Single step in situ formation of porous zinc oxide/PMMA nanocomposites by pulsed laser irradiation: kinetic aspects and mechanisms

Abstract

The simultaneous in situ formation of zinc oxide nanoparticles and controlled porosity in poly(methyl methacrylate) films is presented herein as an innovative method for the preparation of polymer based porous nanocomposites. The process is based on the conversion of zinc acetate in spherical zinc oxide nanoparticles of 9 nm on average, by pulsed laser activated precursor decomposition. The kinetics of the nanoparticles formation, followed by UV-visible spectroscopy, show a remarkable increase of the reaction rate with respect to the conventional thermally activated process. Most importantly, as revealed by scanning electron microscopy investigations, the laser treatment leads to the simultaneous formation of a porous structure in the polymer matrix, which can be ascribed to the yielded gaseous by-products during the zinc oxide nanoparticle formation. The combination of all these characterizations allowed a deeper insight in the kinetic aspects and mechanisms involved in the single step formation of porous poly(methyl methacrylate)/zinc oxide nanocomposites with tailored characteristics that cannot be achieved by the conventional thermal treatment.

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Introduction

In the last two decades the organic–inorganic nanocomposites field has had great success thanks to the possibility of combining some peculiar mechanical^1–4 and functional⁵ properties of both organic polymers and inorganic materials. This combination results in a very large set of accessible composite materials with a wide range of properties opening pathways to prepare customized materials on specific needs. From this point of view polymer based composite materials represent not only an exciting field of basic research, but also offer perspectives for several industrial applications in many different technological fields such as optics, micro-electronics, transportation, health, energy, housing,^5,6 and environmental protection.^7,8

Among the possible inorganic nanoparticles, metal and non-metal oxides represent the most employed class of materials, thanks to their properties that cover, basically, all studied features in materials science.⁹ This makes metal oxides particularly suitable for preparing polymer-based composites⁶ characterized by features that would be otherwise peculiar for polymers, such as electrical¹⁰ or magnetic properties.^11,12

The dispersion of fillers is the most important parameter that has to be carefully controlled in order to obtain homogeneous composite materials. Nevertheless, this is still one of the main problems related to the composites preparation based on the conventional mechanical and/or ultrasound mixing methods. It is well known that nanoparticles arising from flame pyrolysis or wet synthesis cannot be easily re-dispersed in organic matrices by only using conventional mixing methods, due to their strong tendency to form micrometric agglomerates.¹³

For overcoming this problem, an alternative method can be the so-called “in situ synthesis” of nanoparticles (NPs) in which the fillers are directly synthesized in a hosting polymeric solid matrix preventing the NPs aggregation. Typically, a previously dispersed filler precursor is converted in NPs by a simple thermal treatment at a suitable temperature, resulting in the formation of particles homogeneously dispersed in the polymeric matrix.^14–16 This approach cannot be always employed because of the high temperature often required for converting the precursor in the desired NPs that can degrade the polymer. An alternative and innovative approach, for activating the in situ synthesis, is the laser irradiation that also permits to localize the NPs' formation in specific areas of the polymer, obtaining thus nanocomposite patterns. Few papers report this latter method and most of them are focused on in situ synthesis of noble metal nanoparticles,^17,18 cadmium sulphide^19–21 and zinc sulphide,²² whereas the in situ synthesis by laser irradiation of metal oxide NPs in solid polymeric matrices has not been reported yet.

Laser irradiation of zinc nitrate, zinc acetate and zinc acetylacetone that results in zinc oxide (ZnO) NPs has been reported in literature for colloidal systems²³ and thin precursor layers,^24–26 whereas the incorporation of this sort of precursors in polymeric matrices and the consecutive irradiation to obtain oxide NPs has never been considered as potential alternative for the localized formation of organic/inorganic nanocomposites. Nevertheless, ZnO/polymer based nanocomposites are becoming more and more important for many technological applications such as organic solar cells,^27,28 photo-detectors,^29,30 gas sensing devices,³¹ antibacterial^32,33 and photo-catalysis systems^34,35 among others. In all of these applications, localized formation of NPs together with a high surface area of the nanocomposite are often required features, playing an important role to the enhancement of the final material's performance. For this reason, the ZnO NPs are usually combined with porous supports or with materials that can be foamed in order to obtain the needed high surface area.^36,37 The preparation of such nanocomposite systems is often complicated and time consuming, furthermore usually toxic or not sustainable chemicals are employed or released during the process.

Herein, we present a single step green process for the in situ localized formation of porous PMMA/ZnO nanocomposite films by means of laser irradiation of PMMA/zinc acetate solid film. We show that the pulsed laser irradiation induces the conversion of the precursor, previously dispersed in the polymer matrix, to ZnO NPs of controlled size and simultaneously the yielded gaseous by-products lead to a foaming process that results in the formation of micrometric pores in the solid film. This is an important step for the fabrication of tailored polymer-based nano-structured composites where the control is not only on the NPs formation, as done so far, but also on the morphology/texture of the polymeric hosting matrix. The systematic study here reported allows the better understanding of the kinetics and mechanisms at the base of the proposed process in order to control the features of the final material.

Experimental

Materials

Poly(methyl methacrylate) (PMMA, average molecular weight ∼ 350 kDa), zinc acetate dihydrate (Zn(OAc)₂, 99.999%), 2-propanol (LC-MS Chromasolv®), toluene (Chromasolv® Plus), hydrochloridric acid (HCl, 37%, ACS reagent) and nitric acid (HNO₃, 70%, ACS reagent) were purchased by Sigma Aldrich (Milan, Italy). All reported chemicals were high-purity reagents and they were used as received without any further purification.

Preparation of PMMA/zinc acetate

100 mg of PMMA were dissolved in 10 mL of toluene overnight by heating at 60 °C on a hot plate (solution A). 2.7 mg of Zn(OAc)₂ were dissolved in 0.6 mL of 2-propanol by ultrasonic bath (LBS 2, Falc) for 30 min at 59 kHz, and subsequently by stirring for 3 hours at room temperature and 1 hour at 45 °C (solution B).

Solution B was then added to the solution A, and left under vigorous stirring for 15 min at room temperature. Afterwards, the so-obtained solution was sonicated by probe sonicator (Vibra Cel, Sonics) using three pulses (intensity 20 kHz, 15 s each and pulse amplitude of 50%). 5 mL of the final solution were then drop casted in a circular Teflon mould (38 mm diameter × 5.5 mm depth) and left to dry under an aspiration hood for 24 hours. Finally, the dry film was detached from the mould and further dried for 6 hours under dynamic vacuum in order to completely remove the possible entrapped solvent. Following this process films of 2.65 wt% Zn(OAc)₂ in PMMA were prepared for further processing.

Laser irradiation

The in situ localized synthesis of ZnO NPs was activated by pulsed laser irradiation of solid PMMA/Zn(OAc)₂ films (absorption spectrum in Fig. S1†) using the second harmonic of a Nd:YAG nanosecond laser (Quanta-Ray, Spectra-Physics). In particular, the irradiation was performed at 532 nm, with a laser irradiation fluence of 200 mJ cm⁻² (1 cm² circular spot) and repetition rate of 10 Hz (pulse duration of 6 ns). As a comparison, some PMMA/Zn(OAc)₂ samples were annealed in an oven at 110 °C for 48 hours in order to thermally activate the in situ synthesis reaction.

Characterizations

Absorption spectra of the free-standing films were recorded on Varian Cary 6000i UV-visible-NIR spectrophotometer (UV-vis) in double beam configuration using PMMA/Zn(OAc)₂ (2.65 wt%) as a reference sample. The absorption spectra were recorded on exactly the same spot of the film after consecutive irradiation, or heating steps. The films thickness (approximately 30 μm) was measured using a micrometre (Coolant Proof Micrometer, Mitutoyo) in order to normalize the absorbance taking into account the optical path. The optical absorption, at about 271 nm, was then plotted as a function of the pulses number in order to obtain a kinetic curve of the NPs formation.

Thermal gravimetric analysis (TGA) was performed in order to detect the formation of gaseous by-products typical of the Zn(OAc)₂ decomposition. The characterization was carried out on a TGA Q500, TA Instruments at 110 °C for 12 hours in constant air flow (50 mL min⁻¹), starting from about 5 mg of sample.

The actual amount of ZnO in the final sample (irradiated with 108 000 pulses) was estimated by inductively coupled plasma atomic emission spectroscopy (ICP-AES), using an iCAP 6500 spectrometer, Thermo Scientific. For selectively collecting the ZnO NPs the sample was pre-treated as follows: about 0.05 g of the composite (only irradiated area) was dissolved in 7 mL of toluene for 24 hours under shaking at 650 rpm (MultiReax, Heiddolph). The so obtained suspension was then centrifuged by an ultracentrifuge Optima L-100K, Beckman Coulter at 30 000 rpm for 45 min and 20 °C using thick-wall polyallomer, 10.5 mL tubes (Beckman Coulter). The supernatant was carefully removed and the residual solid was left drying at room temperature for several hours. In order to remove the possible unreacted precursor, 7 mL of Milli-Q water were added to the dry sample and consequently sonicated for 30 min by ultrasonic bath at 59 kHz (LBS 2, Falc). The suspension was centrifuged again using the same conditions as described above, and then the aqueous supernatant was carefully removed. The remaining solid at the bottom of the tube was dissolved in 3 mL of a solution of HCl and HNO₃ 3 : 1 (v/v) for 24 hours. Before the ICP-AES analyses, the so dissolved sample was diluted with Milli-Q water in a 10 mL calibrated flask. All chemical analyses performed by ICP-AES were affected by a systematic error of about 5%.

The following characterizations were systematically repeated at different pulses number (600, 6000, 24 000 and 108 000) in order to have a deeper insight on the formation mechanisms and morphological changes occurred during the process.

X-ray diffraction (XRD) analyses were performed on a PANalytical Empyrean X-ray diffractometer equipped with a 1.8 kW CuKα ceramic X-ray tube (λ = 1.5418 Å), PIXcel^3D 2 × 2 mm² area detector and operating at 45 kV and 40 mA. The diffraction patterns were collected in air at room temperature using parallel-beam (PB) geometry and symmetric reflection mode, in the range 30–70° 2θ using a step time of 2000 s and a step size of 0.065°. XRD data analysis was carried out using HighScore 4.1 software from PANalytical.

The samples' morphology was investigated by a JEOL JSM 7500FA high resolution scanning electron microscope (HR-SEM) equipped with a cold field emission gun, applying an accelerating voltage of 10 kV and a chamber pressure of 9.6 × 10⁻⁵ Pa. The cross-sections were prepared by fracturing the specimens in liquid nitrogen and the so-obtained cross-sections were coated with 10 nm thick carbon layer by a carbon coater (Emitech K950X high vacuum turbo system, Quorum Technologies Ltd) in order to impart electrical conductivity.

Morphology and dimensions of the primary particles were investigated by a JEOL JEM-1011 transmission electron microscopy (TEM) equipped with a tungsten thermionic electron source, operating at 100 kV. About 0.005 g of the nanocomposite, was dissolved in 1 mL toluene and left stirring overnight. The obtained solution was sonicated for 1 hour, and then a small drop (5 μL) was placed on a copper grid (300 mesh Cu carbon only) followed by drying at room temperature. The obtained TEM images were analysed and processed (at least 100 particles per sample) by ImageJ open-source software in order to evaluate the particles' size distributions.

Results and discussion

Kinetics

During laser irradiation (wavelength 532 nm), the kinetics of the ZnO NPs formation was followed by monitoring the UV-visible absorption in the same area of the irradiated sample (Fig. 1).

Fig. 1 (a) UV-visible absorption spectra of the sample irradiated with different number of pulses. (b) Optical absorption at 271 nm as a function of the pulses number.

In the absence of Zn(OAc)₂, the PMMA film does not absorb any photons at the specific employed wavelength. However, when the Zn(OAc)₂ precursor is mixed with the polymer the resulting PMMA/Zn(OAc)₂ film absorbs photons in the UV-vis region (ESI Fig. S1†). Therefore, as shown in Fig. 1a, after 300 pulses an absorption band is formed in the near UV region, characteristic of ZnO NPs. Increasing the pulses' number, also the intensity of the typical absorption band increases, whereas after 36 000 pulses no further increment is observed. The change in the absorption intensity has to be solely attributed to the formation of the ZnO NPs since the pristine PMMA film does not show any change in the absorption spectrum even after an irradiation with 108 000 pulses (here not reported). The kinetics of the ZnO NPs formation process, presented in Fig. 1b, is extrapolated by plotting the absorption intensity at 271 nm, a peak attributed to the exciton transition of the small ZnO clusters^27,38,39 shown as a shoulder in our spectra. The kinetic curve (Fig. 1b) presents the typical trend of the first order reaction.⁴⁰ Three main steps are observed, a first sudden increase (up to 6000 pulses) during which the first particles are formed upon laser irradiation, a second step (up to 24 000 pulses) where the formation rate decreases, being strictly dependent on the precursor concentration as typical for a first order reaction, and the last step where the curve reaches a plateau, indicative of a reached chemical equilibrium.

It is noteworthy that when the ZnO NPs synthesis is activated by a conventional thermal treatment of a PMMA/Zn(OAc)₂ film (at 110 °C), longer times are needed if compared to the ones here reported. As shown in Fig. S2 of ESI,† for reaching the plateau, a thermally activated system needs about 10 hours instead of one hour as in the case of the pulsed laser irradiation, and this difference becomes even more relevant considering that the actual interaction time of the laser light with the samples is only 216 μs (considering that the pulse duration is 6 ns and the plateau starts after 36 000 pulses).

XRD, SEM and TEM results

The diffractograms presented in Fig. 2 clearly show that the reflections' intensities, ascribable to Zn(OAc)₂ (JCPDS file 00-056-0569), decrease by increasing the number of pulses, demonstrating that the laser irradiation causes the decomposition of the precursor. At the same time, the reflection peaks due to the presence of the ZnO NPs arise from the background and their intensities increase with the number of pulses. The ZnO NPs obtained after 108 000 pulses, are characterized by hexagonal crystalline structure, typical of zincite phase (JCPDS file 01-079-0206). The most remarkable increase in crystallinity is observed after 24 000 pulses, even though after this number of pulses the UV-vis spectra (Fig. 1) do not show any longer significant changes. This strongly suggests that in the third part of the kinetic curve, presented above, the laser irradiation mainly promotes the crystallization of the previously synthesized particles as similarly observed elsewhere.^24,41 HR-SEM cross-section images of the prepared nanocomposites permit the evaluation of the distribution and dispersion of ZnO NPs and the morphology evolution of the PMMA matrix during the irradiation, as reported in Fig. 3.

Fig. 2 XRD reflections of the irradiated samples by increasing the number of pulses. In the top and bottom part of the figure, the patterns of ZnO (JCPDS file 01-079-0206) and Zn(OAc)₂ (JCPDS file 00-056-0569) are reported, respectively. (✻) indicates the contribution due to amorphous PMMA.

Fig. 3 HR-SEM cross-section images (back-scattered electrons) of the morphology evolution and particle growth, increasing the pulses numbers. (a) PMMA/Zn(OAc)₂ not irradiated (inset TEM image of precursor), (b) irradiated with 600 pulses, (c) irradiated with 6000 pulses, (d) irradiated with 24 000 pulses and (e) irradiated with 108 000 pulses (inset TEM image of ZnO NPs, as representative).

Initially, before any irradiation, the PMMA/Zn(OAc)₂ films present a morphologically homogeneous bulk (Fig. 3a) characterized by the presence of sheet-like sub-micrometric aggregates representative of Zn(OAc)₂, as shown by the TEM image in the inset of Fig. 3a. However, after 600 irradiation pulses few pores are formed in the whole volume of the film, always nearby the precursor crystals (Fig. 3b) indicating that there is a strong correlation between the formation of ZnO NPs and the porous structure. This is further supported by the significant increment of the number and dimensions of the pores, simultaneously with the increase of the ZnO NPs amount on the porous surface, as the number of laser pulses increases (Fig. 3b–d). After 108 000 pulses, the NPs are mainly localized on the surface of the pores (Fig. 3e), where they are homogeneously distributed as clearly shown in the high magnification SEM images of Fig. 4. At the same time, the pores are homogeneously distributed in the whole volume of the film (Fig. 3e and 5b). As displayed in the inset of the Fig. 3e and in Fig. 4, the in situ synthesized NPs present rather spherical uniform shape and average diameter of 9 ± 5 nm, with dimensions negligibly affected by the number of the laser pulses as proved by the particle size distributions for each stage of irradiation, presented in the ESI Fig. S3.†

Fig. 4 HR-SEM cross-section images (back-scattered electrons), showing the NPs distribution on the pores' surface after 108 000 pulses.

At the end of the irradiation process, the final conversion of the precursor in ZnO NPs as determined by ICP-AES analysis, was higher than 70 ± 4 wt%. Taking into account both the systematic instrumental error and the complicated sample preparation (as described in the Experimental section), this result has to be considered only as an indication of high precursor conversion, also in good agreement with both XRD measurements (Fig. 2) that showed the disappearance of the reflections due to the Zn(OAc)₂, and HR-SEM images that showed the sheet-like precursor crystals disappear almost completely after 108 000 pulses, if compared to the earlier stages of irradiation (Fig. 3 and 4).

The formation of the final porous structure can be ascribed to the yielded gasses during the decomposition process of the Zn(OAc)₂ upon irradiation. Indeed, it is well known that Zn(OAc)₂ decomposition leads to the formation of ZnO NPs and different gaseous by-products such as acetone, acetic acid, acetic anhydride or carbon dioxide, depending on the involved reaction pathways.^42–44 This was further confirmed by TGA measurements, where the final weight loss, of PMMA/Zn(OAc)₂ samples annealed at 110 °C, increases by increasing the amount of the precursor suggesting an effective formation of gaseous by-products during the ZnO NPs synthesis (ESI Fig. S4†). In the present case the yielded gases, due to the laser induced precursor decomposition, are trapped close to the reaction points into the bulk of the film. As the reaction proceeds the amount of the yielded gases quickly increases and consequently, in each reaction point, their pressure increases up to causing the formation of the observed cavities into the solid matrix and the “crater-like” structures observed in the sample's surface reported in Fig. 5a.

Fig. 5 Low magnification HR-SEM images (back-scattered electrons). (a) Top-view of the nanocomposite sample obtained after a laser irradiation with 108 000 pulses. (b) Cross-section of the obtained porous structure after a laser irradiation with 108 000 pulses. (c) Top-view of the sample obtained by thermal-treatment at 110 °C for 48 h. (d) Cross-section of the sample obtained by thermal-treatment at 110 °C for 48 h.

It is important to notice that by conventional thermal-treatment (Fig. 5c and d), it is not possible to obtain both the above-described porous structure in the bulk and “craters” on its surface (Fig. 5). Moreover, although the NPs distribution is homogeneous their dispersion is not optimal as a consequence of the process' mechanism, in which the precursor aggregates are converted to ZnO NPs in the reaction points and not spread on the pores surfaces as when the laser irradiation is used for inducing the reaction. As already discussed, the thermal method leads to a significant lower reaction rate and thus slower yield and amount of gaseous by-products. Moreover, the required temperature (110 °C), for the thermal activating system, is considerably close to the glass transition temperature of the PMMA, in contrary to the laser treatment in which the overall temperature of the sample increases up to 40 °C (measured by a thermo-camera). Therefore, the combination of the slower formation rate of the yielded gases and of the higher diffusion coefficient due to the higher treatment temperature, determines an unconstrained diffusion of the gaseous by-products from the reaction points towards the surface of the film, thus preventing the porosity formation and “craters-like” structure on the surface.

Conclusions

Porous ZnO/PMMA nanocomposite films are successfully in situ synthesized by means of a single step pulsed laser irradiation of zinc acetate/PMMA solid films. The presented approach allows the simultaneous realization of porous polymeric structure, in which spherical NPs of average diameter of 9 nm are homogeneously embedded.

The study on the kinetics of this process permitted to have a control on the porosity degree, the pores size and the density of the NPs simply by varying the number of the laser pulses. The porous structure formation is ascribed to the rapid formation of gaseous by-products due to the very fast and localized decomposition of the zinc acetate precursor that lead to a foaming process. This is a unique advantage of the pulsed laser irradiation in contrary to the conventional thermal activation method, which results in the formation of compact and uniform polymer nanocomposites. Moreover, the use of laser light allows the formation of patterned nanostructures on polymeric films simply by using a mask or a high-precision X–Y translation stage normal to the incident beam. The present findings demonstrate the uniqueness and versatility of using laser irradiation for localizing the formation of crystalline metal oxides in porous polymer films, which may find application in many different technological fields in which high surface area and oxide nanoparticles are required such as UV light harvesting, gas sensing, photo-catalysis and organic light emitting diodes.

Acknowledgements

Authors thank Marco Scotto for the technical support about the use of the laser, Sergio Marras for the support on the X-ray diffraction measurements and finally Lara Marini for the technical support and helpful discussion on the gravimetric thermal analysis.

References

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Footnote

† Electronic supplementary information (ESI) available: UV-visible absorption spectra of neat PMMA film and PMMA/Zn(OAc)₂ film before irradiation; kinetic curve of ZnO nanoparticles synthesis by thermal activation; particles size distributions of the samples irradiated with different number of pulses; thermo-gravimetric analyses of neat PMMA and PMMA/Zn(OAc)₂. See DOI: 10.1039/c5ra23125f

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