ZnO Nanostructures Grown by Ultrasonic Synthesis

Ultrasonic nanoparticle synthesis has gained increasing attention due to its ability to produce nanomaterials with controlled size, morphology, and crystallinity under mild reaction conditions. The technique leverages acoustic cavitation to generate localized high temperatures and pressures, promoting enhanced nucleation and growth of nanoparticles. Compared to conventional synthesis methods, ultrasonic synthesis offers advantages such as rapid reaction rates, scalability, and the ability to fine-tune structural properties by modifying reaction parameters.

We use the synthesis of ZnO nanostructures as an exemplary case to highlight the advantages of ultrasonic nanoparticle synthesis with modified structures. The study by Morales-Flores et al. (2013) explores the role of sonochemical synthesis in controlling the morphology of ZnO nanostructures. Utilizing the Hielscher probe-type sonicator UP400St (400 watts, 24 kHz), the researchers demonstrated how variations in reaction conditions, particularly pH, influence the final morphology, structural properties, and photoluminescence behavior of ZnO nanostructures.

Experimental Setup – ZnO Nanoparticle Synthesis using Sonication

Aqueous solutions of zinc acetate (0.068 M) were subjected to ultrasonic irradiation at 40 W dissipated power under argon flow. The reaction pH was adjusted between 7 and 10 using ammonium hydroxide (NH4OH), significantly impacting the morphology of the synthesized ZnO structures. The sonochemical process induced acoustic cavitation, generating localized high-temperature and high-pressure conditions that promoted ZnO nucleation and growth.

Influence of pH on Morphology and Structural Properties

Scanning electron microscopy (SEM) revealed distinct morphologies at different pH levels:

• pH 7.0: Formation of rod-like ZnO nanostructures (86 nm width, 1182 nm length) with a mixed ZnO/Zn(OH)2 phase.
• pH 7.5–8.0: Transition to faceted bar and cup-end bars (~250–430 nm length, 135–280 nm width).
• pH 9.0: Spindle-shaped ZnO nanostructures (~256 nm length, 95 nm width) with high microstrain.
• pH 10.0: Uniform faceted nanobars (~407 nm length, 278 nm width) with reduced defect density.

SEM micrographs of ultrasonically synthesized ZnO nanostructures grown at (a) pH 7, (b) pH 7.5, (c) pH 8, d) pH 9,
and (e) pH 10 of the reaction mixture.
(Study and images: ©Flores-Morales et al., 2013)

X-ray diffraction (XRD) confirmed the presence of hexagonal wurtzite ZnO for pH > 7, with enhanced crystallinity and grain growth at higher pH values.

Optical Properties and Defect Control

Room-temperature photoluminescence (PL) analysis highlighted two main emission bands:

• Ultraviolet emission (~380 nm): Near-band-edge excitonic transitions.
• Visible emission (~580 nm): Associated with structural defects such as oxygen vacancies and interstitial defects.

Notably, increasing the pH led to higher defect-related emission intensity up to pH 9, attributed to increased surface area and lattice imperfections. However, at pH 10, the intensity of defect emissions declined due to reduced surface and lattice defects.

“ZnO nanostructures of different morphologies could be fabricated by ultrasonic hydrolysis of zinc acetate in aqueous solution by controlling its hydrolysis rate through pH adjustment. While a solution pH 7 or lower produces impure ZnO nanostructures mixed with Zn(OH)2 phase, higher pH values of the reaction mixture produce ZnO nanostructures in pure hexagonal phase. Controlling solution pH in between 7.5 and 10, phase pure ZnO nanostructures of varied morphology could be produced and the concentration of their structural and surface defects could be controlled. Utilization of low power ultrasound for the chemical synthesis of ZnO nanostructures efficiently has been demonstrated.”
Flores-Morales et al., 2013

This study illustrates the profound impact of ultrasonic irradiation using the UP400St on ZnO nanostructure synthesis. By tuning the pH, the researchers successfully modulated morphology, crystallinity, and defect density. The findings highlight the potential of sonochemical methods for tailored nanoparticle synthesis, offering pathways for applications in optoelectronics and catalysis.

Get the Best Sonicator for your Nanoparticle Synthesis

Hielscher probe-type sonicators are renowned for their power, reliability, precision, and user-friendliness, making them the ideal choice for nanoparticle synthesis. With cutting-edge technology and robust engineering, these ultrasonic processors offer unparalleled control over sonochemical reactions, ensuring reproducibility and efficiency. The UP400St, for instance, provides precise energy input and customizable settings, allowing researchers to tailor synthesis conditions for optimal nanoparticle morphology and crystallinity. Whether for laboratory-scale research or industrial applications, Hielscher sonicators guarantee high performance and ease of use, solidifying their reputation as a top choice for sonochemical synthesis.
Take advantage of the power of ultrasonics for nanoparticle synthesis!
 

Design, Manufacturing and Consulting – Quality Made in Germany

Hielscher ultrasonicators are well-known for their highest quality and design standards. Robustness and easy operation allow the smooth integration of our ultrasonicators into industrial facilities. Rough conditions and demanding environments are easily handled by Hielscher ultrasonicators.

Hielscher Ultrasonics is an ISO certified company and put special emphasis on high-performance ultrasonicators featuring state-of-the-art technology and user-friendliness. Of course, Hielscher ultrasonicators are CE compliant and meet the requirements of UL, CSA and RoHs.

The table below gives you an indication of the approximate processing capacity of our ultrasonicators:

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