Kinetic Study of Organic Dye Degradation Using ZnO Particles

Zinc oxide (ZnO) particles were successfully synthesized via sol-gel approach using zinc acetate dihydrate (Zn(CH₃COO)₂.2H₂O) and ammonia (NH₄OH) solution as precursors. By adjusting the reaction parameters such as amount of ammonia, reaction time as well as complexing agent aluminium sulphate Al₂(SO₄)₃, ZnO particles with different morphologies i.e. rod-like, rice-like and disk-like could be synthesized. The effectiveness of ZnO particles with different morphologies (rod-like, rice-like and disk-like) on the photocatalytic activity has been studied. The results showed that rod-like ZnO particles were the most effective in degrading the Rhodamine B (RhB) solution under the illumination of ultraviolet (UV) light. The rate constant was found to be first order, with rod-like particles was the highest (0.06329 min^-1), followed by rice-like ZnO particles (0.0431 min^-1) and disk-like ZnO particles  (0.02448 min^-1).

Fig. 1. Rod-like ZnO nanoparticles.

Fig. 2. Cone-like ZnO nanoparticles.

Fig. 3. Disc-like ZnO nanoparticles.

Fig. 4. Degradation of RhB solution by rod-like ZnO nanoparticles.

[Reference: SY. Pung, W.P. Lee, and A. Azizan, “Kinetic study of organic dye degradation using ZnO particles with different morphologies as a photocatalyst”, Int. J. Inorg. Chem., (2012) doi:10.1155/2012/608183.]

Tip-growth mode and base-growth mode Au-catalyzed ZnO NWs

Au could catalyze the growth of ZnO NWs in two possible ways depending on the related position of Au alloy droplets (nanoparticles) to the NWs. The growth was called tip-growth mechanism if the Au alloy nanoparticles were found at the tips of ZnO NWs (Fig. 1), whereas the growth was named base-growth mechanism if the Au alloy nanoparticles were found at the base of the ZnO NWs (Fig. 2).

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Fig. 1 (a) Tip-growth Au-catalyzed ZnO NW, EDS analysis of NW at the (b) body and (c) tip.

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                                         (b) base

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                                         (d) tip

Fig. 2 (a) Base-growth Au-catalyzed ZnO NW, EDS analysis of NW at the (b) base, (c) body and (d) tip.

[Reference: Pung, S. Y.;Choy, K. L.;Hou, X.; J. Crys. Growth 2010, 312, 2049 - 2055.]

Growth of catalyst-free ZnO nanowires

Among semiconductor nanomaterials, ZnO has been one of the most interesting systems due to their extraordinary properties and remarkable multifunction capability. Zinc oxide (ZnO) is a II-VI compound semiconductor with wide band gap (E_g = 3.37 eV) and large exciton binding energy (E_b = 60 meV) at room temperature. The development of ZnO can be traced since 1912. The present renaissance of ZnO research started in the mid 1990s. Due to the unique properties, ZnO nanostructures, particularly NWs, are potential candidates for applications in solar cells, gas sensors, light emitting diodes, ultraviolet lasers and field effect transistors (FETs), etc..

          Various vapor route approaches such as pulse laser deposition, metal-organic chemical vapour deposition and atomic layer deposition, have been used for synthesizing ZnO NWs. Amongst them, CVD is the most popular approach uses by researchers to produce ZnO NWs. Two requirements are needed to produce vertically aligned ZnO NWs. Firstly, substrates/epilayers with small lattice mismatch to ZnO (heteroepitaxial growth) or highly c-oriented ZnO seed layers (homoepitaxial growth/catalyst-free growth) are necessary for facilitating the growth of aligned NWs. Secondly, a moderate synthesis condition is required to maintain the epitaxial relationship between the substrate and the ZnO NWs during the CVD process so as to achieve aligned growth of NWs.
            Figure 1 shows the catalyst-free growth of ZnO NWs on highly c-oriented ZnO seed layer using CVD. 




Fig. 1. (a) Vertically aligned ZnO NWs grown on highly c-oriented ZnO seed layer, (b) Top view of ZnO NWs shows that the tip of NWs are hexagonal shape.

[Reference: Pung, S. Y.;Choy, K. L.;Hou, X. et al.; Nanotechnology 2008, 19, 435609.]