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Ultrasound-assisted preparation of ZnO nanostructures: understanding the effect of operating parameters

  • Niraj R. Sikwal

    Niraj R. Sikwal received his BE degree in chemical engineering in 2010 from PRE College Loni, Pune University, India, and his MTech degree in chemical engineering from the National Institute of Technology, Warangal, India, in 2014. He published a chapter in the Handbook of Functional Nano Materials and filed an Indian patent on the production of iron oxide pigment. His has research interests in the areas of nanotechnology and microreactors.

         , Shirish H. Sonawane

    Shirish H. Sonawane is currently working as an associate professor in the Chemical Engineering Department, National Institute of Technology, Warangal, India. His research interest focused on the synthesis of hybrid nanomaterials, cavitation-based inorganic particle synthesis, sonochemical synthesis of nanolatex, process intensification, and microreactor for nanoparticle production. Dr. Sonawane has published more than 50 research papers in reputed journals, 7 book chapters, and 6 Indian patent applications. He completed 6 consultancy projects from chemical industries and 4 research projects from the government.

     EMAIL logo     , Bharat A. Bhanvase

    Bharat A. Bhanvase is currently working as an associate professor in the Chemical Engineering Department, Laxminarayan Institute of Technology, RTM Nagpur University, Nagpur, Maharashtra, India. His research interests focused on heterogeneous catalysis, nanomaterials and nanocomposites, process intensification, microfluidics, and nanofluids. He has published 35 articles in international journals, 4 in national journals, and presented 6 papers in international conferences and 12 in national conferences. He has written 9 book chapters in internationally renowned books and applied for 3 Indian patents.

         , Kirankumar Ramisetty

    Kirankumar Ramisetty is working as a postdoctoral research associate at the University of Limerick, Ireland. He finished his doctoral research under the guidance of eminent researchers Dr. Parag Gogate and Dr. A. B. Pandit. His extensive work on the application of ultrasonic cavitation on physical processes such as atomization, crystallization, and emulsification contributed to the publication of 5 research articles with 21 citations. His current work is focused on the application of ultrasound to tailor the crystal size distribution for improved formulation properties of API compounds.

         , Dipak V. Pinjari

    Dipak V. Pinjari is currently working as an assistant professor at the Institute of Chemical Technology, Mumbai, India. His research interests focus on process intensification, cavitation engineering and technology, synthesis of nanomaterials, polymers, sonochemistry, and paint technology. He has completed his PhD in chemical engineering in 2012 from the Institute of Chemical Technology, Mumbai, India. He has published 36 articles in international journals. He has filed 3 Indian patents. He has more than 2 years of teaching experience and 9 years of research experience.

         , Parag R. Gogate

    Parag R. Gogate is a versatile chemical engineer with outstanding research work based on the use of alternate energy sources for process intensification, wastewater treatment, and synthesis of nanomaterials. Dr. Gogate has published extensively in journals of high repute, with 202 international journal publications along with over 6500 citations (h-index of 44) as well as has written 15 chapters in edited books. Dr. Gogate has active consultancy projects with many national/international industries and collaborations with many research groups worldwide.

         and Rajulapati Satish Babu

    Rajulapati Satish Babu received his MTech and PhD degrees in biotechnology from the Jawaharlal Nehru Technological University, Hyderabad, India, in 2005 and 2013, respectively. He has been an assistant professor in the Biotechnology Department of the National Institute of Technology, Warangal, India, since 2012. His research interests are in the areas of bioinformatics, bioprocess engineering, environmental biotechnology, modeling, simulation, and optimization of bioprocesses.
Published/Copyright: March 25, 2016
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Green Processing and Synthesis
From the journal Green Processing and Synthesis Volume 5 Issue 2

Abstract

The present work deals with the use of ultrasound atomization for the preparation of zinc oxide nanostructures. The focus of the work is to understand the effect of different operating parameters, such as ultrasonic power dissipation, flow rate, concentration, and surface tension, on the final particle morphology obtained during the synthesis. The prepared nanostructures were observed under scanning electron microscope to understand the morphology of the synthesized nanostructures. It was established that the final characteristics of the nanostructures, in terms of shape and size, can be effectively controlled by controlling flow rate, precursor concentration, surface tension, and ultrasonic power dissipation. The droplet size was found to increase with an increase in the flow rate of an aqueous solution of zinc nitrate, ultrasonic power, and decrease in the loading of surfactant (polyvinylpyrrolidone). The present work has clearly established the utility of ultrasound-assisted synthesis of nanoparticles with clear evidence for obtaining desired characteristics based on controlled application.

Keywords: atomization; droplet size; nanoparticles; operating parameters; ultrasound

1 Introduction

Atomization is an important process applied for the formation of droplets and/or particles based on instantaneous reactions, and usually, the final droplet final particle morphology, which depend on the operating parameters as well as the approach of atomization, are significantly important. Apart from the general application of obtaining liquid droplets, a number of applications in terms of obtaining powders, spraying of coatings on substrate, atomizing fuel in combustors, encapsulation, etc. are also becoming areas of interest depending on the strong requisites for the particle size distribution. For such atomization applications, a number of atomization approaches such as pressure atomization, spinning disc atomization, two-fluid atomization, etc. [1–3] have been generally used. Mechanical atomization processes rely on the use of pressurized liquid where mechanical energy is converted into kinetic energy, and typically, smaller droplets of liquid are obtained though with less control over the distribution. Conventional atomization processes are usually pressure atomization and spinning disk atomization, which do not offer much control over the droplet size and are not energy efficient.

Ultrasound-assisted atomization processes that use a vibrating atomizer surface for the generation of droplets from the liquid spread on the surface through a concentric hole at the center are also becoming common. It has been reported that ultrasound-assisted atomization gives better control over the droplet size characteristics as the formation of droplet and its distribution are governed by the relative contributions of the capillary wave hypothesis and cavitation hypothesis, which in turn are dependent on the operating parameters [1, 4, 5].

Research into developing processes to control the morphology and size of the inorganic nanomaterials has attracted significant attention because the structure and size of nanoparticles play an important role in determining the optical, electrical, and other physicochemical properties of the synthesized materials [6–9]. Nanocrystalline zinc oxide (ZnO) has important applications as catalyst [10], photocatalyst [11], polymer filler and additives for UV screening [12], antiwear additives in oil lubricants [13], and advanced ceramics [14]. Among all nanostructured semiconducting materials, ZnO has the widest band gap (3.37 eV), with a large excitation binding energy (60 meV). Besides its non-toxicity and lower costs, its chemical stability has made ZnO a promising material in various fields including photocatalysis [15], solar cells [16], sensors [17], nanogenerators [18], room temperature UV laser applications [19], and optical waveguides [20]. Considering the important applications of ZnO nanomaterials and the associated dependence of effectiveness of application on particle size distribution, the present work has concentrated on investigations related to the synthesis of ZnO nanomaterials.

Various chemical synthesis methods such as sol-gel method [21], solvo thermal approach [22], microemulsion-based approach [23], and physical methods, including chemical vapor deposition [24], thermal decomposition of organometallic precursors [25], and synthesis approaches based on arc plasma [26], laser ablation [27], and levitation gas condensation [28], have been used for the preparation of ZnO nanoparticles. Improved synthesis routes based on a simple hydrothermal treatment for the fabrication of nanostructured products have also been reported [29]. Compared to chemical methods, physical methods are always advantageous, as such methods do not require the necessary processing steps of wet chemistry such as washing, drying, and calcination and also because physical methods can produce particles with higher purity. Among the physical methods, spray pyrolysis is generally used for the synthesis of nano-sized metal oxides, but this approach requires high-pressure air to atomize precursor solution and thus the overall operation can be highly energy intensive. The present work deals with the use of ultrasound-assisted approach for the synthesis of ZnO nanostructures using aqueous solution of zinc nitrate as the raw material with the objective of process intensification as well as understanding the engineering aspects in terms of the effect of important operating parameters. There have been some studies related to the use of the ultrasound for the synthesis of nanostructures. Khorsand Zak et al. [30] investigated the synthesis of ZnO nanostructures using the ultrasound-assisted method and reported that the sonochemical approach is green and does not require high temperatures or toxic chemicals and that the shape of the particles can be adjusted by varying the frequency of irradiation. Xie et al. [31] also reported a simple, surfactant-free method for the preparation of double pyramid-like ZnO architectures using sonochemistry. A detailed analysis of the literature reveals that ultrasonic atomization-based synthesis of ZnO nanostructures has not been significantly investigated in the past, despite the intensification benefits of the approach, such as lower reaction time and energy requirement as well as better control over the final product characteristics, and hence, the present work forms a significant contribution to the field of process intensification of nanomaterial synthesis. The final size and particle characteristics of the nanostructures will be dependent on the droplet size generated in the atomization process, which in turn depends on the operating parameters. The effect of the various parameters such as ultrasonic power, flow rate, concentration of zinc nitrate aqueous solution, and surface tension has been established in this work and a detailed investigation is presented. The phenomenon of the ultrasonic atomization and the actual process of the formation of droplets have also been captured using photographic analysis.

2 Materials and methods

2.1 Materials

Zinc acetate dihydrate [(Zn(COOCH3)2·2H2O] and zinc nitrate (purity above 95%) have been obtained from Sigma-Aldrich, Mumbai, India, whereas ethanol and acetone were obtained from Hi Theme Chemicals (Hanamkonda, Warangal, India). Polyvinylpyrrolidone (PVP; average molecular weight of 40,000 g/mol; Merck, Mumbai, India) has been used as the capping agent (surfactant). All the chemicals used as received from the supplier without any further purification.

2.2 Experimental setup for ultrasonic atomization

A typical flat surface ultrasonic atomizer operating at 20-kHz frequency and with 130 W rated power output (ATFT, 20–40 kHz, model VCX 134; Sonics and Materials, Newtown, CT, USA) was used in the present work. The atomizer consists of a vibrating surface with a concentric hole (surface diameter of 20 mm and orifice size of 2 mm) for the spread of the introduced liquid. The plate is connected to the transducer driven by an electric generator, which has a provision to vary the power delivered to the atomizer tip by changing the percentage amplitude. Liquid is delivered to the atomizer tip through the concentric hole of 2 mm using a syringe pump, and the effect of flow rate was investigated over the range from 0.1 to 10 mL/s. The droplets ejected from the atomizer were photographed using a Canon camera attached with a 45-mm macro lens with high shutter speed (1/320 s). The use of higher shutter speed enabled freezing of the droplets as soon as they are ejected. An external flashgun was used in the measurements and was kept behind the flow of atomized droplets so as to give adequate light for capturing the fast-moving droplets at higher shutter speeds. The atomizer was kept in a wooden box (which is covered with black paper as a background for photographic analysis) to avoid the effect of external air currents on the movement of the ejected droplets. The schematic representation of the experimental setup has been given in Figure 1.

Figure 1: Schematic representation of the experimental setup.
Figure 1:

Schematic representation of the experimental setup.

2.3 Synthesis of ZnO nanostructures using atomization technique

The selection of substrate for the growth of ZnO is of prime importance because the lattice parameters and crystal structure of the nanostructures and the used substrate should match. This has a considerable effect on crystal growth behavior and quality. Indium tin oxide (ITO)-coated glass substrate has several outstanding properties, and its use improves the initial nucleation as well as subsequent growth of ZnO nanostructure on ITO substrate. Initially, ITO glass surface was cleaned with deionized water followed by acetone and methanol for 10–15 min in the presence of ultrasonic irradiations based on the use of ultrasonic bath. Subsequently, the cleaned glass was heated to a temperature of about 130–150°C. To form the ZnO nanostructures on this plate, the aqueous solution of zinc nitrate at different precursor concentrations was passed through the atomizer. The heated glass plate was moved slowly at a distance of 12–15 cm from the atomizer horn tip to collect the droplets on the glass surface. As the liquid drop strikes the ITO glass surface, it instantly vaporizes because of the high temperature of glass plate, during which zinc nitrate is converted to ZnO with release of the NO2 and O2 gases along with the water vapor. The resulting white nanostructures were observed on the glass plate.

2.4 Analysis

An image analysis technique has been used to measure the droplet size using Image J and Fiji open source software. The Image J macro program was written to intensify the process of measurement of drop size, and with this program, all the processed data, including the area of the droplets, the ratio of minor axis to major axis, circularity, and the Feret equivalent diameter of the droplets, were exported in an Excel sheet (Microsoft, Redmond, WA, USA). The sample representation of the droplets used for the analysis of the droplet size in Image J software is shown by the square box in Figure 2. Individual droplet size measurements were based on the zoom mode of the analysis window, as shown in Figure 3. Scanning electron microscopy (SEM) analysis of the samples was carried out on a JEOL JSM 680LA 15 kV SEM to obtain the surface characteristics of the sample. The surface tension of different solutions was measured using the capillary rise method, whereas the viscosity of the solution was measured using the Brookfield Dial Viscometer, Spindle No. 1 LV.

Figure 2: Droplets used for analysis using Image J software.
Figure 2:

Droplets used for analysis using Image J software.

Figure 3: Image J software result window for droplet size.
Figure 3:

Image J software result window for droplet size.

3 Results and discussion

3.1 Effect of flow rate on droplet size

The effect of flow rate was investigated over the range of 2–8 ml/min, and the obtained results have been given in Figure 4 for the case with 20-kHz frequency and ultrasonic power dissipation of 39 W. It can be seen from the figure that the droplet size (dp) increased from 336 to 472 μm, with an increase in flow rate from 2 to 8 ml/min. The observed results can be attributed to the fact that, as the surface available for the liquid spread is constant, it is likely that the liquid spread is not uniform at higher flow rates and also the thickness of the layer is expected to be higher. This non-uniformity in the spread of liquid leads to irregularities in capillary waves (formation of crust and crumb) and hence gives higher size of the droplets as well as wide distribution. Ramisetty et al. [4] also reported that with an increase in the flow rate from 0.5×10-7 to 4.5×10-7 m3/s, there is an increase in droplet size from 122 to 190 μm. Ranjan and Pandit [32] also investigated the effect of different operating parameters on droplet formation during the ultrasonic atomization process and reported that viscosity, ultrasonic intensity, and liquid flow rate have significant effect on droplet size.

Figure 4: Effect of flow rate on droplet size (power of atomization=39 W; precursor concentration=0.05 m).
Figure 4:

Effect of flow rate on droplet size (power of atomization=39 W; precursor concentration=0.05 m).

3.2 Effect of precursor concentration on droplet size

The observed variation in droplet size with the precursor concentration in the introduced liquid is shown in Figure 5. It can be seen from the figure that droplet size marginally increases with an increase in liquid phase concentration. The obtained results can be attributed to the higher density of liquid arising from the higher concentration of precursor, such that the atomizer requires more energy to disintegrate the liquid layer into droplets. As the supplied energy is constant, higher droplet size is obtained. Similar observations in terms of marginal effect can also be seen in the literature. Wang et al. [33] investigated the effect of concentration of zirconium nitrate aqueous solution on droplet size and distribution over the concentration range of 1–500 mol/m3 for the case of ultrasound-assisted spray pyrolysis and reported that an increase in concentration had a marginal effect on the droplet size. The obtained droplet size at 10-mol/m3 concentration was 5 μm, which was reduced very marginally to 4 μm at 500 mol/m3. Unimodal distribution was also reported with an increase in the concentration of precursor. Chen et al. [34] also investigated the effect of precursor concentration on droplet size as well as the structures of ceria and zirconia nanoparticles and reported that an increase in the concentration of zirconia precursor from 0.1 to 1 wt% leads to a marginal difference in morphology due to change and distribution in the droplet size.

Figure 5: Effect of precursor concentration on droplet size (flow rate=2 ml/min, power of atomization=39 W).
Figure 5:

Effect of precursor concentration on droplet size (flow rate=2 ml/min, power of atomization=39 W).

3.3 Effect of ultrasonic power on the droplet size

The effect of ultrasonic power over the range from 13 to 65 W on the droplet size was investigated, and the obtained results have been reported in Figure 6. The figure shows that the droplet size increased with an increase in ultrasonic power. Initially, the droplet size was 400 μm at 10 W power dissipation, which subsequently increased to >500 μm for 40 W power dissipation. The observed results can be explained based on the dependence of vibration tip amplitude on the power dissipation [4]. With an increase in ultrasonic power, there will be a proportional increase in the amplitude. At smaller power dissipation levels, due to lower amplitude of vibrations, it is likely that the liquid is allowed to uniformly spread on the flat surface, and this thin layer is then broken into small droplets. At higher power dissipations, higher vibration amplitude are expected to result in the generation of droplets without being sufficiently spread on the atomizer surface, and hence, a higher droplet size is observed. The governing equations for explaining the effect of power dissipation are the following:

(1)Power=ρCSU22, (1)

where the speed of sound wave (U) is defined as (Am×ω0)=(Am×2πf), and

(2)I=ρC(Am×2πf)22, (2)
(3)Am=2IρC(2πf)2. (3)
Figure 6: Effect of power of atomization on droplet size (flow rate=2 ml/min, precursor concentration=0.1 m).
Figure 6:

Effect of power of atomization on droplet size (flow rate=2 ml/min, precursor concentration=0.1 m).

3.4 Effect of surfactant concentration on the droplet size

Surface tension has a substantial effect on droplet size during the process of atomization. The size of the droplet is expected to be smaller owing to surface tension effects, and hence, there will be formation of ZnO nanoparticles with smaller size. Considering this hypothesis, zinc nitrate solution was mixed with PVP (average molecular weight, 40,000 g/mol) as surfactant at a loading ranging from 0.5 g/100 to 3 g/100 ml of zinc nitrate solution. The flow rate of the liquid was maintained constant at 2 ml/min so that a thin layer should be formed on the vibrating tip. The effect of surfactant concentration on the droplet size of liquid has been given in Figure 7, which shows that, initially, at the loading of 0.5-g/100-ml solution, the droplet size was >400 μm, which reduced to 350 μm at surfactant loading of 1 g/100 ml. With further increase in surfactant loading to 3 g/100 ml, the droplet size further reduced to 320 μm. The presence of surfactant ensures the layer of liquid to behave as an elastic sheet giving uniform spreading and hence forming a thin layer of liquid on the surface. In addition, owing to changes in surface tension, there is a beneficial effect in terms of enhanced cavitational activity and hence lower droplet size is obtained [35]. The effects of the various parameters on the droplet size have been summarized in Table 1.

Figure 7: Effect of surfactant concentration on droplet size (flow rate=2 ml/min, power of atomization=39 W).
Figure 7:

Effect of surfactant concentration on droplet size (flow rate=2 ml/min, power of atomization=39 W).

Table 1:

Experimentally observed droplet size obtained using ultrasonic atomizer.

Flow rate (ml/min)Concentration (m)Power of atomization (W)PVP surfactant quantity (g)Droplet size (μm)
10.0539–244
20.0539–276
20.0552–281
20.0565–345
20.0539–310
20.1390.1260
20.05390.2234

3.5 Correlation for droplet size using ultrasonic atomization

The correlations for the prediction of droplet size generally considers various parameters such as frequency, flow rate, surface tension, and density in the form of dimensionless numbers. The frequency of irradiation also has an effect on droplet size because the rate of coalescence at lower frequency is significantly higher than that obtained at higher frequency [36]. In the case of ultrasonic atomization, the dimensionless numbers are typically modified to include the ultrasonic parameters. The Weber number has been modified to include the ultrasonic frequency, f, as follows:

(4)We=fQρσ. (4)

The Ohnesorge number is also modified, considering that in the case of ultrasonic atomization, the growth of instabilities is controlled by amplitude, Am:

(5)Oh=μfAm2ρ. (5)

Another dimensionless number, called intensity number, IN, is generally included to consider the effect of energy density on droplet size:

(6)IN=f2Am4CQ. (6)

Using these dimensionless numbers, a correlation was proposed by Ramisetty et al. [4] that is applicable over the following ranges: f=20–130 kHz, ρ=1017–1063 kg/m3, σ=36,948–41,000 N/m, Oh=2.71–161.64, We=14.8–571, IN=3.65×10-13–1.92×10-9, which has been given as follows:

(7)dP=0.0154(πσρf2)0.33[1+(πσρf2)-0.2We0.154Oh-0.111IN-0.033]. (7)

Barba et al. [37] have proposed another correlation for the ultrasonic atomization of alginate solutions:

(8)dP=0.058(πσρf2)0.33We0.151Oh0.192IN-0.02. (8)

The two correlations were applied to predict the size of droplets produced by the 20-kHz ultrasonic atomizer in the present work for the specific case of aqueous solution of zinc nitrate. The obtained data for the actual experimentally obtained values and the predicted values have been given in Table 2. The table shows that the correlation developed by Ramisetty et al. [4] correlates well with the data obtained in the current work, with good matching with the observed droplet size. The properties of different zinc nitrate aqueous solutions used in the work have been given in Table 3, which were considered for the prediction of the droplet size. It has been also conclusively established that a change in the precursor concentration and addition of surfactant result in changes in density, viscosity, and surface tension, which should be taken into account in the analysis.

Table 2:

Comparison between droplet size predicted by correlations (Eqs. 7 and 8) and experimental observation.

Sr. no.Power (W)Frequency (kHz)Flow rate (×10-8 m3/s)dp Observed (μm)Correlation dp (μm)
Eq. 7Eq. 8
139201.66724425272.090
239203.33327628481.158
352203.33328128975.918
465203.33334529272.088
539203.33331028581.125
639203.33326028480.440
739203.33323428280.097
Table 3:

Properties of the zinc nitrate aqueous solution.

Flow rate (ml/min)Concentration (m)Surface tension (N/m)Viscosity (kg/m s)Density (kg/m3)
1.667E-080.050.042700.001031008.3
3.333E-080.050.042700.001031008.3
3.333E-080.050.042700.001031008.3
3.333E-080.050.042700.001031008.3
3.333E-080.100.042000.001061030.0
3.333E-080.100.038670.001101036.0
3.333E-080.100.037280.001121044.8

3.6 ZnO nanosheets formation

ZnO nanosheets on ITO glass were synthesized with the assistance of ultrasonic atomization. The exact chemical reaction for the synthesis can be given as follows:

2Zn(NO3)2→2ZnO+4NO2+O2

For the synthesis of ZnO nanostructure, the aqueous solution with different concentrations of zinc nitrate was passed through the atomizer at different flow rates, i.e. 1 and 2 ml/min, using a syringe pump. The ejected droplets from the atomizer tip were collected on the ITO glass, which was maintained at 130°C–150°C using a hot plate. The deposited ZnO sheets were characterized by SEM to establish surface morphology.

ZnO growth on the glass substrate using the ultrasonic atomization shows different types of crystallographic structures and orientations under different sets of experimental conditions, as the defined structure of the nanoparticles depends on the growth conditions of the aerosols. To investigate the relationship between droplets and their corresponding spray pyrolysis-based particle size, four different experiments were performed by changing the flow rate from 1 to 2 ml/min with two different zinc precursor solution concentrations (0.05 and 0.1 m). Figure 8 shows the SEM images of the ZnO nanostructure obtained from the different optimum processing conditions. Figure 8A depicts the SEM image of the ZnO nanostructure obtained at a flow rate of 1 ml/min and precursor concentration of 0.05 m, where the formation of nanoplatelets with size around 20 nm was observed. The size of the ZnO nanostructures was found to be increased (Figure 8B and C) for the flow rate of 2 ml/min and precursor concentration of 0.05 m. Figure 8D shows the SEM image of the ZnO nanostructures prepared with the flow rate of 2 ml/min and precursor concentration of 0.1 m. The particle size of the ZnO nanostructure is comparatively larger for the precursor loading of 0.1 m, which is in line with the trends obtained for the droplet size. In all the cases, the platelet-like structure was observed. It has been established that the trends for the particle size matched well with the droplet size predictions.

Figure 8: SEM images of the ZnO sample on ITO glass for power of atomization=39 W. (A) flow rate=1 ml/min, precursor concentration =0.05 m, (B) flow rate=2 ml/min, precursor concentration=0.05 m, (C) flow rate=2 ml/min, precursor concentration=0.05 m, and (D) flow rate=2 ml/min, precursor concentration=0.1 m.
Figure 8:

SEM images of the ZnO sample on ITO glass for power of atomization=39 W. (A) flow rate=1 ml/min, precursor concentration =0.05 m, (B) flow rate=2 ml/min, precursor concentration=0.05 m, (C) flow rate=2 ml/min, precursor concentration=0.05 m, and (D) flow rate=2 ml/min, precursor concentration=0.1 m.

Literature reports have attempted to establish quantitative prediction of particle size based on droplet size dependent on the concentration and densities of the precursor solution [31, 32]. However, it is extremely difficult to give the exact degree by which droplet size affects particle size because the temperature and the type of glass plate used for the deposition are the other parameters that affect particle size along with droplet size. The average droplet size of the 0.05-m solution with 1-ml/min flow rate was found to be about 244 μm, and this gave ZnO sheet thickness in the range of 14–23 nm, with an average size of about 20 nm. At a concentration of 0.1 m concentration and droplet size of 260 μm, the ZnO nanosheet thickness was in the range of 26–64 nm.

4 Conclusions

The current work has clearly demonstrated that typical sheet-like ZnO nanostructures can be effectively obtained with good control over the final size based on the variations in the flow rate through the atomizer, the precursor concentration, and the surface tension. It was also established that the ZnO nanosheets with almost uniform size distribution were obtained on ITO glass using the ultrasonic atomization process. To obtain thin ZnO nanostructures, low flow rates, power dissipation, and low precursor concentration are useful. The droplet size (dp) was found to increase from 336 to 472 μm with an increase in flow rate from 2 to 8 ml/min. In addition, the droplet size was marginally affected by the liquid phase concentration. The use of surfactant played an important role in the reduction in the droplet size. Owing to changes in surface tension, there was a beneficial effect in terms of enhanced cavitational activity and hence lower droplet size was obtained. The findings presented in the current work have clearly established that precise distribution of the ZnO nanomaterials with different morphologies can be obtained using ultrasonic atomization process.

Corresponding author: Shirish H. Sonawane, Chemical Engineering Department, National Institute of Technology, Warangal 506004, TS, India, e-mail:

About the authors

Niraj R. Sikwal

Niraj R. Sikwal received his BE degree in chemical engineering in 2010 from PRE College Loni, Pune University, India, and his MTech degree in chemical engineering from the National Institute of Technology, Warangal, India, in 2014. He published a chapter in the Handbook of Functional Nano Materials and filed an Indian patent on the production of iron oxide pigment. His has research interests in the areas of nanotechnology and microreactors.

Shirish H. Sonawane

Shirish H. Sonawane is currently working as an associate professor in the Chemical Engineering Department, National Institute of Technology, Warangal, India. His research interest focused on the synthesis of hybrid nanomaterials, cavitation-based inorganic particle synthesis, sonochemical synthesis of nanolatex, process intensification, and microreactor for nanoparticle production. Dr. Sonawane has published more than 50 research papers in reputed journals, 7 book chapters, and 6 Indian patent applications. He completed 6 consultancy projects from chemical industries and 4 research projects from the government.

Bharat A. Bhanvase

Bharat A. Bhanvase is currently working as an associate professor in the Chemical Engineering Department, Laxminarayan Institute of Technology, RTM Nagpur University, Nagpur, Maharashtra, India. His research interests focused on heterogeneous catalysis, nanomaterials and nanocomposites, process intensification, microfluidics, and nanofluids. He has published 35 articles in international journals, 4 in national journals, and presented 6 papers in international conferences and 12 in national conferences. He has written 9 book chapters in internationally renowned books and applied for 3 Indian patents.

Kirankumar Ramisetty

Kirankumar Ramisetty is working as a postdoctoral research associate at the University of Limerick, Ireland. He finished his doctoral research under the guidance of eminent researchers Dr. Parag Gogate and Dr. A. B. Pandit. His extensive work on the application of ultrasonic cavitation on physical processes such as atomization, crystallization, and emulsification contributed to the publication of 5 research articles with 21 citations. His current work is focused on the application of ultrasound to tailor the crystal size distribution for improved formulation properties of API compounds.

Dipak V. Pinjari

Dipak V. Pinjari is currently working as an assistant professor at the Institute of Chemical Technology, Mumbai, India. His research interests focus on process intensification, cavitation engineering and technology, synthesis of nanomaterials, polymers, sonochemistry, and paint technology. He has completed his PhD in chemical engineering in 2012 from the Institute of Chemical Technology, Mumbai, India. He has published 36 articles in international journals. He has filed 3 Indian patents. He has more than 2 years of teaching experience and 9 years of research experience.

Parag R. Gogate

Parag R. Gogate is a versatile chemical engineer with outstanding research work based on the use of alternate energy sources for process intensification, wastewater treatment, and synthesis of nanomaterials. Dr. Gogate has published extensively in journals of high repute, with 202 international journal publications along with over 6500 citations (h-index of 44) as well as has written 15 chapters in edited books. Dr. Gogate has active consultancy projects with many national/international industries and collaborations with many research groups worldwide.

Rajulapati Satish Babu

Rajulapati Satish Babu received his MTech and PhD degrees in biotechnology from the Jawaharlal Nehru Technological University, Hyderabad, India, in 2005 and 2013, respectively. He has been an assistant professor in the Biotechnology Department of the National Institute of Technology, Warangal, India, since 2012. His research interests are in the areas of bioinformatics, bioprocess engineering, environmental biotechnology, modeling, simulation, and optimization of bioprocesses.

Nomenclature

Am

Tip amplitude (m)

C

Velocity of sound in liquid medium (m/s)

dp

droplet diameter (m)

f

Excitation frequency (1/s)

I

Intensity of ultrasound (W/m2)

Q

Volumetric flow rate (m3/s)

Dimensionless numbers
We

Modified Weber number=fQρσ

Oh

Modified Ohnesorge number=μfAm2ρ

IN

Intensity number=f2Am4CQ

Greek letters
μ

Liquid viscosity (Pa s)

λ

Wavelength (m)

ρ

Density of the liquid (kg/m3)

σ

Surface tension (N/m)

ω0

Oscillation frequency of atomizer tip (1/s)

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Received: 2015-9-3
Accepted: 2016-1-18
Published Online: 2016-3-25
Published in Print: 2016-4-1

©2016 by De Gruyter

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/gps-2015-0072
Keywords for this article
atomization; droplet size; nanoparticles; operating parameters; ultrasound
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Frontmatter
  2. In this issue
  3. Editorial
  4. Science needs passion – science is passion – science gives passion
  5. Original articles
  6. Performance evaluation of various bioreactors for methane fermentation of pretreated wheat straw with cattle manure
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  8. Optimization of the asymmetric synthesis of chiral aromatic alcohol using freeze-dried carrots as whole-cell biocatalysts
  9. Green synthesis of 2-aryl benzothiazole heterogenous catalyzed by MoO3 nanorods
  10. Factorial design study to access the “green” iodocyclization reaction of 2-allylphenols
  11. Stereoselective synthesis of (1S,2S)-1-phenylpropane-1,2-diol by cell-free extract of Lactobacillus brevis
  12. Ultrasound-assisted preparation of ZnO nanostructures: understanding the effect of operating parameters
  13. Study of anti-cancer properties of green silver nanoparticles against MCF-7 breast cancer cell lines
  14. Syntheses of ultra-fine barium carbonate powders by homogeneous precipitation method
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  16. Preparation of TiC by carbothermal reduction in vacuum and acid leaching using blast furnace slag bearing titania
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