Growth, Structural, and Optical Characterization of ZnO-Coated Cellulosic Fibers Gil Gonc¸alves,† Paula A. A. P. Marques,*,‡ Carlos Pascoal Neto,† Tito Trindade,† Marco Peres,§ and Teresa Monteiro§
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 386–390
Department of Chemistry, CICECO, TEMA, Center for Mechanical Technology and Automation, and Department of Physics, I3N, UniVersity of AVeiro, 3810-193 AVeiro, Portugal
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ReceiVed June 9, 2008; ReVised Manuscript ReceiVed September 26, 2008
ABSTRACT: Rod-shaped ZnO particles were grown over wood cellulose fibers using a two-step process. In the first step, the formation of ZnO seeds at cellulose fibers surfaces was induced by the alkaline hydrolysis of aqueous Zn(II); in the second step, the growth of the ZnO seeds into larger nanoparticles was promoted by the controlled hydrolysis of Zn(II)-amine complexes. In particular, we will report the use of hexamethylenetetramine (C6H12N4) and triethanolamine (C6H15NO3) to grow, respectively, ZnO nanorods and microrods at the cellulose fibers surfaces. Photoluminescence measurements performed on the nanocomposite materials showed the typical excitonic ZnO recombination peaked between 3.38 and 3.34 eV, at low temperature. The full width at half-maximum of the excitonic line is dependent on the ZnO particles morphology and can be as narrow as 30 meV for some of the materials investigated.
1. Introduction Numerous nanomaterials based on metals, semiconductors, and dielectrics synthesized by different techniques with unique electrical and optical properties have been the subject of recent studies.1 Zinc oxide (ZnO), possessing a band gap energy of 3.37 eV at room temperature, exhibits optical and electrical properties with interest in a broad range of applications.2 Extensive work on the synthesis of ZnO using wet chemical methods has been reported during the last decades, with a special emphasis on the particles morphological control and its influence on their optical properties.1-3 Recently, metal and semiconductor nanoparticles attached onto vegetable or bacterial cellulosic fibers have been the subject of increasing interest.4-7 Following our own recent research in this field,8-11 we have decided to investigate the preparation and optical properties of such type of nanocomposites derived from coating vegetable cellulose fibers with ZnO nanophases. As such, ZnO was grown by the controlled hydrolysis of Zn(II)-amine complexes. It is stressed that in this synthesis, the amine not only acts as a sequestering agent to avoid the spontaneous formation of bulk ZnO precipitates, at room temperature, but also allows one to control the morphology of the ZnO nanostructures in the final materials. In fact, several authors have described the synthesis of morphological welldefined ZnO particles in the presence of chelating agents12-14 or polymers.15,16 There are few studies concerning the controlled growth of ZnO particles at the surfaces of cellulosic fibers. Nevertheless, interesting examples showing the versatility of these nanocomposites have recently been published, including studies on their antibacterial activity2 and templated mineralization processes.8 The mild temperatures employed in this method are compatible with the use of biopolymers as substrates such as cellulose, one of the most abundant polymers available. We also noted that this method allows one to grow morphological uniform ZnO nanorods whose optical properties have been widely investigated because of their implications in optoelectronics.17,18 The interest * Corresponding author. E-mail:
[email protected]. † Department of Chemistry. ‡ TEMA, Center for Mechanical Technology and Automation. § Department of Physics.
in nanocomposites based on cellulose fibers coated with ZnO nanorods is not restricted to academic studies but may also constitute an important material for practical applications, ranging from the film paint industry to the technological everappealing area of optoelectronic paper. Therefore, we report here the photoluminescence behavior of cellulosic fibers coated with ZnO nanorods.
2. Experimental Section 2.1. Materials. All chemicals were supplied by Sigma-Aldrich and used as received. Wood cellulose fibers (Eucalyptus globulus), ECF bleached kraft pulp, composed essentially of cellulose (∼85%) and glucuronoxylan (∼15%) supplied by Portucel (Portugal), were disintegrated and washed with distilled water before use. 2.2. Characterization Methods. Scanning electron microscopy (SEM) images were obtained using a FEG-SEM Hitachi S4100 microscope operating at 25 kV. Transmission electron microscopy (TEM) was performed using a Hitachi H-9000 operating at 300 kV. The samples for TEM were prepared by depositing an aliquot of the aqueous suspension onto a carbon-coated copper grid and then letting the solvent evaporate. X-ray powder diffraction (XRD) was performed, using a Philips X_Pert instrument operating with Cu Ka radiation (k ) 1.54178 Å) at 40 kV/50 mA. The thermogravimetric (TGA) assays were carried out with a Shimadzu TGA 50 analyzer equipped with platinum cell. Samples were heated at a constant rate of 10 °C/min from room temperature to 800 °C, under air. Steady-state photoluminescence (PL) was generated using the 325 nm light from a cw He-Cd laser, and an excitation power density less than 0.6 W cm-2. The cellulose/ZnO samples were mounted in the coldfinger of a closed cycle helium cryostat, and the sample temperature could be controlled in the range from 7 K to room temperature (RT). The luminescence was measured using a Spex 1704 monochromator (1 m, 1200 mm-1) fitted with a cooled Hamamatsu R928 photomultiplier tube. Resonant Raman scattering was performed under 325 nm excitation conditions using a Jovin Yvon Horiba HR800 UV Raman system. 2.3. Coating of Cellulosic Fibers with ZnO. Two alcoholic solutions containing, respectively, 0.18 g of zinc acetate in 230 mL of 2-propanol (solution A: [Zn(CH3CO2)2] ) 3.5 × 10-3 mol dm-3) and 0.08 g of NaOH in 100 mL of 2-propanol (solution B: [NaOH] ) 2.0 × 10-3) were prepared. Both solutions were heated at 50 °C and then cooled to 4 °C. Cellulose fibers (1 g) were then dispersed in 100 mL of a solution resulting from the slow addition of solution B (20 mL) to
10.1021/cg800596z CCC: $40.75 2009 American Chemical Society Published on Web 12/04/2008
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Figure 1. SEM image of cellulose seeded with ZnO. solution A (80 mL). The resulting suspension was mechanically stirred during 15 min at room temperature. After this period of time, the fibers were isolated by filtration, and the solvent was removed by evaporation at 100 °C. This procedure (immersion of the fibers and drying) was repeated thrice, yielding cellulosic fibers surfaces seeded with ZnO. To promote the growth of these ZnO seeds into larger particles, the cellulose fibers were then dispersed in 100 mL of an aqueous solution 2.5 × 10-2 mol dm-3 in Zn(NO3)2 and equal molar concentration of the amine (hexamethylenetetramine (sample A) or triethanolamine (sample B)). This mixture was then heated at 90 °C during 6 h under stirring. The final nanocomposites were filtered, thoroughly washed with distilled water, and finally dried at 50 °C over 24 h.
3. Results and Discussion The treatment of cellulose fibers with both Zn(CH3CO2)2 and NaOH solutions led to the nucleation and growth of discrete ZnO seeds at the cellulose surfaces. This chemical process involves the alkaline hydrolysis of Zn(II) in which the cellulosic fibers act as hydrophilic substrates for the heterogeneous nucleation of ZnO. In fact, SEM analysis of cellulosic fibers collected after this seeding process showed the presence of discrete ZnO nanoparticles over the fibers surface (Figure 1). Moreover, after the fibers were collected, a neglected amount of ZnO particulates was observed in the supernatant alcoholic solution. Although the powder XRD of the seeded cellulose showed weakly defined peaks, the following Bragg reflections were assigned to ZnO (wurtzite type). The growth of ZnO seeds into larger structures was promoted by the controlled hydrolysis of an aqueous Zn(II) solution in the presence of an amine. Although several amines have been used (including hexamethylenetetramine, ethylenediamine, ethanolamine, triethanolamine, dimethylamine, and triethylamine), for the experimental conditions employed only hexamethylenetetramine (HMT) and triethanolamine (TEA) led to a homogeneous coating of the cellulosic fibers. The nanocomposites obtained in the presence of these two amines were then selected for more detailed studies as presented below. Figures 2 and 3 show the SEM images of the nanocomposites obtained in the presence of HMT (sample A) and TEA (sample B), respectively. For both cases, a dense layer of the ZnO phase coating the cellulose fibers is observed, which is the main difference in the average size of the grown ZnO hexagonal particles. In sample A, and by taking the hexagonal facets (basal plane) of the ZnO nanorods as the measured morphological parameter, there is a homogeneous distribution of nanorods with average dimensions of 34 ( 7 nm, while sample B shows a layer of micrometer-sized ZnO nanorods particles whose average
Figure 2. SEM images of ZnO-coated cellulosic fibers obtained in the presence of hexamethylenetetramine, showing different magnifications (sample A).
dimension is 176 ( 21 nm. Greene et al.19 have suggested that ZnO anisotropic nanostructures grown in the presence of HMT result from a kinetic control of HMT and hydrolyzed species in solution. Moreover, HMT could also coordinate selectively to ZnO crystals, thus hindering the growth of certain crystallographic surfaces. The observed morphological differences can be partially associated with such different surface coordination chemistries when distinct amines have been employed in the synthesis of ZnO. Yet, this is a matter under debate as a number of thermodynamic and kinetic parameters need to be invoked to explain the influence of the solute precursors on the final properties of precipitated solids. Figure 4 shows the powder XRD for the nanocomposites. The observed Bragg peaks correspond to crystalline domains of cellulose type I, broad peaks at 2θ ) 22° and 17°, and the remaining peaks observed are typical of ZnO (wurtzite).20 The narrow full width at half-maximum (fwhm) of the peaks corresponding to sample A shows that ZnO crystals were wellcrystallized. On the other hand, the peaks observed for sample B show some broadening, which in principle might be due to lattice defects and dislocations.16 Figure 5 shows the TGA curves of the ZnO/cellulose nanocomposites, which were recorded under normal atmosphere. For comparative purposes, the TGA curve for cellulose is also presented and shows that the two main steps observed correspond to the thermal degradation of cellulose.21 From this analysis, a percentage of ZnO of approximately 17% (w/w) was verified for both nanocomposites (neglecting the amount of vestigial carbon). Because the Zn(II) precursor solutions were
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Figure 5. Termogravimetric behavior of ZnO/cellulose hybrids, A and B, in comparison with blank cellulose fibers.
Figure 3. SEM images of ZnO-coated cellulosic fibers obtained in the presence of triethanolamine, showing different magnifications (sample B).
Figure 6. SEM (left side) and TEM (right side) images of ZnO particles after calcination of samples A (a) and B (b), at 800 °C.
Figure 4. XRD patterns of nanocomposite samples A and B.
of the same concentration, these results suggest that the extension of hydrolysis had occurred in comparable degrees regardless of the amine used. To obtain more detailed images of the ZnO nanostructures, SEM and TEM were performed on the inorganic residues remaining after firing the nanocomposite samples at 800 °C. In this case, charge effects during the electronic beam approximation to the organic substrate are reduced, and better images can be obtained. Figure 6a and b shows the ZnO particles resultant, respectively, from calcination of samples A and B. ZnO nanorods with approximately 130 nm length and 20 nm width
are present in sample A, while dumbbell-shaped ZnO particles predominate in sample B. Note in the latter the typical hexagonal crystallographic habit of the wurtzite structure in some of the ZnO particles. Also, it is interesting to note that in this sample, both single microrods and dumbbell-shaped rods were observed. These morphologies could be present already in the starting nanocomposites samples but could also result from the heat treatment applied during the calcination process. In this case, the dumbbell-shaped ZnO particles could result from sintering of the individual rods, or, conversely, the single rods could result from the breakage of previously existing dumbbell particles. The same relative intensity of diffraction peaks of wurtzite observed for the noncalcinated samples was observed in the calcinated ones, which indicates that temperature treatment did not promote the change of initial structure and morphology of ZnO nanoparticles. The cellulosic nanocomposites described above were further characterized by optical means, Raman and PL techniques. Figure 7 shows the room temperature absorption, PL, and resonant Raman spectra for both nanocomposite samples. The first-order LO phonon peak is located at 573 cm-1 consistent with the 574 cm-1 A1(LO) phonon scattering observed in wurtzite bulk samples.22 As expected from the dimensions of the particles, the absence of quantum confinement effects (only expected for nanosized crystals with sizes com-
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currently observed in ZnO from the data, we cannot exclude that the blue band could be originated from the cellulose fibers. With the same used excitation conditions, the intensity of the excitonic luminescence is higher for the A sample, suggesting a higher optical quality for this hybrid sample.
4. Conclusions
Figure 7. RT Raman scattering for both ZnO/cellulose hybrids. Also shown are the RT PL and absorption spectra for the A sample.
Figure 8. ∼10 K PL spectra obtained with above band gap excitation for the ZnO/cellulose samples.
parable to the 2.34 exciton Bohr radius)23 is also observable from the RT mirror image PL and absorption spectra, which gives a band gap similar to that observed in bulk crystals (∼3.37 eV at RT).24 With the above band gap excitation (He-Cd laser), the low temperature (∼10 K) PL spectrum of the ZnO/cellulose samples is dominated by a pronounced band edge recombination and a deep level emission peaked at ∼2.8 eV as shown in Figure 8. In the case of the band edge recombination, the peak position and full width at half-maximum (fwhm) are found to be sampledependent. In particular, in the case of the nanocomposite prepared with HMT, the peak position of the excitonic recombination occurs at ∼3.38 eV, and a narrowest (∼40 meV) fwhm of the emission is observed. For the nanocomposite prepared in the presence of TEA, an enlargement and red shift of the band edge emission is clearly seen. The variety of ZnO nanostructures morphologies are known to rise to different peak positions of the band edge recombination, even without the presence of quantum confinement effects, mainly due to the different native defect concentrations expected to occur in nanostructures with different sizes due to different surface/ volume ratios.18 Typically, the deep level recombination of ZnO samples is dominated by blue, green, orange, and red broad unstructured bands that are tentatively associated with native defects.25-27 In the nanocomposites analyzed, the deep level luminescence exhibited by both samples occurs in the blue, peaked at ∼2.8 eV. Despite the fact that broad bands are
ZnO/cellulose nanocomposites have been prepared using a two-step synthesis: first the nucleation of ZnO seeds was done at the cellulose surface, and second the growth of ZnO nanostructures was promoted by the controlled hydrolysis of Zn(II)-amine complexes. The morphologies of ZnO structures depend on the type of amine used during the synthesis: HMT allowed the growth of uniform ZnO nanorods that perfectly cover the cellulose fiber, while in the presence of TEA ZnO microrods were obtained. From the XRD and Raman analysis, it is clear that the dominant phase of the ZnO nanostructures corresponds to the stable wurtzite phase. Furthermore, and as expected for ZnO low dimensional structures for which no confinement effects are observable, the ∼3.37 eV energy was found for the RT band gap. Low temperature PL measurements show that narrow excitonic lines are observed with a fwhm that is sampledependent. Also, a broad blue band that is likely to be originated from the cellulose matrix was observed. These new ZnO-cellulose-based composite materials with photoluminescence ability may find interesting applications, as in photoluminescent papers and as reinforcing/photoluminescent agents in polymeric matrices. Further studies on the antibacterial activity of these composites are underway. Acknowledgment. We thank the European Commission (SUSTAINPACK IP-500311-2) and National Project (POCI/ CTM/55945/2004) for the financial support of this work. M.P. thanks the University of Aveiro for a Ph.D. grant.
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