# Solar Explosive Evaporation Growth of ZnO Nanostructures
## 3 Results  
SEM images of ZnO nanostructures deposited on different substrates by MEE using concentrated solar radiation for precursor decomposition are present in Figure 2. As can be seen from Figure 2a (region 1), ZnO hexagons grown on Au/Si substrates have two different geometric sizes. The large ones (area 2 on Figure 2a) that were heterogeneously grown on the Au/Si substrate have diameters of about 1200-1300 nm. Having diameters about of 200-220 nm, the small hexagons (area 3 on Figure 2a) uniformly covered the Au/Si substrate. The estimated height of the large hexagons was about 40 nm, and the height of small hexagons was about 6 nm, as determined by the use of the SEM and EDX equipment software. At similar deposition conditions, nanostructures in the form of the spheres (region 1 on Figure 2b) and in the form of \"mushrooms\" (see region 2 on Figure 2b) were grown on Ag/Si substrates. The deposited structures had sizes of about 500-2500 nm. EDX measurements confirmed that the whole Ag/Si substrates were covered by ZnO, having a spherical morphology in some regions. Therefore, the application of zinc acetylacetonate precursor led to the growth of hexagons on Au/Si substrates opposite to the growth of the spheres on Ag/Si substrates. The deposition of nanostructures by MEE using the carbothermal reduction for ZnO growth led to growth of the nanoneedles on unpolished Si substrates in the shape of \"hedgehogs in the grass\" (see region 1 on Figure 2c). The grown structures had crystal-like basis with diameters about 200-250 nm which ended with a needle of diameter \(\sim\)40 nm (region 2 on Figure 2c). EDX examinations of the needles determined that latter consist of ZnO. Figure 1: Solar concentrator of Geliocenter of the Institute for Problems in Material Science (Crimea, Ukraine). The analysis of EDX results allowed us to determine that all grown nanostructures were ZnO (see Supplementary Materials Figure S1a-c). The presence of carbon in large amounts was found on the surface of ZnO hexagons and ZnO spheres (Figure S1a,b). However, the surface of the ZnO nanoneedles contained the least amount of carbon (Figure S1c). Therefore, the application of mixture of ZnO with graphite powders precursor was more favorable than the zinc acetylacetonate precursor. Additional investigation of the influence of the technological parameters (e.g., boat-substrates distance) on the degree of carbon contamination is required. XRD patterns presented in Figure 3 confirm EDX data: all deposited nanostructures have wurtzitic structure of ZnO. Additionally, the reflection peaks of Au(111), Au(200), Au(220), as well as those of Ag(111), Ag(200), and Ag(220) are shown in the XRD patterns. No other diffraction peaks are shown in Figure 3.  
Figure 3: XRD patterns of ZnO nanostructures grown on (**1**) Au/Si and (**2**) Ag/Si substrates by zinc acetylacetonate decomposition, as well as on (**3**) unpolished Si substrates by the carbothermal reduction of mixtures of ZnO + C powders. Figure 2: SEM images of ZnO (**a**) hexagons, (**b**) spheres, and (**c**) needles deposited by the method of explosive evaporation (MEE) on Au/Si, Ag/Si, and unpolished Si substrates, respectively. The parameters of XRD peaks, approximated by Gaussian function, are presented in Table 1. Each type of ZnO nanostructure has its own set of crystallographic planes: hexagons--(002), spheres--(002), (103), (112), and needles--(002), (101), (102), (103) (JCPDS card number 36-1451). The small magnitude of the full width at half maximum (FWHM) of diffraction peak ZnO (002) of 0.12-0.18 degrees (Table 1) results in high magnitude of grain size as estimated by [PERSON]'s formula [10] about 45-50 nm for the hexagons and the needles, respectively. This confirms their high crystalline quality.