Synthesis and Characterization of Cu-doped ZnO Nanorods

Cu-doped ZnO nanorods were synthesized by sol-gel method using zinc nitrate tetrahydrate, methenamine and cupric acetate monohydrate as precursors. The as-synthesized ZnO nanorods have a twin-rod structure. The polar (002) surface of ZnO nanorods, which could be either negatively charge (O-terminated) or positively charged (Zn-terminated), was responsible for the formation of twin-rod structure. The results showed that the size, aspect ratio, crystallinity and c-lattice parameter of Cu doped ZnO nanorods decreased with increasing of Cu dopant concentration. In fact, the presence of Cu retarded the growth of ZnO nanorods in its preferred growth direction, i.e. (0001). The XPS analysis indicates that Cu ions were oxidized (Cu2+) and substituted into the ZnO lattice at the Zn2+ site. The presence of Cu reduced the optical bandgap of ZnO from 3.34 eV (undoped ZnO nanorods) to 3.31 eV (20 mol% Cu doped ZnO). Besides, it induced a visible PL emission at 2.97 eV, which could be related to the transition of electrons from conduction band (Ec) to Cu acceptor energy level (Ev + 0.45 eV) radiatively.

Fig. 1: ZnO nanorods doped with different Cu dopant concentration (a) 1 mol%, (b) 5 mol%, (c) 10 mol% and (d) 15 mol%.

[Reference: S.Y. Pung, C.S. Ong, K. Mohd Isha and M. H. Othman, “Synthesis and characterization of Cu-doped ZnO nanorods”, Sains Malaysiana 43 (2) (2014) 273-281.]

Degradation of Organic Dye using ZnO nanorods based Continuous Flow Water Purifier

Zinc oxide (ZnO) is a common semiconductor material uses in waste water treatment. However, utilizing of ZnO particles could be easily drained away by water and charged into the water system during the photocatalytic treatment. This could result of forming secondary pollutionin the water system. Hence, it is necessity to grow ZnO nanorods on polyethylene terephthalate (PET) fiber to minimize the above mentioned problem. In this work, ZnO nanorods were grown on the flexible PET fiber in large quantity using a sol gel method at low temperature (90 ^oC). A layer of 1-dodacanethiol polymer was per-coated on the PET fiber to improve the deposition of ZnO seed layer prior to the growth of ZnO nanorods. The PET fiber was covered with high areal density of ZnO nanorods (10.2 ± 0.8NRs/μm²). Subsequently, this PET fiber was inserted into a glass tube for the setup of continuous flow water purifier. The photocatalytic study for degradation of Rhodamine B solution using this setup indicated that the reaction followed 1^st order kinetic with rate constant of 1.28 h^-1. The ZnO nanorods were still intact with the fiber after the photocatalytic study. Thus, it is possible to upscale this setup as water purifier to purify the water system.

Fig. 1: Schematic of continuous flow photocatalytic water purifier.

 

 Fig. 2: SEM images of ZnO nanorods grown on PET fiber (1 kX) (Inset: ZnO nanorods on fiber at 10 kX)

[Reference: Y.L. Chan, S.Y. Pung and S. Sreekantan, “Degradation of Organic Dye using ZnO nanorods based Continuous Flow Water Purifier, J. Sol-Gel Sci. Tech. 6 (2013) 399-405.]

 

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).

                                          (a)

                                          (b)

                                           (c)

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

                                         (a)

                                         (b) base

                                         (c) body

                                         (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.]