Morphology controllable synthesis of ZnO crystals—pH-dependent growth

doi:10.1016/j.matchemphys.2010.03.067

Materials Chemistry and Physics

Volume 123, Issue 1, 1 September 2010, Pages 104-108

https://doi.org/10.1016/j.matchemphys.2010.03.067 Get rights and content

Abstract

ZnO crystals were grown through a solution-based chemical route at ambient pressure and low temperature. It was found that the solution pH is a dominative factor in determining the morphology of crystals: Rod-like ZnO crystals are apt to be formed at near neutral condition, whereas flower-like structured ZnO crystals are preferred to be formed at higher solution pH. By monitoring the reaction intermediates during the ZnO growth process, it was realized that the crucial role of solution pH in determining the morphology of ZnO crystals is to control the structure of the primary reaction intermediates at the early stage of ZnO growth. Moreover, by appreciate controlling the solution pH together with Zn²⁺ concentration, various ZnO crystals of rod-like, dumbbell-like and even more complex flower-like structures were obtained without any template, and a pH-dependent morphology controllable growth mechanism is suggested.

Introduction

ZnO is an important direct semiconductor with bandgap of 3.37 eV and large exciton bind energy of 60 meV at room temperature. Besides, its nontoxicity, inexpensive and chemical stability has made ZnO a promising material in various fields including room-temperature ultraviolet laser [1], [2], [3], field-effect transistor [4], [5], photodetector [6], gas sensor [7], photocatalyst [8] as well as solar cells [9], [10]. To fulfill the increased demands for those applications, efforts have been devoted to obtain ZnO nanocrystals with controlled size and architecture because the optical, physicochemical and electric properties of ZnO crystals are intimately dependent on its size and shape. Up till date, various synthetic methods have been developed to fabricate ZnO nanocrystals with controlled morphologies, where the physical route such as thermal evaporation and pulsed laser deposition, etc. has been proven to be one of the most successful ways [11], [12], [13], [14].

Compared to the physical route where harsh condition such as high temperature (>1000 °C) or special equipment is generally required, the solution-based chemical approach is more attractive because of its low growth temperature and easy preparation of large batches. By controlling the experimental parameters, ZnO nanocrystals of various shapes such as rods [15], [16], [17], tube [18], [19], wires [20], rings [21], etc., have been obtained. More recently, Yu et al. obtained ZnO nanotube bundles via a solution route [22]. Basically, the formation of ZnO crystal in solution is divided into two stages of nucleation and crystal growth, where external conditions such as solution pH, the kinds of precursors and additives, etc. were found to influence both the size and morphology of the resulted ZnO crystals by altering the kinetics of both nucleation and crystal growth [23], [24], [25], [26], [27], [28]. However, challenges are still remaining in understanding the roles of the experimental parameters for accurate controlling of the morphology of ZnO crystals via solution-based approaches.

In this paper, the effect of solution pH on the morphology of ZnO crystals was investigated by monitoring the reaction intermediates produced during the whole ZnO growth processes. It was found that the primary reaction intermediates formed at the early stage of ZnO growth were strongly pH dependent, which lead to form ZnO crystals with varied morphologies. Furthermore, by appreciate controlling the solution pH and Zn²⁺ concentration various ZnO crystals of rod-like, dumbbell-like and even more complex flower-like structures were obtained, and a pH-dependent morphology controllable ZnO growth mechanism was suggested.

Section snippets

Experimental

All chemical reagents in the experiments are of analytical grade and used as received without further purification. Distilled water was used in all experiments.

ZnO crystals were synthesized through a solution-based chemical route at low temperature and ambient pressure. Typically, 25 ml aqueous solution containing certain amount of hexamethylenetetramine (HMT, (CH₂)₆N₄, Shantou Xilong Chemicals) was added to 25 ml of zinc nitrate (Zn(NO₃)₂·6H₂O, Shantou Xilong Chemicals) aqueous solution with the

Results and discussion

Fig. 1 shows the SEM images of ZnO crystals obtained by refluxing of Zn²⁺ and HMT in aqueous solution at different solution pHs. As can be seen, separated ZnO rods of well-defined hexagonal shapes are formed at lower solution pH (Fig. 1A), which is attributed to the anisotropic growth character of ZnO crystals [29]. Interestingly, when ZnO was growing in solution at higher pH, the obtained ZnO crystals are of complicated “peony” shaped structure (Fig. 1B). As the other parameters except for the

Conclusions

In summary, ZnO crystals were synthesized through a simple solution route at low temperature and ambient pressure. It was found that the solution pH plays a decisive role in determining the morphology of ZnO architecturally, in which near neutral condition favors to form rod-like ZnO nanocrystals and higher alkali condition prefers to form flower-like structured ZnO crystals. By monitoring the reaction intermediates produced during the whole ZnO growth processes, direct observation of ZnO

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Grant No. 20473010).

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Morphology controllable synthesis of ZnO crystals—pH-dependent growth

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgment

J. Cryst. Growth

Sens. Actuators B

Solar Energy Mater. Solar Cells

J. Cryst. Growth

Mater. Lett.

J. Cryst. Growth

Adv. Mater.

Science

Appl. Phys. Lett.

J. Phys. Chem. B

Nano Lett.

Adv. Mater.

Cryst. Growth Des.

Chem. Mater.

J. Phys. Chem. B

Appl. Phys. Lett.

J. Am. Chem. Soc.