Promising Piezoelectric Properties of New ZnO@Octadecylamine

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Promising Piezoelectric Properties Of New ZnO@Octadecylamine Adduct Simona Bettini, Rosanna Pagano, Valentina Bonfrate, Emanuela Maglie, Daniela Manno, Antonio Serra, Ludovico Valli, and Gabriele Giancane J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06013 • Publication Date (Web): 10 Aug 2015 Downloaded from http://pubs.acs.org on August 18, 2015

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The Journal of Physical Chemistry

Promising Piezoelectric Properties Of New ZnO@Octadecylamine Adduct Simona Bettini, Rosanna Pagano, Valentina Bonfrate, Emanuela Maglie, Daniela Manno, Antonio Serra, Ludovico Valli, Gabriele Giancane* a

Department of Biological and Environmental Sciences and Technologies, University of Salento, Via per Monteroni, I-73100, Lecce, Italy b

Department of Engineering for Innovation, Via per Monteroni, I-73100 Lecce, Italy

c

Department of Cultural Heritage, Via D. Birago 84, I-73100 Lecce

ABSTRACT: A supramolecular adduct formed by the interaction among octadecylamine (ODA) and zinc oxide nanostructures was promoted. A stable dispersion of the ZnO@ODA adduct was obtained and characterized by means of thermogravimetric analysis, infrared and Raman spectroscopy. Then, the supramolecular adduct was spread at the air/water interface of a Langmuir trough. The presence of octadecylamine gave amphiphilic features to the ZnO@ODA adduct that was transferred from the air/water interface to solid substrates by Langmuir-Schaefer (LS) method. The transferred film was characterized by tunnel electron microscopy that highlighted rectangular well-shaped structures assembled by nanostructure of ZnO arranged in an ODA matrix. Piezoelectric feature of large LS film (1 cm2) was tested and a very promising response was observed as a consequence of the application of a pressure of 1 kPa.

INTRODUCTION Piezoelectric effect was discovered in 1880 by Pierre and Paul-Jacques Curie1. Compression of crystals without a symmetry center can induce the formation of a negative charge on a crystal face and a positive one on the other side of the crystal. As a consequence of such an anisotropy, a bias is generated and an electrical current can flow on an external load. Most common piezoelectric materials are represented by SiO2 quartz,2 gallium orthophosphate (GaPO4), tourmaline,3 barium titanate (BaTiO3) 4 and aluminium nitride.5 In addition to these materials, polyvinylidene fluoride (PVDF) 6 and zinc oxide (ZnO) are largely employed 7, 8 In particular, ZnO appears very appealing for its biocompatibility that proposes such a material for application in the biomedical field 9, in addition to the more traditional uses as pressure sensor or transduction element. ZnO is wurtzite structured material that shows a piezoelectric response along (0001) axis 10 and it is usually deposited by sputtering.11 Many deposition methods are used to obtain ZnO thin films, such as sol-gel processes,12 chemical vapor deposition, atomic layer deposition, molecular beam epitaxy 13. In order to decrease crystal defects and to improve the quality of the grains, high temperature deposition is often used. Anyway, the deposition process is a crucial step to obtain crystalline thin films with good piezoelectric response. In the present paper, we propose the synthesis, functionalization and deposition of ZnO-based thin film by means of the Langmuir-Schaefer (LS) method 14, where octadecylamine@ZnO (ZnO@ODA) adduct is formed and transferred on solid supports. LS technique allows an accurate control of the deposition parameters and it permits to transfer uniform films on a wide range of substrates 15. The possibility to obtain thin piezoelectric films covering a large area by means of a simple and fast method can represent a very intriguing chance to realize devices on square centimeter area that can be employed in many applications. For example, a 1 cm2 of piezoelectric thin film could be used as a pressure sensor for touch-screen, as active tactile sensor system (human-like skin), as implantable device for biomedical applications. 16-18

RESULTS AND DUSCUSSIONS Characterization of ZnO nanostructure and ZnO@ODA adduct In this work we employed a reported controlled precipitation method to obtain zinc oxide nano-adducts. 19 The method involves fast and spontaneous reduction of a solution of a zinc salt using a reducing agent, to limit the growth of particles with specified dimensions, followed by precipitation of the hydroxide precursor of ZnO from the solution. The process is controlled by parameters such as temperature, concentration of reagents and time of precipitation. In the first step of the synthesis procedure ZnO precursor is formed. In the next step, such a precursor undergoes thermal treatment and it is

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converted into ZnO nanocrystals. In addition, ZnO crystals synthesis was also performed in presence of ODA, in order to make the obtained adduct able to be transferred onto a solid substrate by means of the Langmuir-Schaefer technique. In order to check the formation of ZnO during the synthesis, the obtained white powder at different times was characterized by Raman spectroscopy. More in particular, figure 1 shows Raman spectra of the nanoadduct before and after the thermal treatment at 100°C for 3 and 6 hours. Spectra evidence the five active Raman modes of ZnO 20, that resulted more intense and resolved after the second step of synthesis, with a relevant dependence on thermal treatment time. In details, in both gray and light gray spectra, the signal located at 450 cm-1 became more intense and well-defined. Such a peak was ascribed to the E2H vibration mode of the characteristic wurtzite hexagonal phase band of ZnO nanostructure, with the orientation in the c-axis 21-22, highlighting the formation of this kind of nanocrystalline structure. On the other hand, this peak appears broader and less intense in the spectrum of the sample obtained before the annealing procedure (black line, figure 1), confirming the presence of just the ZnO precursor at this point. Furthermore, two weak signals are shown between 325 -355 cm-1 and 375-380 cm-1 that can be assigned to E2H-E2L (multiphonon) and A1T modes, respectively 22. In the same way, these two peaks appeared well-defined only after the thermal treatment as well as the E2H vibration mode (gray and light gray spectra, figure 1a). Similar trend can be pointed out regarding the signal centered between 100-115 cm1 , imputable to E2L vibration mode 23, stronger in the sample after the thermal treatment (gray spectra in figure 1). It is worth to observe that the signal centered at around 560 cm-1, that can be attributed to a combination of A1 and E1 vibration modes 21-22, appears broad and weak in all the spectra. The weak intensity of this peak is related to the presence of an ordered wurtzite structure, as also confirmed by the 450 cm-1 peak resolution 21, 24. Such high structured crystal conformation was further confirmed and deeper studied by means of TEM analysis. Figure 1b depicts Raman spectra of ODA and the ZnO@ODA adduct as obtained after 6 hours at 100 °C (gray line). Also in this case, the typical ZnO Raman profile evidences the formation of ZnO. The signal at about 450 cm-1 became stronger after the thermal treatment, confirming wurtzite generation even in presence of octadecylamine. Moreover, ODA Raman peaks appear unchanged in the ZnO@ODA adduct spectrum, suggesting that the octadecylamine molecules were not affected by the thermal treatment as well as embedded in the nanocrystalline adduct.

Figure 1. a) Raman spectra of the ZnO precursor of (black line); ZnO powder after 3 hours thermal treatment (gray line) and ZnO powder after 6 hours thermal treatment (light gray line). b) Raman spectra of ZnO@adduct (gray line) and ODA (black line). The evidence of the interaction between octadecylamine and ZnO nanostructure was studied by FTIR (figure S1). In the region of high wavenumbers, asymmetric and symmetric stretching modes of primary ODA amine are present at 3330 cm1 and 3255 cm-1, respectively. Both bands appeared shifted after the formation of the supramolecular adduct. The interaction between the organic alkyl amine and metal oxide nanostructure seems to be driven by the ZnO chelation due to the primary amine group of ODA 25-26. A further confirmation of the formation of non-covalent bounds among amines and

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ZnO is given by the presence of both asymmetric and symmetric –NH stretching modes in the ZnO@ODA adduct spectrum. A covalent bound should induce the formation of a secondary amine characterized by a single stretching mode. Thermogravimetry analysis (TGA) were performed on octadecylamine (figure 2a), zinc oxide nanostructure and ZnO@ODA adduct (figures 2b and 2c, respectively). The typical profile of an organic compound was observed for the ODA samples, a complete combustion with a total mass loss of the aliphatic amine is obtained for temperature higher than 400 °C. ZnO thermogravimetric profile shows a first mass loss corresponding to water evaporation (in the range of temperatures comprised between 80 °C and 140 °C) and a second one of 1% up to 400 °C, corresponding with to zinc hydroxide specie loss 27-28. TGA curve profile of ZnO@ODA adduct did not perfectly correspond to a linear combination of the curve of ODA and ZnO, demonstrating an interaction between the two species. Furthermore, the percent of ODA in the ZnO@ODA adduct represents the 40% of the entire mass of the synthesized supramolecular species.

Figure 2. Thermogravimetric analysis profiles carried out on ODA (box a), ZnO (box b) and ZnO@ODA adduct (box c). Thin film characterization ZnO nanoparticles were suspended in CHCl3 with a concentration of 0.14 mg/ml and spread, by means of a gas tight syringe, on the ultrapure water subphase of the Langmuir trough (figure S2). The spread solution was compressed by means of two Teflon barriers with a compression rate of 5 mm min -1. The low surface activity of ZnO nanostructure was highlighted recording the surface pressure vs through area curve of the spread film. A surface pressure of only 4 mN/m was reached by the barriers compression, suggesting that the ZnO nanostructures diffuse in the water subphase. The experiments were repeated by spreading a chloroform suspension of ZnO@ODA adduct (0.1 mg/mL) on the ultrapure subphase. A maximum surface pressure value of 20 mN/m was reached in the correspondent surface pressure vs trough area curve, unequivocally demonstrating the surface activity of the ZnO@ODA adduct. The ZnO@ODA adduct floating film was transferred by means of LS method on different supports, such as Si/SiO2 for AFM and ellipsometry, quartz for piezoelectric measurements, gold for IR and Raman measurements. Furthermore, the transfer was repeated on MICA and PDMS supports, confirming the high versatility of our approach. Figure 3 show the XRD spectra (picture 3a) and the SAED pattern (picture 3c). The (002) and (101) peaks of ZnO crystals are evident in both figures. Regarding the relative intensity of the reflections recorded in the XRD spectrum (figure 3a), a ratio between I101 and I002 of 5 was obtained, on the contrary the reflection (100) is absent. In the ZnO polycrystalline material I101/I002 = 39.77 and I101/I100 = 52.57 29. It is worth to stress that the nanocrystalline grains have a preferential (101) orientation and, as shown in figure 3b, also arranged onto the wafer surface with the c-axis parallel to the surface, as also reported in the literature 30. The SAED pattern (figure 3c) shows a weak contribution (002) planes and a strong reflection originated from the (101) lattice planes, all absolutely consistent with the X-ray diffraction. In addition, diffraction maxima do not form complete rings, as in the case of randomly arranged crystallites, but form well-defined and localized arches, suggesting the strong texturing of the obtained material.

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Figure 3 XRD spectra (picture a) and SAED pattern (box c). In the box b) a schematic representation of the LS film of ZnO@ODA adduct crystallite orientation is proposed. Figure 4a shows that the crystallites self-assembled into well-defined rectangular islands on the substrate. It should be noted that the preferential crystallographic orientation, evidenced by X-ray diffraction and electron diffraction, causes the observed morphological organization. Figure 4b depicts magnification of the highlighted area selected in figure 4a. The alignment of the partially welded nanocrystals into rectangular domains is shown and indeed confirms the texturing evidenced by electron diffraction.

Figure 4 Regular-shaped islands of ZnO@ODA adduct on solid substrate (box a). In figure b a magnification of such a structure is reported. Statistical evaluations of morphological features (picture c, d, e) are reported Both the size of ZnO nanocrystals rectangular islands was determined by the EMIP software (HITACHI High Technology corporation). As clearly evidenced by figure 4c and 4d, the islands have an average size of about 900 x 325 nm2. The ZnO nanocrystals have an average diameter = 24 nm with a standard deviation σ = 8 nm (figure 4e). Ellipsometry measurements were carried out on samples prepared by 2, 5 and 10 LS runs. The experimental data were fitted supposing a mixed matrix of octadecylamine and ZnO. The inorganic moiety of the hybrid film was fitted by Forouhi-Bloomer model 31 and the ODA contribute was fitted by a Cauchy model 32. The correlation between the two species was carried out using a Maxwell-Garnett model 33 (in figure S3a the simulated and experimental data are reported). The thickness of the film of ZnO@ODA adduct obtained by 2 LS runs was estimated to be (65±4) nm, which implying an average thickness of 32 nm for each LS run and a ratio of 56% of ODA was estimated. The difference between the ODA percent estimated by TGA measurement and the value simulated by means of ellipsometry spectroscopy can be rationale supposing that an excess of ZnO diffuse in the water subphase during the Langmuir experiment. Then, a higher ratio of ODA on solid samples is revealed. The linear correlation between the film thickness and the number of LS runs is report-

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ed in figure S3b and it testifies the good control of deposition procedure ensured by our approach. Furthermore, according with TEM observation, we can suppose that nanosheets of ZnO@ODA adduct were transferred on the solid substrate and an almost bi-dimensional plate was initially formed at the air/water interface and then transferred on the solid substrate. The good covering of the substrate surface increasing the number of layers prompted us to check the film piezoelectric properties. When the naked ZnO nanostructures and the drop casted ZnO@ODA adduct were transferred onto the substrates, no piezoelectric effect was obtained as a consequence of the not complete covering of the surface. On the contrary, ZnO@ODA film transferred on a quartz substrate by 10 runs LS exhibited a relevant piezoelectric effect. In figure 5a, a typical measured voltage response of a ZnO LS film under cycling pressure of 1 kPa at 0.1 Hz is shown. During the compression phase of the film, the measurements show periodic alternation of positive and negative output peaks, corresponding to the application and release of pressure stress, respectively (figure 5b). Tests up to 1000 cycles of pressure and relaxing revealed no significant changes in output voltage or current. The piezoelectric response of the ZnO@ODA adduct solid film was checked at two different frequencies of pressure application (figure S4). Mechanical strain can induce a polarization due to piezoelectric bound charges, which results in a potential difference at the two ends of the film. Since the film has a large internal resistance, we can reasonably assume that the system has a little charge leakage/loss during the short period of the strain-hold-release experiments 6. During the strain process, the piezoelectric bound charges induced an internal potential in the film layers. In response, the external free charges are moved to the film to neutralize this potential. At a constant strain, both the charges and the internal potential decrease up to zero as the piezoelectric bound charges are balanced by the free charges. When the strain is released, the piezoelectric potential decreases and the free charges accumulated at both ends of the film produce a contrary potential. The free charges slowly flow back in a contrary direction to the accumulation process, and the current decreases to zero. This behaviors can be well interpreted by a conventional piezoelectric model, considering a resultant component of piezoactive dipoles projected along the film surface, as induced by the deformation. For an applied compression normal to the surface of the film, the following relation then gives the maximum output voltage value 34 V=Ap where the parameter A is function of elastic, piezoelectric and dielectric constants and for the examined devices the ratio value is in the range of 1-3×10-3 V/Pa (figure 5c).

Figure 5. a) Voltage response of the ZnO@ODA adduct solid film under cycling pressure of 1 kPa at 0.1 Hz. b) Detail of the output voltage following the process of strain and release in the piezoelectric tests. c) Linear correlation between the applied pressure and the measured voltage. Points represent the experimental data, continuous line is the linear fit of the measured values. It is worth to observe that 1 kPa corresponds with 10 grams/cm2, lower than the average pressure of a human finger touch (about 40 grams/cm2). These evidences propose the LS film of the supramolecular adduct as transduction element for touch-screen of electronic devices, in biomedical application and nanogenerators.

CONCLUSIONS

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A simple method was used to synthetize zinc oxide and to supramolecular functionalize the ZnO nanostructures by an organic amphiphilic compound, that is octadecylamine. The adduct was characterized by FTIR and Raman spectroscopy highlighting the wurtzite crystal form of the ZnO and the role of amino group of octadecylamine in the capping of zinc oxide. Furthermore, the presence of ODA allowed to form a Langmuir film on a water subphase and to transfer the floating layer of ZnO@ODA adduct on different solid substrates (gold, mylar, PDMS, quartz and silicon dioxide) by means of LS method. Deposited films were characterized by ellipsometry, TEM and XRD pointing out the presence of large (some micrometer in length) and flat (about 32 nm for each LS run) aggregates. Then, the piezoelectric features of films of the ZnO@ODA adduct transferred 10 LS runs were tested and a very interesting response was recorded. The easiness of the procedure used to form the piezoelectric supramolecular adduct, the versatility of deposition method and the large area covered by the film all confer to the ZnO@ODA LS layer very intriguing properties as piezoelectric transductor or energy nanogenerator converting a pressure (mechanical energy) in electric impulses.

EXPERIMENTAL SECTION Zinc sulfate heptahydrate, sodium hydroxide and octadecylamine (ODA) were purchased from Sigma Aldrich®, and were used as received without further purification. Ultrapure MilliQ grade water was used during the synthesis. Nanocrystalline ZnO was prepared by means of a modified precipitation method reported in the literature. 19 Sodium hydroxide solution (100 mM) was added dropwise to a 50 mM zinc sulfate solution and reaction was carried out under vigorous stirring for 12h at room temperature. The obtained precipitate was washed several times with MilliQ grade water by centrifugation at 3000 rpm for 5 min. Finally the precipitate was dried in an oven at 100 °C for 6 h. For the synthesis of ZnO@ODA adduct, the organic compound and ZnSO4 were dissolved in a 2-propanol/milliQ grade water mixture (50:50) at the starting point. In this case the final washings were performed with chloroform. Thermogravimetric analysis (TGA) was performed by a Mettler Toledo TGA/DSC1 Star-e System instrument using Stare software. The heating rate was 10 °C/min under a constant oxygen flow. Dry samples were scanned from 25°C to 800°C. A Horiba MicroRaman Xplora (laser at 785 nm, power 0.7 mW) and a FTIR Spectrum One (Perkin Elmer) in ATR mode were used to characterize the synthetized ZnO@ODA adduct. Langmuir curves were obtained by means of a NIMA trough apparatus (area of 450 cm2) and by spreading 120 µL of ZnO@ODA adduct chloroform suspension (0.1 mg/mL) onto an ultrapure water subphase. After the chloroform evaporation, the floating film was compressed at the air/water interface at a speed of 5 mm/min. The floating film was transferred on the solid supports by means of Langmuir–Schaefer (LS) technique, the horizontal variation of Langmuir–Blodgett method. The substrates were horizontally lifted on the water subphase and a multilayered film was obtained on the solid substrates. A transmission electron microscope Hitachi 7700 was used to perform Transmission Electron Microscopy (TEM) analysis. In order to characterize with the highest accuracy the morphological and structural features of the material, a bilayer film has been prepared for TEM observations. Moreover, the film was protected with a thin layer of amorphous carbon, then was chemically removed from the substrates after exposure to hydrofluoric acid vapours and finally deposited onto suitable grids. To determine the crystalline structure of the nanoparticles, small area electron diffraction (SAED) was obtained by selecting a suitable spot size, convergence angle and condenser aperture to get the diffraction patterns from chosen area of about 1000 nm in diameter with an approximately parallel beam. X-ray Diffraction (XRD) data were collected using a Rigaku Miniflex diffractometer, operating in step-scan mode and employing Cu Kα radiation at 30 kV and 100 mA. The measurements were collected from 25 to 80°, with a 65° step size 0.02° and a scan speed of 0.25° min-1. The acquisition of the signals output voltage deriving from the mechanical deformation induced by the movement down-up of a piston connected to an electromagnet were carried out by means of an electrometer. The devices was assembled simply by establishing sputtered platinum electrical contacts to the ends of the film transferred on glass support. A quartz transducer was employed, to quantitatively evaluate the sensitivity while the electrical response was measured. Ellipsometry measurements were performed by means of EP4 Accurion Nulling Ellipsometer using a varying incident angle of 50 degrees with an excitation source from 420 nm to 650 nm with a step of 10 nm.

ASSOCIATED CONTENT Additional infrared spectra, Langmuir isotherms and ellipsometric simulations of ODA and ZnO@ODA adduct are reported in the Supporting Information file available free of charge on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author * Gabriele Giancane, Via D. Birago, 84, I-73100 Lecce, Italy, phone number: +39 0832 297372 email: [email protected] Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest

ACKNOWLEDGMENT This research was supported by the Projects PON 254/Ric. Potenziamento del “CENTRO RICERCHE PER LA SALUTE DELL’UOMO DELL’AMBIENTE” Cod. PONa3_00334, PRIN 2012 Nanostrutture gerarchiche fotosintetiche per la produzione di energia and the Project PON S.I.Mi.S.A. Cod. PON02_00186_3417512., Project PON Pro.Ali.Fun Cod. PON02_00186_2937475, by Regione Puglia, Costituzione di Reti di Laboratori Pubblici di Ricerca, Progetto Esecutivo 09, WAFITECH.

SUPPORTING INFORMATION This information is available free of charge via the Internet at http://pubs.acs.org

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Promising Piezoelectric Properties of New ZnO@Octadecylamine

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