Morphological, Structural, and Optical Bandgap Characterization of Extracted ZnO Nanoparticles from Commercial Paste

Abstract

ZnO nanoparticles (NPs) were extracted from a commercial paste in both colloidal and precipitate forms. The Zetasizer analysis performed on the colloid showed ZnO NPs ranging from ∼30 nm to ∼100 nm. Thin films of ZnO were deposited on glass substrates by spin-coating technique from a mixture of the extracted colloid and precipitate. The scanning electron microscope (SEM) images showed uniformly arranged, mesoporous, and nanostructured ZnO particles of different shapes, with an estimated film thickness of 0.67 μm. Analysis by energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction analysis (XRD) confirmed the presence of ZnO in the films, with no impurities or remnants of other materials. The XRD analysis showed a polycrystalline nature of the films and identified a pure phase formation of the hexagonal wurtzite structure. The average crystallite size calculated from the diffraction peaks is ∼43.25 nm. The calculated crystal tensile strain is 1.954 × 10⁻³, which increases the crystal volume by 0.728% compared with the crystal volume of standard ZnO. The calculated crystal parameters are a = b = 3.258 Å and c = 5.217 Å. The calculated dislocation density (d) and bond length Zn–O (L) are 5.35 × 10⁻⁴ nm⁻² and 2.695 Å, respectively. Ultraviolet-visible absorption spectra showed an optical band gap of ∼3.80 eV.

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1. Introduction

Zinc oxide (ZnO) is a compound of group II–VI. It is a broadband semiconductor oxide with a direct bandgap of approximately 3.37 eV at room temperature [1]. ZnO has a high chemical and mechanical stability. In addition, it is nontoxic and abundantly available in nature. ZnO became a material of interest for different technological applications due to its high optical transmission in the visible light range. Such applications include optical devices, optoelectronics, flat screens, liquid crystal displays, transparent electronics, and thin-film photovoltaic devices [2–4]. With the advance of nanotechnology, considerable research efforts have studied the properties of ZnO in its nano-dimensional scale, most commonly as thin films. ZnO films attract a lot of interest for their applications in optoelectronics and photovoltaics. One of its applications is acting as an electron transport layer (ETL) in third-generation solar cells due to its high electron mobility [5]. Thin layers of nanostructured ZnO can function as ETL in solar cells that include organic [6], dye sensitized [7], and perovskite solar cells [8], respectively. In addition, when obtained as a mesoporous structure, it could have a possible industrial application for conjugated materials [9, 10]. Several deposition techniques are used to grow ZnO films, such as thermal evaporation [11], spray pyrolysis [12], sputtering [13], laser ablation [14], and sol-gel [15] among others. In this work, we extracted the ZnO nanoparticles (NPs) from a commercial paste, in a colloidal form similar to that obtained through the conventional sol-gel chemical process. We obtained ZnO thin films by depositing the extracted ZnO NPs on corning glass substrates by spin-coating. Then, we used different characterization methods to assess their possible applications in third-generation solar cells. This work opens the door to the recycling and reuse of nanostructured oxides after recovering them from their original products. The recovered oxides can be used in different applications, such as the fabrication of solar cells to produce clean, cheap, and abundant energy.

2. Materials and Methods

3.36 grams of potassium hydroxide (KOH) were dissolved in 200 ml of water, while 30 grams of the commercial ZnO paste (bacitracin zinc 500 [USP'U] in 1 g, polymyxin B sulfate 10000 [USP'U] in 1 g, neomycin sulfate 3.5 mg in 1 g, pramoxine hydrochloride 10 mg in 1 g, Genomma lab USA Inc) in a laboratory beaker were placed on a magnetic stirring hot plate, applying a low temperature. When the paste melted, the temperature rose to 60°C, and the KOH solution was dripped slowly using a Pasteur pipette. Magnetic stirring was applied during the KOH dripping, staring at 200 rpm, which increased with increasing the volume of the mixture in the beaker until it reached almost 400 rpm. After completing the dripping of the KOH solution, the sample remained under magnetic stirring of 400 rpm at 60°C for 30 minutes and then removed from the magnetic stirring hot plate and placed in a stable place to cool down. During the cooling process, three layers appeared in the beaker as shown in Figure 1: a top layer of the organic materials found in the commercial paste, a layer of ZnO precipitate (PPT) at the bottom, and a colloid of KOH solution along with ZnO NPs in the middle. After twenty-four hours from the previous process, the top layer was separated from the beaker, and a sample was taken from the colloid layer using a pipette. Then, a magnetic stirring of 400 rpm was applied to the colloid and the precipitate in the beaker for 18 hours. After the stirring period, the beaker was left 24 hours to stabilize, and a second sample of the colloid was taken. The two colloid samples were then characterized by the Zetasizer analysis.

[figure omitted; refer to PDF]

Thin films of ZnO were deposited on clean glass substrates of 1 cm² using a mixture from the colloid and the precipitate. The mixture was stirred for 30 min at 400 rpm before the deposition to form a homogeneous suspension. The uniformity of the deposited thin films was improved after various experimental trials, changing the spin-coating parameters without changing the preparation method of the mixture. The films were deposited by a series of successive layers, using dynamic spin-coating with a rotation speed of 3000 rpm, rotation time of 15 seconds, and 5 minutes of preheating between layers at 120°C. After the deposition of all stacked layers, the films were annealed at 550°C for 60 minutes. In this work, we report the results of ZnO thin film deposited in 5 stacked layers. The film was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and optical absorption spectra measurements.

Malvern Zetasizer Nano ZS DL was used to determine the size distribution of ZnO NPs in the colloidal solution. A JEOL JSM7600-F SEM at 15 kV was used to examine the surface morphology of the film. The percentage of constituent elements was evaluated by EDS. The structural properties were determined by XRD in the grazing incidence geometry, with an inclination of 1°, and recorded in 2θ = 0.02° steps. The XRD beam under CuKα filtered monochromatic radiation (l = 0.15418 nm) at 40 kV with 35 mA and an aperture diaphragm of 0.2 mm, using a D5000 Siemens X-ray diffractometer. Based on the data obtained from the XRD analysis, we calculated the crystallographic properties of the film and compared them with the standard ZnO values. Optical transmittance measurements in the range of 350–1100 nm were performed by using a Jobin-Yvon/Spex H20-Vis spectrometer with a 0.1 nm resolution. Based on the data obtained from the optical measurement, we calculated the bandgap of the thin film, considering only direct transitions.

3. Results and Discussion

3.1. Zetasizer

The Zetasizer analysis was used to determine the size range of the NPs extracted from the paste. Figure 2(a) shows the result of the analysis performed on the first colloid sample, taken before the magnetic stirring. Two curves are observed: the first shows NP sizes ranging from 45 nm to 190 nm, which represents 92.4% of the amount of ZnO NPs, and the second shows NP sizes ranging from 190 nm to 825 nm, which represents 7.6% of the amount of ZnO NPs in the colloid. Figure 2(b) shows the result of the Zetasizer analysis performed on the second sample, taken after applying a magnetic stirring of 400 rpm for 18 hours. There is a significant decrease in the volume of the second curve (particle sizes from 190 nm to 825 nm), while the first curve became sharper (particle sizes from 35 nm to 100 nm), representing 97.5% of the amount of ZnO NPs in the colloid. These results demonstrate that the developed extraction process successfully extracted the ZnO NPs from the commercial paste, while the subsequent magnetic stirring homogenized the extracted particles and reduced their dimensions.

[figures omitted; refer to PDF]

3.2. SEM

SEM images were used to study the surface morphology of the deposited ZnO films. Figures 3(a) and 3(b) show SEM micrographs of the ZnO film at different spots and magnifications. The images show a mesoporous arrangement of nano and submicronic particles. There are variations in the shape, diameter, and crystal size of the particles. The smaller particles represent the NPs in suspension within the colloid as per the Zetasizer analysis, while the larger particles represent the submicronic particles from the precipitate. Particles have different orientations, which indicate the existence of different crystalline planes of the material.

[figures omitted; refer to PDF]

To identify the size range of the precipitate particles, we measured the larger ZnO particles using a high-quality image with a magnification of ×50000, as shown in Figure 4. The particles size varied from less than 100 nm to just over 400 nm.

[figure omitted; refer to PDF]

Given the particle size range detected in the Zetasizer analysis (35–100 nm) and the average crystalline size calculated for the deposited ZnO film (43.25 nm), the higher optical bandgap value could be due to quantum confinement, which modifies the bandgap value of semiconductor materials. In general, the structural and optical properties of the sample coincide with what other studies reported for ZnO synthesized by sol-gel and deposited by spin-coating [23–25]. In this work, we recovered ZnO NPs from a commercial product through a process that implies simplicity and low cost.

4. Conclusions

We chemically extracted the ZnO particles from a commercial product (Dr. Bell’s Pomade) and deposited them on glass substrates by spin-coating. The ZnO thin film is mesoporous, with uniformly arranged nanostructured particles of different shapes. The film is formed by pure hexagonal wurtzite ZnO, with no impurities or remnants of other materials. The average crystalline size of the ZnO particles is ∼43.25 nm. The particles showed a slight volume expansion of 0.728% due to a tensile strain of 1.954 × 10⁻³. All calculated structural parameters are similar to those of the standard ZnO values. The estimated bandgap from the transmittance spectrum is larger than the standard bandgap of ZnO. A quantum confinement effect could explain the larger bandgap. Our work opens the door to the recycling of nanoparticles, an area that has little research to date but great importance and potential for the future [26]. The reported ZnO film has suitable properties for optoelectronic and photovoltaic applications. One of the possible applications is as an ETL in third-generation solar cells.

Acknowledgments

The authors are grateful to CONACYT & SENER (Secretariat of Energy) of Mexico for the financial support to carry out this work under Project No. 254667 Consolidation of LENERSE (Laboratory of Energy Renewable of SudEst). The authors wish to thank Dr. Patricia Quintana for the help provided by their experimental characterization laboratories of National Council for Science and Technology of Mexico & Mixed Fund, Yucatan State Government with contract 2008-108160 and National Council for Science and Technology of Mexico under Project Nos. 2009-01-123913, 29-(2692, 4643), 188345, and 204822. The authors I. V. Perez-Quintana, M. Acosta, and J. A. Mendez-Gamboa would like to thank National Council for Science and Technology of Mexico & Secretariat of Energy (Mexico) under Project No. 254667 Consolidation of LENERSE (Laboratory of Energy Renewable of SudEst).

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