Ultra-low Concentration of Cellulose Nanofibers (CNFs) for Enhanced Nucleation and Yield of ZnO Nanoparticles

Abstract

Cellulose nanofibers (CNFs) were used in aqueous synthesis protocols for zinc oxide (ZnO) to affect the formation of the ZnO particles. Different concentrations of CNFs were evaluated in two different synthesis protocols producing distinctly different ZnO morphologies (flowers and sea urchins) as either dominantly oxygen- or zinc-terminated particles. The CNF effects on the ZnO formation were investigated by implementing a heat-treatment method at 400 °C that fully removed the cellulose material without affecting the ZnO particles made in the presence of CNFs. The inorganic phase formations were monitored by extracting samples during the enforced precipitations to observe changes in the ZnO morphologies. A decrease in the size of the ZnO particles could be observed for all synthesis protocols, already occurring at small additions of CNFs. At as low as 0.1 g/L CNFs, the particle size decreased by 50% for the flower-shaped particles and 45% for the sea-urchin-shaped particles. The formation of smaller particles was accompanied by increased yield by 13 and 15% due to the CNFs' ability to enhance the nucleation, resulting in greater mass of ZnO divided among a larger number of particles. The enhanced nucleation could also be verified as useful for preventing secondary morphologies from forming, which grew on the firstly precipitated particles. The suppression of secondary growths' was due to the more rapid inorganic phase formation during the early phases of the reactions and the faster consumption of dissolved salts, leaving smaller amounts of metal salts present at later stages of the reactions. The findings show that using cellulose to guide inorganic nanoparticle growth can be predicted as an emerging field in the preparation of functional inorganic micro/nanoparticles. The observations are highly relevant in any industrial setting for the large-scale and resource-efficient production of ZnO.

Figure 1

Schematic illustrating the post-treatment procedures with the removal of the CNF fraction through heat treatment demonstrated at the bottom and the freeze-drying procedure of the ZnO/CNF hybrid material at the top. The micrographs represent the flower-shaped ZnO particles produced in the presence of 0.1 g/L of CNFs after the heat-treatment (bottom) and freeze-drying (top), respectively, with the arrows highlighting the remaining CNFs.

Figure 2

Micrographs of ZnO particles referred to as flowers a and b (run 1 and run 3 (Table 1)) and sea urchins c and d (run 4 and run 6 (Table 1)) after calcination at 400 °C. The effect of performing the synthesis in the presence of 0.1 g/L of CNF is demonstrated. The histograms show the distribution of particle diameters in micrometers for the flower-shaped particles (a: average size 3.52 μm and b: average size 1.76 μm) and the sea urchin particles (c: average size 2.45 μm and d: average size 1.34 μm) consisting of nanosheets and hexagonally faceted rods, respectively. The red lines represent the average particle sizes. All particles (regardless if synthesized with CNF) were calcinated for accurate comparison.

Figure 3

Micrographs of the freeze-dried samples from the reaction producing dominantly rod-based sea urchin structures in a CNF concentration of 0.1 g/L (see Table 1).

Figure 4

X-ray diffractograms of freeze-dried samples of the (a) flower particles and (b) sea urchin particles at CNF contents of 0 to 0.1 g/L. All diffraction peaks solely correspond to the ZnO wurtzite phase.

Figure 5

Micrographs of ZnO particles referred to as (a–c) flower-shaped and (d–f) sea-urchin-shaped. The effect of different amounts of CNF, 0.05 and 0.1 g/L, present during the ZnO-synthesis is shown for (b, c) flower structures and (e, f) sea urchin structures. The CNF was removed via thermal degradation at 400 °C before microscopy imaging.

Figure 6

Flower-shaped ZnO particles obtained (a–c) in the absence of CNF and (c–e) with CNF present at 0.1 g/L for reaction times of (a, d) 1 min, (b, e) 15 min, and (c, f) 60 min. (g–l) The appearance of the sea urchin structures obtained (g–i) without the incorporation of CNF and (j–l) when incorporating 0.1 g/L of CNF at the same reaction times. The calcination was made at 400 °C with the purpose of highlighting the morphological ZnO differences after temperature exposure that solely removed the cellulose with unaffected ZnO structures.

Figure 7

Micrographs showing the freeze-dried samples of the (a, b) flower-shaped and (c, d) sea-urchin-shaped ZnO structures made in the presence of CNF at a concentration of 0.1 g/L. The micrographs show the appearance of the freeze-dried ZnO/CNF material (a, c) 1 min and (b, d) 15 min into the reaction. The arrows highlight the presence of cellulose nanofibers in the extracted aliquots.