−1 (ascribed to dodecyl sulfate counterions) corresponding to CH2 stretching vibrations, along with
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Table 1 The phase composition and the structural data of Zn2.5Al, PZn2.5Al-DS and RZn2.5Al-DS powders. The contact angle on pressed pellets used as targets in PLD depositions is included. Powder samples
Zn2.5Al PZn2.5Al-DS RZn2.5Al-DS
Phase composition
LDH phase DS-LDH phase Partial DS intercalation
Contact angle (◦ )
Structural data
Main phase: DS-LDH Secondary phase: LDH
a (nm)
c (nm)
IS (nm)
D00l (nm)
D110 (nm)
0.306 0.305 0.305 0.305
2.250 7.440 7.457 2.238
0.27 2.00 2.01 0.27
23 21 15 17
28 28 25 25
11 108 101
Fig. 3. The AFM images of Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS thin films grown by PLD and MAPLE. Contact angle is shown.
the bending vibration at 1468 cm−1 . The SO4 symmetric vibrations could also be evidenced in the 1197–1058 cm−1 region [27]. Some of carbonates are still present in the organo-modified LDS powder, observable for example by the presence of the peaks around 1360 cm−1 , which is the CO3 2− – asymmetric stretching mode (3). The peak is intense for the RZn2.5Al-DS powder due to the existence of the secondary unmodified carbonate-LDH phase. The presence of carbonate in the PZn2.5Al-DS sample in the absence of a carbonateLDH phase is due to the coexistence of DS and carbonate anions in the same interlayer space. The result is to be expected due to affinity of carbonate for LDHs. The water contact angles values measured on pressed pellets are comprised in Table 1, revealing the change from hydrophilic surface for Zn2.5Al (11◦ ) to a hydrophobic one for PZn2.5Al-DS and RZn2.5Al-DS (108◦ and 101◦ , respectively). 3.2. Thin films We compare the films deposited via the two techniques, PLD and MAPLE. The morphological aspect of the films depicted in the AFM images is characterized by rough surfaces, with big grains and large voids between them (Fig. 3). Unmodified Zn2.5Al and RZn2.5Al-DS films deposited via PLD display smaller grains and larger voids which could be associated with smaller contact angles. The other films exhibit a combination of small and large grains. This aspect apparently favored higher contact angles. The film deposited via PLD exhibit, in generally, slight higher thickness in
comparison with films deposited using MAPLE due to the dilution of the MAPLE target (Table 2). However, for these experiments, the numbers of pulses used for the deposition by MAPLE were high enough to obtain films having thicknesses comparable to those obtained by PLD. The roughness of the films, expressed as root mean square (RMS) deviation, shows smaller values for the film deposited from the pristine Zn2.5Al LDH target, irrespective to the technique used, while for organo-LDH composite films the roughness is considerable higher. It should be mentioned that due to these relatively high roughness, AFM analysis encountered difficulties in measuring such as unstable feedback, tip alteration during scanning, etc. The XRD patterns of the films deposited via PLD and MAPLE from the three LDH based targets are presented in Fig. 4a–c. Only the (0 0 l) reflections are visible in the XRD patterns of the films deposited from the pristine Zn2.5Al LDH target. Both techniques, PLD at 1064 nm and MAPLE at 266 nm produced oriented ZnAlLDH films, similar to MgAl-LDHs [18] or NiAl or CoAl-LDHs [28]. The peak intensities, recorded under the same conditions, are smaller in the case of MAPLE deposition due to smaller amount of deposited material/smaller thickness of film. The films deposited from the DS-intercalated LDH target prepared by co-precipitation, PZn2.5AlDS, exhibits only the basal reflections, shifted toward small angles, as a consequence of DS anions intercalation. The lower intensities for the MAPLE deposited film indicate a smaller film thickness. The films deposited from the organo-modified LDH by reconstruction, RZn2.5Al-DS, exhibit, in the case of PLD deposition, the same phase composition as the target, i.e. intercalated DS-LDH phase
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Table 2 The phase composition and the structural data of as deposited Zn2.5Al, PZn2.5Al-DS and RZn2.5Al-DS thin films deposited by laser techniques. The roughness, thickness (from AFM analysis) and the contact angle of the obtained films are presented. Thin films
Zn2.5Al PZn2.5Al-DS RZn2.5Al-DS
Deposition technique
PLD MAPLE PLD MAPLE PLD MAPLE
Phase composition
LDH-phase LDH-phase DS-LDH phase DS-LDH phase DS-LDH phase LDH-phase DS-LDH phase
XRD structural data c (nm)
IS (nm)
D00l (nm)
2.263 2.284 7.801 7.798 7.960 2.285 7.584
0.27 0.28 2.12 2.12 2.17 0.28 2.06
20 22 20 25 20 14 18
and an unmodified LDH phase. Both phases are c-oriented. In the case of MAPLE deposition, the amount of transferred material was not enough to evidence by XRD the eventual presence of a secondary unmodified LDH phase. The structural data are included in
Fig. 4. The XRD pattern of Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS thin films grown by PLD on silicon substrates at ablation wavelength 1064 nm from pellets target of Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS. The thin films deposited by MAPLE from Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS powders (10% w/w) dissolved and then frozen in water and ethanol (1:1). MAPLE deposition were done at wavelength 266 nm.
Roughness RSM (nm)
Thickness (nm)
Contact angle (◦ )
85 87 307 190 218
495 420 667 640 689
105 161 136 147 115
270
657
171
Table 2. The c lattice parameters are slightly higher for organo-LDH films in comparison with their corresponding targets, while for the pristine LDH films they remained almost unchanged. The effect evidenced that the strong electrostatic interaction between the sheets when carbonate anionic species and water molecules are present in the interlayer is weakened by the DS intercalation. The growth of oriented films increases the accessibility in the LDH gallery and
Fig. 5. The FTIR spectra of the Zn2.5 Al, PZ2.5Al-DS and RZn2.5Al-DS films for both PLD and MAPLE depositions.
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probably contributes to the enlargement of interlayer distances. This combination between intercalation and orientation increase the propensity of the films to change their hydrophilic properties or to easily be functionalized by co-intercalation of other organic molecules with appropriate functional groups. The FTIR spectra of the films are presented along with the FTIR spectrum of their corresponding targets. The FTIR spectra of the Zn2.5Al films (Fig. 4a) exhibit similar characteristic as the respective targets. The increased broadening and splitting indicate a slight alteration of the interlayer carbonate anions, probably due to the oriented growth. The effect is stronger for the MAPLE film. The broad band around 3200–3500 cm−1 , assigned to O H stretching, became broader and asymmetric, while the smaller shoulder around 3000 cm−1 , attributed to H-bonded O H stretching vibration [29], almost disappeared. The FTIR spectra of the PZn2.5Al-DS films, for both PLD and MAPLE depositions (Fig. 4b), almost mimic the PZn2.5Al-DS powder. The CH2 stretching bands, as well as symmetric vibration of S O characteristic for SDS, are clearly visible. The peaks assigned to carbonate counterions are almost absent, confirming the formation of an organo-LDH nanocomposite film. The DS intercalation in PZn2.5Al-DS films is nearly complete for both deposition techniques. The same comments could be delivered for the FTIR spectra of the RZn2.5Al-DS films (Fig. 4c). The CH2 stretching bands and symmetric vibration of S O are distinguishable. The CO3 2− asymmetric stretching mode is also visible in both cases meaning that although the carbonate-LDH phase is not detected by XRD in the MAPLE film, lack of a long range order, the secondary phase is present (Fig. 5). The contact angles are also included in Table 2. In general, superhydrophobic materials require a combination of a hierarchical micro/nanostructure and low-surface-energy. The roughness of the surface of all the films increases considerably the film contact angles, in comparison with the respective targets. The values are higher for all the samples obtained via MAPLE, more likely due to a higher homogeneity of film surface topology. During the deposition by PLD at 1064 nm, the wavelength penetration depth is higher and therefore larger cluster are ejected from the target and deposited onto the substrate. This mechanism secured the transfer of the very same material as the target, but could create sometimes non-homogenous surfaces. The DS-LDH films exhibit highly hydrophobic or superhydrophobic properties, except for the RZn2.5Al-DS film deposited by PLD. The lower angle contact for this sample can be explained by its non-homogeneity of the surface, probably as a result of the mixed phase composition of the target. 4. Conclusions Hybrid nanocomposites films could be prepared by laser techniques using both PLD at 1064 nm or MAPLE at 266 nm. Dodecyl sulfate intercalated ZnAl-LDH materials were transferred as thin films by PLD and/or MAPLE with the conservation of the organic guest anions in the interlayer space. The protective role of LDH’s gallery system could be considered for the transfer of other organic or even biological molecules of interest in thin film composition. All the films were well oriented. A combination of appropriate morphologies exhibiting high roughness and targets compositions
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having increased hydrophobicity allowed the obtaining of superhydrophobic films. PLD is a more time-efficient and facile deposition technique, but the eventual occurrence of non-homogeneous surface topographies is more difficult to eliminate through the adjustment of growth parameters. On the other hand, by using MAPLE we obtained films with the same composition and structure as their respective targets, as well as increases hydrophobicity. The organo-layered double hydroxides composite thin films could be used for further functionalizations, for example by anion exchange with proper functional molecules, due to the effect of DS intercalation which weakens the electrostatic force between the layers. Acknowledgments This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PCCA 137/2012. The authors thank Dr. Simona Brajnicov for the AFM analysis. References [1] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today 11 (1991) 173–301. [2] M. Mokhtar, S.N. Basahel, Y.O. Al Angary, J. Alloys Compd. 493 (2010) 376–384. [3] R. Marangoni, M. Bouhent, C. Taviot-Gueho, F. Wypych, F. Leroux, J. Colloid Interface Sci. 333 (2009) 120–127. [4] J.J. Yu, J. Cheng, C.Y. Ma, H.L. Wang, L.D. Li, Z.P. Hao, Z.P. Xu, J. Colloid Interface Sci. 333 (2009) 423–430. [5] M. Halma, K.A.D.F. Castro, V. Prevot, C. Forano, F. Wypycn, S. Nakagaki, J. Mol. Catal. A: Chem. 310 (2009) 42–50. [6] S.P. Newman, W. Jones, New. J. Chem. 22 (1998) 105–155. [7] N. Nhlapo, T. Motumi, E. Landman, S.M.C. Verryn, W.W. Focke, J. Mater. Sci. 43 (2008) 1033–1043. [8] L. Moyo, N. Nhapo, W.W. Focke, J. Mater. Sci. 43 (2008) 6144–6158. [9] F.R. Costa, M. Saphianniakova, U. Wagenknecht, G. Heinrich, Adv. Polym. Sci. 210 (2008) 101–168. [10] U. Constantino, V. Ambrogi, M. Nocchetti, L. Perioli, Microporous Mesoporous Mater. 107 (2008) 149–160. [11] F. Zhang, L. Zhao, H. Chen, S. Xu, D.G. Evans, X. Duan, Angew. Chem. Int. Ed. 47 (2008) 2466–2469. [12] J. Wang, D. Li, X. Yu, X. Jing, M. Zhang, Z. Jiang, J. Alloys Compd. 494 (2010) 271–274. [13] L. Zang, Y. Lin, S. Xu, R. Li, X. Xheng, F. Zhang, Appl. Clay Sci. 48 (2010) 641–645. [14] D. Shan, S. Cosnier, C. Mousty, Anal. Chem. 75 (2003) 3872–3879. [15] X. Li, J. Liu, X. Ji, J. Jiang, R. Ding, Y. Hu, X. Huang, Sens. Actuators B 147 (2010) 241–247. [16] X. He, K. Kobayashi, M. Takahashi, G. Villemure, A. Yamagishi, Thin Solid Films 397 (2001) 255–265. [17] R.G. Buchheit, H. Huanm, J. Coat. Technol. Res. 1 (2004) 277–290. [18] A. Matei, R. Birjega, A. Nedelcea, A. Vlad, D. Colceag, M.D. Ionita, C. Luculescu, M. Dinescu, R. Zavoianu, O.D. Pavel, Appl. Surf. Sci. 257 (2011) 5308–5311. [19] A. Matei, R. Birjega, A. Vlad, C. Luculescu, G. Epurescu, F. Stokker-Cheregi, M. Dinescu, R. Zavoianu, O.D. Pavel, Appl. Phys. A 110 (2013) 841–846. [20] R. Birjega, A. Matei, B. Mitu, M.D. Ionita, M. Filipescu, F. Stokker-Cheregi, C. Luculescu, M. Dinescu, R. Zavoianu, O.D. Pavel, M.C. Corobea, Thin Solid Films 543 (2013) 63–68. [21] W.T. Reichle, Solid State Ionics 22 (1986) 135–141. [22] D. Carriazo, M. del Arco, E. Garcia-Lopez, G. Marci, C. Martin, L. Palmisano, V. Rives, J. Mol. Catal. A: Chem. 342–343 (2011) 83–90. [23] R. Birjega, O.D. Pavel, G. Costentin, M. Che, E. Angelescu, Appl. Catal. A: Gen. 288 (2005) 185–193. [24] B. Du, Z. Guo, Z. Fang, Polym. Degrad. Stab. 94 (2009) 1979–1985. [25] S. Miyata, Clays Clay Miner. 23 (1975) 369–376. [26] D.G. Evans, D.A. Xue, Chem. Commun. (2006) 485–496. [27] C. Yilmazet, U. Unal, H.Y. Acar, J. Solid State Chem. 187 (2012) 295–299. [28] R. Birjega, A. Matei, M. Filipescu, F. Stokker-Cheregi, C. Luculescu, D. Colceag, R. Zavoianu, O.D. Pavel, M. Dinescu, Appl. Surf. Sci. 278 (2013) 122–126. [29] P.S. Braterman, Z.P. Xu, J. Mater. Chem. 13 (2003) 268–273.
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