Synthesis of Zinc Oxide Nanostructures on Graphene/Glass Substrate via Electrochemical Deposition: Effects of Potassium Chloride and Hexamethylenetetramine as Supporting Reagents

Abstract

The effects of the supporting reagents hexamethylenetetramine (HMTA) and potassium chloride (KCl) mixed in zinc nitrate hexahydrate (Zn(NO3)2·6H2O) on the morphological, structural, and optical properties of the resulting ZnO nanostructures electrodeposited on graphene/glass substrates were investigated. The supporting reagent HMTA does not increase the density of nanorods, but it does remarkably improve the smoothness of the top edge surfaces and the hexagonal shape of the nanorods even at a low temperature of 75 °C. Hydroxyl (OH−) ions from the HMTA suppress the sidewall growth of non-polar planes and promote the growth of ZnO on the polar plane to produce vertically aligned nanorods along the c axis. By contrast, the highly electronegative chlorine (Cl−) ions from the supporting reagent KCl suppress the growth of ZnO on the polar plane and promote the growth on non-polar planes to produce vertical stacking nanowall structures. HMTA was found to be able to significantly improve the crystallinity of the grown ZnO structures, as indicated by the observation of much lower FWHM values and a higher intensity ratio of the emission in the UV region to the emission in the visible region. Equimolar mixtures of Zn(NO3)2·6H2O and the supporting reagents HMTA and KCl seem to provide the optimum ratio of concentrations for the growth of high-density, uniform ZnO nanostructures. The corresponding transmittances for such molar ranges are approximately 55–58 % (HMTA) and 63–70 % (KCl), which are acceptable for solar cell and optoelectronic devices.

Effect of Annealing Temperature on ECD Grown Hexagonal-Plane Zinc Oxide

“…The overall reaction for nucleation was dependent on the deposition process to the cathode surface. Hence, considering the primary growth reaction of ZnO via ECD with Zn(NO 3 ) 2 ($\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_{2}6{\mathrm{H}}_2\mathrm{normalO}$) and KCl as reagents, they can be broken down in the chemical reactions listed below [45,46,47,48]:$\mathrm{Zn}{({\mathrm{NO}}_3)}_2+2\mathrm{KCl}\leftrightharpoons {\mathrm{ZnCl}}_2+2{\mathrm{KNO}}_3$…”

“…Additionally, excessive amounts of KCl could promote other growth species, such as KNO 3 and ZnCl 2 while 0.1 M is proven to be sufficient for the purpose of capping the (0001) plane [52]. Thus, 0.1 M was chosen as the standard growth condition in this work [45,53]. Through extraction of the reduction current (${\mathrm{I}}_{\mathrm{normalP}\left(\mathrm{re}\right)}$) and oxidation current (${\mathrm{I}}_{\mathrm{normalP}\left(\mathrm{ox}\right)}$) from Figure 3b into Figure 3c, there is a clear enhancement trend shown with increasing KCl concentration, while presented in Figure 3d is the ratio of ${\mathrm{I}}_{\mathrm{normalP}\left(\mathrm{ox}\right)}/{\mathrm{I}}_{\mathrm{normalP}\left(\mathrm{re}\right)}$, which remains relatively constant as a function of [KCl].…”

Si Nanocrystals/ZnO Nanowires Hybrid Structures as Immobilized Photocatalysts for Photodegradation

“…Various forms of ZnO nanostructures have attracted considerable attention as one of the most efficient catalysts for photocatalytic degradation, due to their high specific surface-area, direct bandgap, rapid carrier transport, and strong oxidation ability [1,2]. ZnO is also an abundant, environmentally friendly material, and it is easy and inexpensive to grow in nanostructures [3,4]. Under UV light irradiation, organic pollutants in waste water streams can be decontaminated in the presence of ZnO nanostructures, where the photogenerated electrons and holes travel to the catalyst surface and produce highly oxidizing species such as hydroxyl radicals and super oxide ions responsible for degradation reactions [5].…”

Synthesis of a zinc oxide/graphene hybrid material by the direct thermal decomposition of oxalate

“…Creating strong electronic interaction between nanoscale graphene and ZnO in a composite can significantly modify the ZnO band structure, leading to an array of exciting new properties for the bulk material. For example, they have seen use as photocatalysts [4][5][6][7][8][9][10], sensors [11][12][13][14], capacitors [15][16][17][18][19][20], and as transparent conductive thin-films [21] to name just a few. Equally as diverse as their applications are the methods of synthesizing ZnO/graphene hybrid materials.…”

“…Equally as diverse as their applications are the methods of synthesizing ZnO/graphene hybrid materials. By far, the most popular fabrication methods involve either solvothermal or sol-gel techniques [4,9,14,16,20,[22][23][24][25][26][27], however there have been a fair number of studies involving either electrodeposition of the material [7,11,21,28], or solutionbased synthesis involving pH manipulation of a Zn 2+ ion precursor [15,29,30].…”