Chemical Bath Deposition of ZnO Nanowires at Near-Neutral pH Conditions without Hexamethylenetetramine (HMTA): Understanding the Role of HMTA in ZnO Nanowire Growth

Abstract

Chemical bath deposition (CBD) is an inexpensive and reproducible method for depositing ZnO nanowire arrays over large areas. The aqueous Zn(NO(3))(2)-hexamethylenetetramine (HMTA) chemistry is one of the most common CBD chemistries for ZnO nanowire synthesis, but some details of the reaction mechanism are still not well-understood. Here, we report the use of in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to study HMTA adsorption from aqueous solutions onto ZnO nanoparticle films and show that HMTA does not adsorb on ZnO. This result refutes earlier claims that the anisotropic morphology arises from HMTA adsorbing onto and capping the ZnO {10 1 0} faces. We conclude that the role of HMTA in the CBD of ZnO nanowires is only to control the saturation index of ZnO. Furthermore, we demonstrate the first deposition of ZnO nanowire arrays at 90 °C and near-neutral pH conditions without HMTA. Nanowires were grown using the pH buffer 2-(N-morpholino)ethanesulfonic acid (MES) and continuous titratation with KOH to maintain the same pH conditions where growth with HMTA occurs. This semi-batch synthetic method opens many new opportunities to tailor the ZnO morphology and properties by independently controlling temperature and pH.

Influence of anneal atmosphere on ZnO-nanorod photoluminescent and morphological properties with self-powered photodetector performance

“…(1) and (2)). 13,43,44 For $pH 11 solutions, production of ZnO occurs via the equilibrium reactions shown in Eqs. (3) and (4).…”

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“…(Results on the formation of undoped ZnO-GFs as a baseline for the synthesis of CZO-GFs are also reported for completeness.) The growth of nanostructured ZnO by a solution synthesis pathway has been a topic of keen research interest for the past decade or so [10][11][12][13], motivated by the need for an inexpensive means of preparing such material with the potential for scale-up [14]. The context of our work is that of non-electrochemical routes to NW synthesis where the general method used falls into the category of chemical bath deposition (CBD).…”

“…The CBD method has typically been based on Zn(NO 3 ) 2 -HMTA chemistry (where HMTA is hexamethylenetetramine, (CH 2 ) 6 N 4 ) [11] and also involves the use of a thin-film or particulate seed layer of ZnO [12] or a metal catalyst. Recent insight on the reaction chemistry using attenuated total reflection Fourier transform infrared spectroscopy has revealed that the role of HMTA in the anisotropic deposition chemistry is one of controlling the saturation index of ZnO as described by McPeak et al [13]-these authors go on to demonstrate the growth of ZnO-NWs without HTMA, but using a pH buffer in combination with continuous KOH titration where the seeding substrate comprises a high-rugosity ZnO layer prepared by drop-coating from a ZnO nanoparticle suspension in ethanol/water. In fact, the method employed in our work is based on an even simpler technique of a repeated two-step, dropcasting and annealing sequence-where the drop-casting is akin to the seed layer preparation method of ref [13] , but doesn't involve attendant issues of that method such as pH control.…”

“…Recent insight on the reaction chemistry using attenuated total reflection Fourier transform infrared spectroscopy has revealed that the role of HMTA in the anisotropic deposition chemistry is one of controlling the saturation index of ZnO as described by McPeak et al [13]-these authors go on to demonstrate the growth of ZnO-NWs without HTMA, but using a pH buffer in combination with continuous KOH titration where the seeding substrate comprises a high-rugosity ZnO layer prepared by drop-coating from a ZnO nanoparticle suspension in ethanol/water. In fact, the method employed in our work is based on an even simpler technique of a repeated two-step, dropcasting and annealing sequence-where the drop-casting is akin to the seed layer preparation method of ref [13] , but doesn't involve attendant issues of that method such as pH control. In addition, there is no specific, separate process stage to form a seed growth layer, or any constraint of substrate crystallinity to promote epitaxy, yet the growth of doped ZnO nanowires proceeds remarkably successfully on an amorphous (quartz) substrate.…”

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“…Firstly, -NH 2 groups of PEI can react with Zn 2+ and Eu 3+ to form their chelates. Then, the chelates react with [29]. The growth speed of Eudoped ZnO crystals depends on the releasing speed of those ions.…”