# Low-Cost, High-Yield ZnO Nanostars Synthesis for Pseudocapacitor Applications

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Synthesis of ZnO Nanostars

#### 2.2. Characterization

## 3. Results and Discussion

#### 3.1. Material Characterization

#### 3.2. Electrochemical Measurements

^{−1}) on a $1\text{}{\mathrm{cm}}^{2}$ graphene paper substrate, as shown in Figure 3a. The electrode was then dried on a hot plate at 60 °C in air, obtaining a mass of 0.2 mg, measured with a Mettler Toledo (Columbus, OH, USA) MX5 Microbalance (sensitivity: 0.01 mg). It should be noted that particular care was taken in order to have electrodes with the same mass so as to easily compare the electrochemical performances.

_{c}, mC) can be determined from the CV curves as follows [44]:

_{s}, ${\mathrm{F}\text{}\mathrm{g}}^{-1}$) can be determined from the CV curves as follows [45]:

_{s}for all the growth times. In all the cases, a marked dependence on the scan rate was observed, as for the 10 min growth time discussed above. Indeed, the growth time significantly affected the C

_{s}, with 6 and 10 min grown ZnO NSs exhibiting the largest values.

_{s}of the 10 min NSs as a function of the scan rate in 1 M ${\mathrm{Na}}_{2}{\mathrm{SO}}_{4}$ (magenta curve), 1 M KCl (green curve), and 1M NaCl (purple curve). While NaCl and KCl showed very similar results, ${\mathrm{Na}}_{2}{\mathrm{SO}}_{4}$ evidenced a larger specific capacitance at a lower scan rate.

_{s}(υ = 5 ${\mathrm{mV}\text{}\mathrm{s}}^{-1}$) as a function of growth time. The C

_{s}values exhibited a clear bell-shaped trend. Figure 5b shows the impedance modulus and phases angle amplitudes at 1 Hz as a function of growth time. Focusing on the impedance modulus (blue), a funnel-shaped trend can be recognized. The 10 min growth point had the lowest impedance value (56 Ω), which is specular with the highest value of C

_{s}(94 ${\mathrm{F}\text{}\mathrm{g}}^{-1}$) found for the same sample. The impedance module is inversely related to capacitance [49]. Hence, these two quantities being inversely proportional, lower |Z| values mean higher capacitance values. It is unequivocal that the impedance modulus trend (magenta curve, Figure 5b) is the C

_{s}bell trend's mirror image (red curve, Figure 5a).

_{s,GCD}can be calculated from the GCD as follows [45]:

_{s,GCD}as a function of the scan rate (CV, blue curve) and as a function of current density (GCD, red curve). The C

_{s,GCD}trend matched well with the values of the CV analyses, hence confirming again that all the data were consistent.

## 4. Conclusions

## Supplementary Materials

_{2}SO

_{4}at different scan rates as a function of potential values. Figure S6. Bode plot from the EIS analyses acquired at 0.3 V: (a) impedance modulus and (b) phase angle amplitudes for all growth times analyzed. Data for the GP substrate are also reported (GP grey, 0.5 min black, 1 min red, 3 min green, 6 min blue, 10 min light blue, 20 min magenta, and 30 min Bordeaux lines and circles).

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

- IEA. Electricity. 2020. Available online: www.iea.org (accessed on 1 June 2022).
- Abas, N.; Kalair, A.; Khan, N. Review of Fossil Fuels and Future Energy Technologies. Futures
**2015**, 69, 31–49. [Google Scholar] [CrossRef] - Electricity-Market-Report-January-2022. Available online: www.iea.org (accessed on 1 June 2022).
- Schmidt, O.; Hawkes, A.; Gambhir, A.; Staffell, I. The Future Cost of Electrical Energy Storage Based on Experience Rates. Nat. Energy
**2017**, 2, 17110. [Google Scholar] [CrossRef] - Höök, M.; Tang, X. Depletion of Fossil Fuels and Anthropogenic Climate Change—A Review. Energy Policy
**2013**, 52, 797–809. [Google Scholar] [CrossRef][Green Version] - Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: A Review. Energy Environ. Sci.
**2011**, 4, 3243–3262. [Google Scholar] [CrossRef] - Kim, T.; Song, W.; Son, D.Y.; Ono, L.K.; Qi, Y. Lithium-Ion Batteries: Outlook on Present, Future, and Hybridized Technologies. J. Mater. Chem. A
**2019**, 7, 2942–2964. [Google Scholar] [CrossRef] - Vangari, M.; Pryor, T.; Jiang, L. Supercapacitors: Review of Materials and Fabrication Methods. J. Energy Eng.
**2013**, 139, 72–79. [Google Scholar] [CrossRef] - Sharma, P.; Bhatti, T.S. A Review on Electrochemical Double-Layer Capacitors. Energy Convers. Manag.
**2010**, 51, 2901–2912. [Google Scholar] [CrossRef] - Banerjee, S.; De, B.; Sinha, P.; Cherusseri, J.; Kar, K.K. Applications of Supercapacitors; Springer: Cham, Switzerland, 2020; Volume 300. [Google Scholar] [CrossRef]
- Kim, B.K.; Sy, S.; Yu, A.; Zhang, J. Electrochemical Supercapacitors for Energy Storage and Conversion. In Handbook of Clean Energy Systems; Yan, J., Ed.; John Wiley & Sons: Hoboken, NJ, USA, 2015. [Google Scholar] [CrossRef]
- Najib, S.; Erdem, E. Current Progress Achieved in Novel Materials for Supercapacitor Electrodes: Mini Review. Nanoscale Adv.
**2019**, 1, 2817–2827. [Google Scholar] [CrossRef][Green Version] - Fleischmann, S.; Mitchell, J.B.; Wang, R.; Zhan, C.; Jiang, D.E.; Presser, V.; Augustyn, V. Pseudocapacitance: From Fundamental Understanding to High Power Energy Storage Materials. Chem. Rev.
**2020**, 120, 6738–6782. [Google Scholar] [CrossRef] - Dunn, B.; Kamath, H.; Tarascon, J.M. Electrical Energy Storage for the Grid: A Battery of Choices. Science
**2011**, 334, 928–935. [Google Scholar] [CrossRef][Green Version] - Jiang, Y.; Liu, J. Definitions of Pseudocapacitive Materials: A Brief Review. Energy Environ. Mater.
**2019**, 2, 30–37. [Google Scholar] [CrossRef][Green Version] - Yu, Z.; Tetard, L.; Zhai, L.; Thomas, J. Supercapacitor Electrode Materials: Nanostructures from 0 to 3 Dimensions. Energy Environ. Sci.
**2015**, 8, 702–730. [Google Scholar] [CrossRef][Green Version] - Yi, C.-Q.; Zou, J.-P.; Yang, H.-Z.; Leng, X. Recent Advances in Pseudocapacitor Electrode Materials: Transition Metal Oxides and Nitrides. Trans. Nonferrous Met. Soc. China
**2018**, 28, 1980–2001. (In English) [Google Scholar] [CrossRef] - Wang, L.; Zhang, G.; Liu, Q.; Duan, H. Recent Progress in Zn-Based Anodes for Advanced Lithium Ion Batteries. Mater. Chem. Front.
**2018**, 2, 1414–1435. [Google Scholar] [CrossRef] - Ridhuan, N.S.; Abdul Razak, K.; Lockman, Z.; Abdul Aziz, A. Structural and Morphology of ZnO Nanorods Synthesized Using ZnO Seeded Growth Hydrothermal Method and Its Properties as UV Sensing. PLoS ONE
**2012**, 7, e50405. [Google Scholar] [CrossRef] - Özgür, Ü.; Alivov, Y.I.; Liu, C.; Teke, A.; Reshchikov, M.A.; Doǧan, S.; Avrutin, V.; Cho, S.-J.; Morkoҫ, H. A Comprehensive Review of ZnO Materials and Devices. J. Appl. Phys.
**2005**, 98, 041301. [Google Scholar] [CrossRef][Green Version] - Klingshirn, C. ZnO: From Basics towards Applications. Phys. Status Solidi Basic Res.
**2007**, 244, 3027–3073. [Google Scholar] [CrossRef] - Kong, X.Y.; Wang, Z.L. Spontaneous Polarization-Induced Nanohelixes, Nanosprings, and Nanorings of Piezoelectric Nanobelts. Nano Lett.
**2003**, 3, 1625–1631. [Google Scholar] [CrossRef][Green Version] - Chang, P.C.; Fan, Z.; Wang, D.; Tseng, W.Y.; Chiou, W.A.; Hong, J.; Lu, J.G. ZnO Nanowires Synthesized by Vapor Trapping CVD Method. Chem. Mater.
**2004**, 16, 5133–5137. [Google Scholar] [CrossRef] - Li, Y.; Cheng, G.S.; Zhang, L.D. Fabrication of Highly Ordered ZnO Nanowire Arrays in Anodic Alumina Membranes. J. Mater. Res.
**2000**, 15, 2305–2308. [Google Scholar] [CrossRef][Green Version] - Kamalasanan, M.N.; Chandra, S. Sol-Gel Synthesis of ZnO Thin Films. Thin Solid Films
**1996**, 288, 112–115. [Google Scholar] [CrossRef] - Strano, V.; Greco, M.G.; Ciliberto, E.; Mirabella, S. Zno Microflowers Grown by Chemical Bath Deposition: A Low-Cost Approach for Massive Production of Functional Nanostructures. Chemosensors
**2019**, 7, 62. [Google Scholar] [CrossRef][Green Version] - Hodes, G. Semiconductor and Ceramic Nanoparticle Films Deposited by Chemical Bath Deposition. Phys. Chem. Chem. Phys.
**2007**, 9, 2181–2196. [Google Scholar] [CrossRef] [PubMed] - Liao, F.; Huang, Y.; Ge, J.; Zheng, W.; Tedsree, K.; Collier, P.; Hong, X.; Tsang, S.C. Morphology-Dependent Interactions of ZnO with Cu Nanoparticles at the Materials’ Interface in Selective Hydrogenation of CO
_{2}to CH_{3}OH. Angew. Chem. Int. Ed.**2011**, 50, 2162–2165. [Google Scholar] [CrossRef] [PubMed] - Najib, S.; Bakan, F.; Abdullayeva, N.; Bahariqushchi, R.; Kasap, S.; Franzò, G.; Sankir, M.; Demirci Sankir, N.; Mirabella, S.; Erdem, E. Tailoring Morphology to Control Defect Structures in ZnO Electrodes for High-Performance Supercapacitor Devices. Nanoscale
**2020**, 12, 16162–16172. [Google Scholar] [CrossRef] [PubMed] - Jayachandiran, J.; Yesuraj, J.; Arivanandhan, M.; Raja, A.; Suthanthiraraj, S.A.; Jayavel, R.; Nedumaran, D. Synthesis and Electrochemical Studies of RGO/ZnO Nanocomposite for Supercapacitor Application. J. Inorg. Organomet. Polym. Mater.
**2018**, 28, 2046–2055. [Google Scholar] [CrossRef] - Luo, Q.; Xu, P.; Qiu, Y.; Cheng, Z.; Chang, X.; Fan, H. Synthesis of ZnO Tetrapods for High-Performance Supercapacitor Applications. Mater. Lett.
**2017**, 198, 192–195. [Google Scholar] [CrossRef] - Guo, Y.; Chang, B.; Wen, T.; Zhao, C.; Yin, H.; Zhou, Y.; Wang, Y.; Yang, B.; Zhang, S. One-Pot Synthesis of Graphene/Zinc Oxide by Microwave Irradiation with Enhanced Supercapacitor Performance. RSC Adv.
**2016**, 6, 19394–19403. [Google Scholar] [CrossRef] - Kim, J.H.; Lee, Y.S.; Sharma, A.K.; Liu, C.G. Polypyrrole/Carbon Composite Electrode for High-Power Electrochemical Capacitors. Electrochim. Acta
**2006**, 52, 1727–1732. [Google Scholar] [CrossRef] - Strano, V.; Smecca, E.; Depauw, V.; Trompoukis, C.; Alberti, A.; Reitano, R.; Crupi, I.; Gordon, I.; Mirabella, S. Low-Cost High-Haze Films Based on ZnO Nanorods for Light Scattering in Thin c-Si Solar Cells. Appl. Phys. Lett.
**2015**, 106, 3–9. [Google Scholar] [CrossRef] - Schneider, C.A.; Rasband, W.S.; Eliceiri, K.W. NIH Image to ImageJ: 25 Years of Image Analysis. Nat. Methods
**2012**, 9, 671–675. [Google Scholar] [CrossRef] [PubMed] - Bruno, L.; Strano, V.; Scuderi, M.; Franzò, G.; Priolo, F.; Mirabella, S. Localized Energy Band Bending in ZnO Nanorods Decorated with Au Nanoparticles. Nanomaterials
**2021**, 11, 2718. [Google Scholar] [CrossRef] [PubMed] - Yang, H.; Teng, F.; Gu, W.; Liu, Z.; Zhao, Y.; Zhang, A.; Liu, Z.; Teng, Y. A Simple Post-Synthesis Conversion Approach to Zn(OH)F and the Effects of Fluorine and Hydroxyl on the Photodegradation Properties of Dye Wastewater. J. Hazard. Mater.
**2017**, 333, 250–258. [Google Scholar] [CrossRef] [PubMed][Green Version] - Seehra, M.S.; Geddam, U.K.; Schwegler-Berry, D.; Stefaniak, A.B. Detection and Quantification of 2H and 3R Phases in Commercial Graphene-Based Materials. Carbon
**2015**, 95, 818–823. [Google Scholar] [CrossRef] [PubMed][Green Version] - Barbagiovanni, E.G.; Reitano, R.; Franzò, G.; Strano, V.; Terrasi, A.; Mirabella, S. Radiative Mechanism and Surface Modification of Four Visible Deep Level Defect States in ZnO Nanorods. Nanoscale
**2016**, 8, 995–1006. [Google Scholar] [CrossRef] [PubMed] - Dai, M.; Xu, F.; Lu, Y.; Lui, Y.; Xie, Y. Synthesis of Submicron Rhombic ZnO Rods via ZnOHF Intermediate using Electrodeposition Route. Appl. Surf. Sci.
**2011**, 257, 3586–3591. [Google Scholar] [CrossRef] - Tian, H.; Li, Y.; Zhang, J.; Ma, Y.; Wang, Y.; Wang, Y.; Li, Y. High Pressure Induced Phase Transformation through Continuous Topology Evolution in Zinc Hydroxyfluoride Synthesized via a Hydrothermal Strategy. J. Alloys Compd.
**2017**, 726, 132–138. [Google Scholar] [CrossRef] - Komarneri, S.; Bruno, M.; Mariani, E. Synthesis of ZnO with and without Microwaves. Mater. Res. Bull.
**2000**, 35, 1843–1847. [Google Scholar] [CrossRef] - Guo, Y.; Liu, N.; Sun, T.; Cui, H.; Wang, J.; Wang, M.; Wang, M.; Tang, Y. Rational Structural Design of ZnOHF Nanotube-Assembled Microsphere Adsorbents for High-Efficient Pb
^{2+}removal. CrystEngComm**2020**, 22, 7543–7548. [Google Scholar] [CrossRef] - Zubieta, L.; Bonert, R. Characterization of Double-Layer Capacitors for Power Electronics Applications. IEEE Trans. Ind. Appl.
**2000**, 36, 199–205. [Google Scholar] [CrossRef][Green Version] - González, A.; Goikolea, E.; Barrena, J.A.; Mysyk, R. Review on Supercapacitors: Technologies and Materials. Renew. Sustain. Energy Rev.
**2016**, 58, 1189–1206. [Google Scholar] [CrossRef] - Tielrooij, K.J.; Garcia-Araez, N.; Bonn, M.; Bakker, H.J. Cooperativity in Ion Hydration. Science
**2010**, 328, 1006–1009. [Google Scholar] [CrossRef] - Wang, J.; Polleux, J.; Lim, J.; Dunn, B. Pseudocapacitive Contributions to Electrochemical Energy Storage in TiO
_{2}(Anatase) Nanoparticles. J. Phys. Chem. C**2007**, 111, 14925–14931. [Google Scholar] [CrossRef] - Lindstrom, H.; So, S.; Solbrand, A.; Hjelm, J.; Hagfeldt, A.; Lindquist, S. Li Ion Insertion in TiO
_{2}(Anatase), Voltammetry on Nanoporous Films.Pdf. J. Phys. Chem. B**1997**, 2, 7717–7722. [Google Scholar] [CrossRef] - Bard, A.J.; Faulkner, L.R.; White, H. Electrochemical Methods: Fundamentals and Applications; John Wiley: New York, NY, USA, 2004; Volume 677, pp. 368–375. [Google Scholar]

**Figure 1.**Kinetic study of ZnO NS growth. SEM images of nanostructures sampled after a growth of 0.5 (

**a**), 10 (

**b**), and 30 min (

**c**); distribution of arm length as a function of time (

**d**). The dashed red line in (

**c**) indicates the arm length of an NS.

**Figure 2.**(

**a**) XRD pattern of NSs and AnnNSs grown for 10 min; (

**b**) room-temperature photoluminescence spectra; (

**c**) visible emission band fitting with blue, green, and orange contributions, and (

**d**) histogram of fit contributions for both NSs and AnnNSs.

**Figure 3.**(

**a**) Schematic of sample preparation for electrochemical characterization. CV curves in 1 M ${\mathrm{Na}}_{2}{\mathrm{SO}}_{4}$ of GP substrate (grey line); nanostars as prepared (magenta line) and after annealing (green line) at 20 mV/s (

**b**); CV curves in 1 M ${\mathrm{Na}}_{2}{\mathrm{SO}}_{4}$ of NSs with 0.5, 1, 3, 6, 10, 20, and 30 min growth times and GP at 20 mV/s (

**c**); CV curves in 1 M ${\mathrm{Na}}_{2}{\mathrm{SO}}_{4}$ of as-prepared NS at different scan rates (

**d**).

**Figure 4.**(

**a**) Stored charge in GP (black symbols), 10 min ZnO NSs on GP (total, full magenta symbols) and their difference (net) (open magenta symbols); (

**b**) specific capacitances extracted from CV for 0.5, 1, 2, 3, 6, 10, 20, and 30 min ZnO NSs in Na

_{2}SO

_{4}; and (

**c**) specific capacitances of 10 min ZnO in NaCl, KCl, and Na

_{2}SO

_{4}(purple, green, and magenta symbols respectively).

**Figure 5.**(

**a**) C

_{s}trend from CV curves acquired at 5 ${\mathrm{mV}\text{}\mathrm{s}}^{-1}$ for all growth times analyzed and (

**b**) impedance modulus and phase angle amplitude (magenta and blue symbols, respectively) trends (F = 1 Hz) as a function of growth time.

**Figure 6.**(

**a**) GCD curves (0.5 A g

^{−1}black, 1 A g

^{−1}red, 1.5 A g

^{−1}green, 3 A g

^{−1}blue, 5 A g

^{−1}light blue, and 10 A g

^{−1}magenta lines) and (

**b**) specific capacitance obtained by GCD (red dashed line and circles) and CV (light blue dashed line and stars) curves, trends of 10 min NS.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. |

© 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## Share and Cite

**MDPI and ACS Style**

Di Mari, G.M.; Mineo, G.; Franzò, G.; Mirabella, S.; Bruno, E.; Strano, V. Low-Cost, High-Yield ZnO Nanostars Synthesis for Pseudocapacitor Applications. *Nanomaterials* **2022**, *12*, 2588.
https://doi.org/10.3390/nano12152588

**AMA Style**

Di Mari GM, Mineo G, Franzò G, Mirabella S, Bruno E, Strano V. Low-Cost, High-Yield ZnO Nanostars Synthesis for Pseudocapacitor Applications. *Nanomaterials*. 2022; 12(15):2588.
https://doi.org/10.3390/nano12152588

**Chicago/Turabian Style**

Di Mari, Gisella Maria, Giacometta Mineo, Giorgia Franzò, Salvatore Mirabella, Elena Bruno, and Vincenzina Strano. 2022. "Low-Cost, High-Yield ZnO Nanostars Synthesis for Pseudocapacitor Applications" *Nanomaterials* 12, no. 15: 2588.
https://doi.org/10.3390/nano12152588