Pressure training

132 The results of the size distribution by volume proved that ZnO nanoparticles synthesized with LABS were of good quality.

133 3. Size Quality Analysis of ZnO Nanoparticles produced with LABS

Figure 4.17 is a graph of size distribution by intensity of ZnO nanoparticles produced with LABS. Figure 4.18 is a graph of raw correlation data of the distribution. It was evident that there was a relationship between the size distribution and the intensity of the ZnO nanoparticles. The positive correlation coefficients indicate a positive linear relationship between the size distribution and intensity of the ZnO nanoparticles present in the sample. Hence, the result met the quality criteria.

Figure 4.16 Size distributions by intensity of ZnO nanoparticles with LABS 1

2

3

134 Figure 4.17 Graph of particles size distribution by intensity of

ZnO nanoparticles produced with LABS

Figure 4.18 Graph of raw correlation data

135 The value of Z-average of the sample with LABS was very close to Z-average 735 nm of ZnO nanoparticles dispersed in water. Z-average of 354 nm was close to Z-average of 340 nm for ZnO nanoparticles dispersed in ethylene glycol (Marsalek, 2014) and 386.4 nm for pure ZnO nanofluid (Chai et al., 2018). However, the results differ greatly from Z-average of 145.1 nm with PdI of 0.189 for ZnO particles synthesized Using Ixora Coccinea Leaf Extract (Yedurkar et al., 2016), Z-Average size of 46.61 nm and Polydispersity index of 0.552 from extracellular synthesis of zinc oxide nanoparticle using seaweeds of gulf of Mannar, India (Nagarajan et al., 2013), average size of 62.94 nm for surface modified ZnO nanofluid (Chai et al., 2018) and Z-average of 64.4 nm and PdI of 0.418 from ZnONPs from Vitis vinifera Peel Extract (Divya et al., (2018).

The disparities in Z-averages and Polydispersity Indices could be because the results depended on the dispersant used, its refractive index and viscosity, the measurement technique, synthesis techniques and the purpose for which the ZnO nanoparticles was be used.

4.5 Comparison of ZnO Nanoparticles Synthesized with SLS and LABS

Table 4.3 was used for the comparison of the qualities of ZnO nanoparticles produced with SLS and the ZnO nanoparticles produced with LABS. The table shows the results of all the characterizations using the different characterization techniques. The results revealed that the ZnO nanoparticles produced with SLS and LABS were both of good quality but the ZnO particles produced with LABS yielded better results

136 Table 4.3 Summary of results from the characterization techniques used for the analysis of

ZnO nanoparticles produced with SLS and LABS ZnO

Sample

Characterization Technique

Raman BET TGA PSD

Material /structure

Intensity (a. u.)

Surface area (m²/g)

Pore diameter (d.nm)

Degradation temperature (⁰C)

Thermal stability

Z-average (nm)

PdI % vol

% inten-sity SLS ZnO NPs/

wurtzite crystal

1300 284.286 2.411 325 high 354.0 0.605 68.5 66.3 LABS ZnO NPs/

wurtzite crystal

1400 288.421 2.433 420 higher 715.2 0.431 97.3 90.9

In both ZnO samples, Raman results indicated that the two samples were ZnO nanoparticles of wurtzite crystal structure. The difference in the Raman results was that the ZnO nanoparticles in LABS sample had higher intensity revealing that it had greater concentration of ZnO nanoparticles. Therefore, it had greater yield of the ZnO nanoparticles.

BET results demonstrated that the ZnO nanoparticles produced with LABS had greater surface area and larger pore size diameter. It implied that ZnO nanoparticles in LABS sample had better adsorption properties.

TGA results discovered that degradation temperature of ZnO nanoparticles produced with LABS was higher than that of ZnO particles produced with SLS. It depicted that the ZnO nanoparticles in LABS sample had higher thermal stability and was more reliable than the ZnO nanoparticles in SLS sample.

PSD results show that Z-average diameter of ZnO nanoparticles produced with LABS was higher with less PdI than the Z-average of the ZnO nanoparticles produced with SLS.

Considering the percentage volumes and percentage intensities of the two samples, the ZnO nanoparticles in LABS sample exhibited similar identities in terms of their size properties.

137 97.3 % by volume and 90.9 % by intensity of the ZnO particles in LABS sample fell within the same size range compared to 68.5 % by volume and 66.3 % by intensity in SLS sample. The analysis showed that the ZnO particles produced with LABS had better particle size distribution.

The Raman, BET, TGA and PSD analyses of the ZnO nanoparticles produced with SLS and LABS demonstrated that the ZnO nanoparticles produced with LABS had good structure with higher concentration of ZnO nanoparticles, larger surface area and pore diameter, higher thermal stability and reliability, and better size properties than the ZnO nanoparticles produced with SLS.

Based on this analysis the ZnO nanoparticles produced with LABS was used in the subsequent parts of this research.

4.6 Calibration of the Nanostructured ZnO Capacitive Relative Humidity Sensors The nanostructured ZnO capacitive humidity sensors were calibrated at room (laboratory) temperatures ranging from 27 ⁰C to 30 ⁰C. Stable relative humidity environments were maintained using saturated solutions of lithium Chloride (LiCl), Potassium carbonate (K2CO3), potassium chloride (KCl), potassium sulphate (K2SO4) and sodium chloride (NaCl). Each of the humidity environments was used to calibrate the unannealed sensor and the sensors annealed at 150 ⁰C and 200 ⁰C. Linear regression equation (equation 3.5) was used for the calibration of the sensors (Hsuan-Yu and Chiachung, 2019).

y = b0 + b1x 4.4

where y is the standard humidity value measured by DHT11, x is the capacitance value measure by nanostructured ZnO capacitive RH sensor, b0 and b1 are constants. In each case, the calibration equation was established to build the relationship between the standard RH (reference

138 RH) values measured by DHT11 RH sensor and the corresponding capacitance values of the nanostructured ZnO capacitive relative humidity sensors. The capacitance corresponding to the standard %RH of each of the saturated salt solutions were determined from the adsorption curve.

Desorption curve was also obtained. The coefficient of determination (R²) for each calibration equation was determined for each set of data. Positive values of R² indicated good linearity, which showed good linear relationship between the capacitance values and relative humidity values. Negative values of R² indicated poor linearity. The standard deviation was used to determine how widely the capacitance values were dispersed from the mean capacitance of the sensor. The sensitivity, the response and recovery times and the accuracy of the sensors were also determined.

4.6.1 Calibration of the Unannealed Nanostructured ZnO Capacitive Relative Humidity (RH) Sensor

The unannealed nanostructured ZnO capacitive relative humidity sensor was calibrated using the saturated salt solutions of LiCl, K2CO4, NaCl, KCl and K2SO4.

1. Calibration of the Unannealed Nanostructured ZnO Capacitive RH Sensor Using