# Fabrication and Photo-Detecting Performance of 2D ZnO Inverse Opal Films
## 3 Results and Discussion
### Fabrication and Characterization of 2D ZnO IO Films  
The as-prepared monolayer PS/CA-ZA films were further annealed in air to remove the PS templates, hence obtaining 2D ZnO IO films. Figure 3 shows the SEM images of the 2D ZnO IO films fabricated at different CA/ZA molar ratios. From Figure 3a,b one can see that the IO film fabricated without CA had rough walls and a broken bottom; cracks across the film indicate poor connectivity. In contrast, when the molar ratio of CA/ZA was 0.2:1, the 2D ZnO IO film had thinner walls and a continuous bottom, despite of some holes (Figure 4c,d). The inner pore size was calculated to be about 280 nm, slightly smaller than the diameter of template microspheres, which may be due to framework shrinkage during the calcination process. When the CA/ZA molar ratio further increased to 1:1, a film with rough walls and a continuous bottom was obtained (Figure 4e,f). In this work, CA was used as a complexing agent to form homogeneous precursor gel with ZA, which \"glues\"  
Figure 1: Scanning electron microscope (SEM) images of the poly(styrene-co-acrylic acid) (PSA) microspheres assembled in (**a**) zinc acetate (ZA) solution and (**b**) citric acid-zinc acetate (CA-ZA) solution (CA/ZA molar ratio = 1:1). Figure 2: SEM images of polystyrene (PS)/precursor films with different CA/ZA molar ratios: (**a**) 0:1; (**b**) 0.2:1; (**c**) 1:1. PS microspheres together to yield crack-free IOs [24]. Similar results were reported in a previous study [25]. The gelation during drying also narrows the gaps between PS microspheres, and the addition of CA may provide a better adhesion to the substrate. As a result, more ZnO nanoparticles grow from the substrate rather than interstices between PS microspheres with increasing CA/ZA molar ratio. Figure 4: X-ray diffraction (XRD) spectrums of annealed ZnO powder at different CA/ZA molar ratios: (**a**) 0:1; (**b**) 0.2:1; (**c**) 1:1. Figure 3: SEM images of ZnO IO films on SiO\({}_{2}\)/Si substrates fabricated at different CA/ZA molar ratios: (**a**,**b**) 0:1; (**c**,**d**) 0.2:1; (**e**,**f**) 1:1. Figure 4 shows the XRD spectra of annealed ZnO powder at different CA/ZA molar ratios. All of the peaks can be indexed to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes of hexagonal wurtzite ZnO structure according to JCPDS No. 36-1451. These results indicate that the films fabricated at different CA/ZA molar ratios have the same crystal structure as hexagonal wurtzite ZnO. To further confirm the microstructure of the 2D ZnO IO film, HRTEM and SAED patterns were used to examine the ZnO nanoparticles taken from the film with the CA/ZA molar ratio of 1:1. Figure 5b shows the HRTEM image of the nanoparticles scraped from the film. The interplanar distances of 0.25 nm and 0.26 nm are attributed to the (101) and (002) planes of hexagonal wurtzite ZnO, respectively. The SAED pattern shows seven diffraction rings corresponding to (100), (002), (101), (102), (110), (103), and (200) planes of hexagonal wurtzite ZnO (Figure 5c). These results also reveal the polycrystalline wurtzite ZnO structure of the film. Figure 6a shows the UV-Vis absorbance spectrums of the 2D ZnO IO films with different CA/ZA molar ratios. All films are highly transparent in the visible region and have strong UV absorption below 370 nm. Their bandgaps can be estimated by the following equation:  
\[(\alpha\hbar\
u)^{2}\ =\ C(\hbar\
u-E_{\mathrm{g}}), \tag{1}\]  
where \(\alpha\) is the absorption coefficient, \(\hbar\
u\) is the light energy, C is a constant, and \(E_{\mathrm{g}}\) is the band gap energy [26]. Figure 6b shows the corresponding \((\alpha\hbar\
u)^{2}\)-\(\hbar\
u\) curves. The bandgaps are estimated to be 3.25, 3.25, and 3.22 eV when the CA/ZA molar ratios are 0:1, 0.2:1, and 1:1, respectively. There is little change in bandgap despite differences in the CA/ZA molar ratio.