Enhanced Quality, Growth Kinetics, and Photocatalysis of ZnO Nanowalls Prepared by Chemical Bath Deposition

ZnO nanowalls (NWLs) represent a nontoxic, abundant, and porous material, with promising applications in sensing and photocatalysis. They can be grown by low-cost solution methods on Al (covered) substrates; Al­(OH)₄^– generated in situ is assumed to be responsible for engendering the NWL morphology. Here, we grew ZnO NWLs by chemical bath deposition (at 70–95 °C). The roles of pH, concentration of Al­(OH)₄^–, and growth time on the thickness and quality of NWL film were experimentally investigated, and the growth kinetics was explained in terms of a self-screening model. Increasing the chemical bath pH from 5.7 to 7.4 led to a 40% thicker film and more NWLs per unit area of the substratedue to increased concentration of Al­(OH)₄^–but these were accompanied by the presence of embedded micro-/nanoparticles. We propose the use of anodized Al as a way to enhance the growth rate and density of the NWLs with no detrimental effect on film quality. Compared with non-anodized Al, NWL film grown on anodized Al (at the lower pH) showed a higher growth rate, an excellent film quality, and a higher photocatalytic activity in the degradation of toxic methyl orange

Double Role of HMTA in ZnO Nanorods Grown by Chemical Bath Deposition

ZnO nanorods (NRs) grown by chemical bath deposition (CBD) are among the most promising semiconducting nanostructures currently investigated for a variety of applications. Still, contrasting experimental results appear in the literature on the microscopic mechanisms leading to high aspect ratio and vertically aligned ZnO NRs. Here, we report on CBD of ZnO NRs using Zn nitrate salt and hexamethylenetetramine (HMTA), evidencing a double role of HMTA in the NRs growth mechanism. Beyond the well-established pH buffering activity, HMTA is shown to introduce a strong steric hindrance effect, biasing growth along the <i>c</i>-axis and ensuring the vertical arrangement. This twofold function of HMTA should be taken into account for avoiding detrimental phenomena such as merging or suppression of NRs, which occur at low HMTA concentration

Role of Au_<i>x</i>Pt_1–<i>x</i> Clusters in the Enhancement of the Electrochemical Activity of ZnO Nanorod Electrodes

This study quantitatively elucidates the role of metal clusters in the electrochemical activation of metal-oxide nanostructured electrodes. Through the deposition of nearly monodisperse Au_<i>x</i>Pt_1–<i>x</i> (<i>x</i> = 0, 0.5, 1) clusters, smaller than 3 nm, on the ZnO nanorod (NR) electrode surface, a controlled enhancement of charge transfer and activation of electrocatalytic processes was achieved. The interfacial electrical states of the hybrid electrodes were probed by electrochemical impedance spectroscopy (EIS). Analysis of the charge-transfer resistance and interface capacitance, estimated by modeling EIS curves in different bias regimes, indicated the presence of a large amount of active donor states (∼10²⁰ cm^–3) at the surface of the ZnO NRs. Decoration of the ZnO NRs with Au_<i>x</i>Pt_1–<i>x</i> clusters strongly increased the charge-transfer process at the cluster–ZnO/electrolyte interface. This induced a more effective depletion of the electron charge available in the donor states of the ZnO NRs, leading to the formation of a positively charged layer at the interface between ZnO and the clusters. These two effects, intrinsically linked with the alignment between the electronic states of the Au_<i>x</i>Pt_1–<i>x</i> clusters and ZnO, strongly enhance the interface reactivity of the ZnO NR electrodes toward the redox reaction of potassium ferricyanide. This is particularly relevant for understanding and improving the performance of electrochemical biosensors