Results and Discussions

Tailoring the active surface sites of ZnO nanorods on the glass substrate for photocatalytic activity enhancement

3. Results and Discussions

The XRD patterns of all the samples are shown in Figure 1. Based on the reference JCPDS No. 79-2205, the peaks confirm that ZnO has a polycrystalline hexagonal wurtzite structure. Each pattern demonstrates five prominent peaks at 2𝜃 of 31.7^o, 34.4^o, 36.25^o, 47.5^o, and 56.5^o that correspond to (100), (002), (101), (102), and (110) lattice planes, respectively. No peaks related to other zinc complexes or other impurities were seen, confirming the phase purity of ZnO [45,46]. The ZnO diffraction patterns also show that the (002) plane is the highest peak for all the samples. This indicates that the (002) plane is the preferred growth orientation. In order to further identify a preferred growth orientation quantitatively, a parameter texture coefficient (TC) was defined by using the following simple calculation [47]:

𝑇𝐶_ℎ𝑘𝑙 =

𝐼𝑚(ℎ𝑘𝑙) 𝐼𝑜 (ℎ𝑘𝑙) 1

𝑛[∑ ^{𝐼𝑚(ℎ𝑘𝑙)} 𝐼𝑜 (ℎ𝑘𝑙)

𝑛1 ] 1

where n is the number of peaks and Im and Io are the intensity of measured and standard peaks, respectively. Generally, a TC value less than 1 means that the growth orientation is random, while the highest TC value means that the lattice plane is dominant and can be considered to be the preferred orientation. As shown in Table 1, the TC value of all the peaks of the ZnO grown for 2 h is less than 1; it reflects the random growth orientation. While for other samples, the (002) lattice plane has the highest TC value thus it can be concluded that the c-axis direction is the preferred growth orientation. Moreover, ZnO nanorods that were grown for 6 h have the highest TC value of the (002) plane among all samples.

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Figure 1 X-ray diffraction patterns of ZnO nanorods with four diferrent growth times

Table 1 Texture coefficient of ZnO nanorods with four different growth times Growth

time (hours)

Texture Coefficient (TC)

(100) (002) (101) (102) (103) (112)

2 0.392 0.666 0.301 0.497 0.298 0.525

4 0.877 1.491 0.674 1.113 0.667 1.177

6 0.169 2.801 0.385 1.290 0.877 0.477

8 0.303 1.573 0.477 1.638 0.932 1.072

The lattice parameters were then evaluated using the (002) lattice plane data and presented in Table 2. The measured lattice parameters of ZnO for all the samples are similar, namely a = 3.25 Å and c = 5.20 Å. These findings are in good agreement with the standard data for wurtzite ZnO (a = 3.249 Å, c = 5.205 Å). By using Scherer’s formula [48], the average crystallite size of ZnO grown for 2, 4, 6 and 8 h are 98.67 Å, 136.88 Å, 241.12 Å, and 246. 94 Å, respectively. Generally,

20 30 40 50 60

2Theta (Degree)

2 h 002

Intensity (a.u)

70 4 h 6 h 8 h

101 102 103

112

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this result demonstrates that the increase in the growth times increases the crystallinity of ZnO.

Table 2 ZnO lattice parameters in (002) lattice plane Growth time

(hour) 2θ Volume

(ų) a (Å) c(Å) Crytallite Size (Å)

FWHM

2 34.488 47.57 3.25 5.20 98.67 0.28817

4 34.525 47.68 3.25 5.20 136.88 0,46298

6 34.494 47.66 3.25 5.20 241.12 0.32746

8 34.552 47.64 3.25 5.20 246.94 0.30995

The FESEM images of ZnO nanorods with four different growth times are shown in Figure 2. It is clearly observed that generally the ZnO nanorods have hexagonal shape and grow perpendicularly to the substrates. ZnO nanorods grown for 2 h and 4 h have a slightly random growth direction, while when the growth time is increased to 6 h and 8 h, ZnO nanorods are grown more perpendicularly to the substrate. The density of the ZnO nanorods are also increased from around 15, 22, 24 up to 25 nanorods/µm² substrate area for the ZnO grown with 2, 4, 6 and 8 h, respectively. This indicates that the nucleation of ZnO on the glass substrate still continue during the growth time. Moreover, the diameter of ZnO nanorods is also increased as the increase of growth time. The average diameter of the ZnO nanorods was approximately 90 -165 nm and 120–167 nm for the growth time of 2-4 h and 6-8 h, respectively.

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Figure 2 FESEM images of ZnO nanorods with four different growth times. Scale bar 500 nm

Cross-sectional images of the ZnO nanorods are shown in Figure 3. It is clearly seen that the height of the nanorods increases significantly as the growth time increases. The average height of the ZnO nanorods grown for 2, 4, 6 and 8 h is 353, 1067, 1573 and 1800 nm, respectively. By combining with the surface image in Figure 2, it can be obtained that the calculated surface area of ZnO nanorods grown for 2, 4, 6 and 8 h is around 14, 20, 25 and 27 m² per m² substrate, respectively. The FESEM images clearly show that during the growth process, the Zn and O ions in the precursor solution dominantly continue to grow on the existing ZnO nanorods in c-axis direction; while the growth on a-b plane and new nucleation are also occur. This is in accordance with the XRD pattern in Figure 1 which demonstrates that the TC value of (002) plane increase in the increase of growth time. From the FESEM images and the XRD patterns, it can be concluded that the increase in the growth time from 2 h to 8 h greatly enhance the

2 h

6 h

4 h

8 h

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surface area of ZnO nanorods but the highest TC value of the (002) plane is ZnO nanorods grown for 6 h.

Figure 3 FESEM cross-sectional images of ZnO nanorods with four different growth times. Scale bar is 500 nm

Figure 4 shows the typical room temperature optical absorption spectrum of ZnO nanorods based on various growth times. Strong absorption in the UV region with an absorption edge at a wavelength about 390 nm is the ZnO characteristics as a wide band gap semiconductor [49]. Generally, the absorption intensity in the UV and visible regions increases as the growth time increases, but there is no difference in UV absorption intensity between ZnO nanorods grown for 6 h and 8 h although the surface of both is slightly different. Although ZnO nanorods grown for 6 hours have a smaller surface area but have more atoms in the (002) plane, so their electrons can absorb the UV light more effectively and excited into the conduction band.

2 h 4 h

6 h 8 h

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Figure 4 UV-VIS optical absorption spectra of ZnO nanorods grown with four different growth times.

Figure 5 (a) shows the UV–Vis diffuse-reflectance spectra recorded at room temperature. Each spectra show a sharp edge at a wavelength about 390 nm, which correlates with the absorption edge in the absorbance spectra. The reflectance spectra were then used to calculate the band gap energy (Eg) using the Kubelka-Munk equation [50,51]:

𝛼ℎ𝑣 = 𝐴(ℎ𝑣 − 𝐸𝑔)^1/2

(3)

𝛼 = 𝐹(𝑅) = ^(1−𝑅)2

2𝑅

(2) Where α, h, 𝑣, A, Eg and R are the absorption coefficient, the Plank constant, the light frequency, the constant, the bandgap energy and % reflectance, respectively.

The value of F(R) is proportional to an absorption coefficient α in the Tauc equation, so α can be replaced by F(R).

750 700 600

550 450 400 350

300 650

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0

Wavelength (nm)

Absorbance

2 h 4 h 6 h 8 h

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Figure 5 (a) Reflectance spectra and (b)Tauc plot of ZnO nanorods with four different growth times.

Bandgap energy was determined using the Tauc plot, which is the plot between (𝛼ℎ𝜈)² and the photon energy (ℎ𝜈) as shown in Figure 5 (b). By extrapolating the linear part of the plot, the band gap is defined when 𝛼 = 𝐹(𝑅) = 0 or at the intersection of the linear slope with the photon energy axis [52]. By

4 h

6 h

3.10 3.15 3.20 3.25 3.30

3.10 3.15 3.20 3.25 3.30

3.15

8 h

3.10 3.15 3.20 3.25 3.30

2 h

3.10 3.15 3.20 3.25 3.30

hυ (eV) hυ (eV)

hυ (eV) hυ (eV)

( ɑh υ )

2

( ɑh υ )

2

( ɑh υ )

2

( ɑh υ )

2

3.23 eV 3.24 eV

3.24eV 3.24 eV

350 400

300 450 500

50 45 40 35 30 25 20 15 10 5 0