# 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

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**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.

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**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