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 § Department of Chemistry, CICECO, TEMA, Center for Mechanical Technology and Automation, and Department of Physics, I3N, UniVersity of AVeiro, 3810-193 AVeiro, Portugal 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 (C 6 H 12 N 4 ) and triethanolamine (C 6 H 15 NO 3 ) 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 well- defined ZnO particles in the presence of chelating agents 12-14 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 activity 2 and templated mineraliza- tion processes. 8 The mild temperatures employed in this method are compat- ible 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 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 ever- appealing 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 disin- tegrated 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 photomul- tiplier 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(CH 3 CO 2 ) 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 * Corresponding author. E-mail: paulam@ua.pt. † Department of Chemistry. ‡ TEMA, Center for Mechanical Technology and Automation. § Department of Physics. CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 386–390 10.1021/cg800596z CCC: $40.75  2009 American Chemical Society Published on Web 12/04/2008