Formation Mechanism and Properties of Foam-Structured ZnO Films Prepared Using Catalyst-Free Carbothermal Method

Foam-structured ZnO films were formed on c-axis Al2O3 substrates by using a catalyst-free carbothermal method. The morphological properties of the films strongly depended on their substrate position in the reactor, implying that the quantity of Zn vapor transported played a crucial role in determining the film structure. The formation mechanism of the films is proposed to be as follows: with increased transport of Zn vapor, vaporized Zn elements transformed into liquid Zn nanoparticles, and the liquid Zn elements on the rods promoted the new nucleation of ZnO nanorods. The repeated growth of ZnO rods at the nucleation sites led to the foam-like structure. The as-formed foam-structured ZnO films exhibited the wurtzite crystal structure, and the atomic composition ratio of Zn to O was 1 : 0.85. The main photoluminescence emissions were centered at 3.3 and 2.5 eV, and they were attributed to band-to-band and defect-related transitions, respectively.


Introduction
Zinc oxide (ZnO) has been receiving increasing attention owing to its promising properties, such as a bandgap that is direct and wide (≈3.4 eV), an exciton binding energy that is large (≈60 meV), high transparency in the visible region, high stability against high-energy radiation, and ease of etching in both acid and base media [1][2][3][4][5]. In particular, ZnO nanostructures have been attracting considerable interest because of their peculiar properties. They have been grown using various techniques such as chemical vapor deposition (CVD), physical vapor deposition, and solution techniques [1][2][3][6][7][8][9]. In particular, in CVD methods, vapor-liquid-solid (VLS) and vapor-solid (VS) processes have been used widely to produce ZnO nanorods (NRs) [4,[10][11][12]. While the VLS process usually requires a metal catalyst layer, the VS process facilitates direct deposition by transporting the metal oxide vapor from a high temperature to a low temperature.
In the present study, we focused on the formation of foam-structured ZnO (FS-ZnO) films, which are characterized by a network of ZnO NRs, on c-axis Al 2 O 3 substrates in a one-step VLS process. Furthermore, the properties of the films were investigated. The VLS process includes the formation of Zn vapor in a carbothermal reaction, nucleation of liquid Zn nanoparticles on ZnO NRs, and formation of ZnO NRs in an oxidation process. In this paper, in addition to a description and discussion of the aforementioned study, we propose a formation mechanism for the FS-ZnO films.

Materials and Methods
FS-ZnO films were formed on c-axis Al 2 O 3 substrates (4 cm × 4 cm) by using a catalyst-free carbothermal method. A schematic of the experimental setup used for the preparation of the nanostructural ZnO films is shown in Figure 1. A mixture of ZnO (99.9%, Sigma-Aldrich) and graphite (99.9%, Sigma-Aldrich) powders with a molar ratio of 7 : 1 was used as the precursor for the film preparation. The precursor was placed at the center of the alumina boat, and the substrates were positioned on the left (L) and right (R) sides of the precursor as well as at its center. The substrates on the left and right were at a distance of approximately 4 mm from the precursor. Prior to the growth process, the reactor used for the carbothermal reaction was purged with Ar gas for 1 h and subsequently heated up to 800°C at a rate of 5°C/min. After the desired temperature was attained, the growth time of ZnO films was 1 h. During ZnO film growth, a mixture of Ar and O 2 (Ar: 87%; O 2 : 13%) was introduced in the reactor, and the partial pressure of each gas was controlled by flowmeters.
The morphological structure of the nanostructural ZnO films formed was characterized using a scanning electron microscope (SEM, Hitachi S-4800), and elemental analysis of the films was performed using an electron probe microanalyzer (EPMA, Shimadzu EPMA-1600) and energy dispersive X-ray spectroscopy (EDX, Hitachi S-4800). The structural properties of the nanostructural ZnO films were investigated using an X-ray diffractometer (XRD, PANalytical X'Pert MPD), and the optoelectronic properties at room temperature were measured using photoluminescence (PL) spectroscopy (LabRAM HR, Sci-Tech Instruments). All the data was taken after more than five times of experiments. Figure 2 shows the XRD spectra of nanostructural ZnO films formed at different substrate positions, which are indexed as L, C, and R zones. For all samples, diffraction peaks were mainly observed at 31.8°, 34.4°, and 36.9°, corresponding to the (100), (002), and (101) diffraction planes of hexagonal ZnO with the wurtzite structure (JCPDS Card No. 01-089-0510). The degree of preferred orientation for the films is indicated by the texture coefficient (TC), which is given by [4,13] TC hkl = I hkl /I 0 hkl 1/n∑ n n=1 I hkl /I 0 hkl , 1

Results and Discussion
where I hkl and I 0 hkl are the measured and standard intensities of the (hkl) peak, and n is the number of diffraction peaks. The calculated TC values for the (002) diffraction peak for the as-deposited nanostructural ZnO films formed in the L, C, and R zones were 65%, 23%, and 29%, respectively. These values implied that the nanostructural ZnO films grown in the L zone showed a strong preferred orientation in the c-axis direction, while those prepared in the C and R zones did not exhibit any preferred orientation. Figure 3 shows SEM images of nanostructural ZnO films formed in the L, C, and R zones. It is noteworthy that the sample grown in the L zone (Figure 3(a)) exhibits a hexagonal NR structure, whereas the samples prepared in the C and R zones (Figures 3(a) and 3(d)) show a network structure and foam-like structure, respectively.   Equation (2) represents the reduction process of ZnO in the carbothermal reaction, equation (3) describes ZnO formation in the reaction between the vaporized Zn element and O 2 gas, and equation (4) represents NR formation by additional nucleation of the liquid Zn particles on ZnO NRs. 3 Journal of Nanomaterials Figure 5 shows PL spectra of nanostructural ZnO films grown at L, C, and R zones. As illustrated in Figure 5, the emission peaks in the spectrum were mainly observed at 380 nm (3.3 eV) and 500 nm (2.5 eV) for all samples. UV peaks centered at a wavelength of 380 nm (Peak 380 ) were attributed to the band-to-band transition, and the visible peaks centered at 500 nm (Peak 500 ) originated from the defect-related transitions [1,5]. The calculated values of Peak 380 /Peak 500 ratio for as-grown nanostructural ZnO films formed at L, C, and R zones were 1.43, 0.63, and 1.22, respectively. Therefore, one can infer that the nanostructural ZnO films grown at the L zone have a relatively strong UV emission, probably due to a strong preferred orientation toward the c-axis, compared to the nanostructural ZnO films formed at the C and R zones exhibiting a weak preferred orientation as argued by TC values in Figure 2. The surface area of the foam-like structure increased dramatically, unlike other nanostructural ZnO films. Furthermore, the atomic ratio of Zn to O in the films was 1 : 0.85, and this was confirmed by the EPMA and EDX. Therefore, it is strongly suggested that the defect-related transitions around 500 nm were mainly caused by a surface defect and an oxygen vacancy. Thus, in FS-ZnO films, oxygen-deficient and surface-derived defects are strongly related to the visible emission of the PL spectrum of the films. Even though we have no experimental data at present, the defect-related peaks are expected to lessen by a heat treatment of samples in the oxygen atmosphere.

Conclusions
Foam-structured ZnO (FS-ZnO) films were formed on c-axis Al 2 O 3 substrates by using a catalyst-free carbothermal method. The as-formed films showed the wurtzite crystal structure, and the atomic composition ratio of Zn to O was approximately 1 : 0.85. Photoluminescence emissions centered at 3.3 and 2.5 eV were attributed to band-to-band and defect-related transitions, respectively. The morphological properties of the nanostructural ZnO films strongly depended on their substrate positions in the reactor, implying that the quantity of Zn vapor transported played a crucial role in determining the film structure. Differences in the morphological structures of the nanostructural ZnO films could be explained on the basis of the quantity of vaporized Zn elements transported. The transport of Zn vapor gradually increased from the L zone to the R zone, and this was followed by the flow of Ar-O 2 gas mixture from the L zone to the R zone. We propose the following formation mechanism for the FS-ZnO films: with increased transport of Zn vapor, vaporized Zn elements transformed into liquid Zn nanoparticles. The liquid Zn elements on the rods promoted the new nucleation of ZnO rods, and the repeated growth of ZnO rods at the nucleation sites led to a foam-like structure.   Journal of Nanomaterials

Data Availability
All the data of this manuscript used to support the findings of this study are available from the corresponding author upon request. Anyone who has questions about the results of the study could contact the authors freely.

Conflicts of Interest
The author(s) declare(s) that they have no conflicts of interest.