Low-temperature precipitation synthesis of flower-like ZnO with lignin amine and its optical properties
© Miao et al.; licensee Springer. 2013
Received: 9 July 2013
Accepted: 2 October 2013
Published: 17 October 2013
A facile precipitation method has been developed to synthesize ZnO with [bis(2-aminoethyl)amino]methyl lignin (lignin amine) that is chemically modified from low-cost pulp industrial lignin. The obtained ZnO crystallites have been characterized to exhibit a hexagonal wurtzite structure, and their sizes have been determined at ca. 24 nm (mean value). These ZnO nanocrystallites are of high purity and well crystallized. Our present synthetic approach apparently exempts the commonly used calcining purification procedure. It is found that the morphology of ZnO and its specific surface area are capable of being tuned by varying the added lignin amine amount. Using the optimal 10 mL lignin amine, the synthesized ZnO exhibits flower-like morphology with proper specific surface area. Additionally, photoluminescence property of the obtainable ZnO displays two emissive bands at 383 nm (sharp) and in the range of 480 to 600 nm (broad) at room temperature. Their intensities were revealed to depend on the added lignin amine amount as well as on the molar ratio of Zn2+/OH-. The present investigation demonstrates that our method is simple, eco-friendly, and cost-effective for the synthesis of small-size ZnO materials.
Zinc oxide is an important n-type semiconductor with a wide band gap of 3.37 eV, allowing for its wide applications in optoelectronic and microelectronic devices [1–3]. Due to its small size and large specific surface area, nanoscale ZnO has showed superior performance relative to the bulk one [4, 5]. Up to date, improving the synthetic methods of ZnO nanomaterials as well as developing new ones has been increasingly attractive. These methods do affect the properties of materials such as field emission, optics, piezoelectricity, and catalysis [6–8]. Although ZnO materials with different morphologies were synthesized [1, 9–14], some disadvantages still remain in the present methods. Therefore, a facile, cost-effective, as well as environment-friendly approach is highly demanded, especially the approach that produces ZnO in a large scale.
In this work, the controllable synthesis of ZnO nanocrystallites has been developed using a facile precipitation method by tuning the amount of [bis(2-aminoethyl)amino]methyl lignin (i.e., lignin amine). The pure flower-like ZnO nanocrystallites with proper size have been fabricated and fully characterized. Since the used lignin amine is low cost and derives from chemical modification of lignin which is a by-product of pulp industry, a potentially commercial prospect on producing ZnO nanocrystallites is expected.
The alkaline lignin (industrial purity) was supplied by Qianjin Fuli Limited Company in Jilin province of China. Formaldehyde and diethylenetriamine (analytical purity) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. (Tianjin City, China). Zinc acetate (Zn(CH3COO)2·2H2O, 99% purity) and sodium hydroxide (NaOH, flakes, 97% purity) were used as zinc source and precipitant, respectively. All the chemicals were used as received without any purification.
In our experiment, lignin amine (0.3 g mL-1) was synthesized using a previously reported method . Alkaline lignin of 20.0 g was mixed with 40 mL deionized water and 60 g formaldehyde (40% by weight). Diethylenetriamine (40 mL) was added into the stirring mixture solution dropwise at room temperature. After this, the mixture was stirred at 75°C for 3 h by reflux. Then the lignin amine solution with a concentration of 0.3 g mL-1 was obtained.
Zinc acetate of 2.7 g was dissolved in 25 mL deionized water. Then a 25-mL NaOH solution (0.08 g mL-1) and lignin amine were added to the zinc acetate solution. The mixture solution was stirred for 5 h in the 80°C water bath. Then the mixture solution was cooled to room temperature, and the ZnO particles were precipitated. The precipitate was filtered and washed with deionized water, and the ZnO particles were obtained after drying at 30°C for 12 h. To study the effect of lignin amine on the morphology of the as-prepared ZnO, experiments using 0, 5, 10, and 15 mL lignin amine were carried out. The products were labeled as ZnO-0, ZnO-5, ZnO-10, and ZnO-15. Additionally, ZnO-10 has been chosen to examine whether the commonly used calcining purification procedure is necessary for our synthesized materials. We calcined ZnO-10 in air with a heating rate of 10°C min-1 and allowed it to stand at 500°C for 2 h using a compact muffle furnace (KSL-1700X, MTI Corporation, Richmond, CA, USA).
The crystallinity and purity of the prepared samples were analyzed by X-ray diffraction (XRD; Rigaku D/Max-RC, Tokyo, Japan) using CuKα radiation. Scans were performed from 5° to 80° (2θ) at a rate of 4° min-1. Scanning electron microscopy (SEM) images were taken with a field emission microscope (S-4800, Hitachi, Ltd., Chiyoda-ku, Japan). Transmission electron microscopy (TEM) imaging and high-resolution TEM (HRTEM) imaging of the samples were performed on a JEM-2100 electron microscope (JEOL, Tokyo, Japan) with an acceleration voltage of 200 kV. Carbon-coated copper grids were used as the sample holders. Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption experiments were carried out on the automated surface area and pore size analyzer. The photoluminescence performance was examined using a fluorescence spectrophotometer (FLS920, Edinburgh Instruments Ltd., Royston, UK) with a Xe lamp at room temperature at an excitation wavelength of 325 nm.
Results and discussion
We also determined the PL spectra of the ZnO prepared with 10 mL lignin amine at various molar ratios of Zn2+/OH- as the ratio affects the defects of the synthesized materials. From Figure 7b, we can see that the basic concentration has a more pronounced effect on the intensity of the visible emission than on that of the UV emission. Upon increasing the Zn2+/OH- from 1:2, 1:6, to 1:10, the intensity of the visible emission is generally quenched, accompanied with slight variation of UV emission. It is worth noting that the ZnO prepared at 1:4 displays the highest visible emission, as seen in Figure 7b. Associated with the above assignment of emissions, it is suggested that the ZnO prepared at 1:4 of Zn2+/OH- tends to have more surface defects.
With the aid of lignin amine, the flower-like ZnO nanomaterials featured with tapered petals have been synthesized by a facile precipitation method. The results of combined XRD and SEM have shown that the non-calcined and calcined ZnO nanomaterials have quite similar crystallinity, morphology, and particle size. This confirms that the non-calcined ZnO nanocrystallites are of high purity and well crystallized. Thus, applying our synthetic approach, the high-temperature calcining purification procedure that is usually used in many syntheses is no longer required. Furthermore, our approach greatly simplifies the synthesis of ZnO nanomaterials.
It has been revealed from the SEM images that the morphology and size of the synthesized ZnO crystallites can be tuned by the added lignin amine. We also found that both the lignin amine amount and molar ratio of Zn2+/OH- have a significant effect on the PL spectra of ZnO, especially for the visible emission. The ZnO prepared with 10 mL lignin amine and Zn2+/OH- of 1:4 has the favorable flower-like morphology with the proper crystalline size and also has the most defects, for it displays the most intensive visible emission band.
In brief, the lignin amine used in the present synthesis was obtained by chemically modifying abundant and cost-effective lignin. Therefore, the present study not only provides the possibility of large-scale production of ZnO nanoparticles but renews pulp industrial lignin waste and reduces environmental contamination as well.
This work was supported by the Fundamental Research Funds for the Central Universities (DL12EB05-02) and the Natural Science Foundation of China (30901136, 21273063). The Foundations for the Returned Overseas Chinese Scholars of Heilongjiang Province (LC2011C22) and State Education Ministry are greatly acknowledged.
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