## 4 Diamond-Based 1D Metal Oxide Heterojunction Classes
### D N-ZnO/P-Diamond Heterojunction
#### 4.1.2 1 D N-ZnO/P-Diamond Related Optoelectronic Devices  
In the past, the existing ZnO types were usually deposited on a diamond substrate in the form of thin-film, which was used for surface acoustic wave (SAW) filters applications [65], examining structural and electrical properties [66; 67; 72], and for constructing p-n junction diode [68]. In the last ten years, composite 1D ZnO related-nanostructures and diamond nano-optoelectronic p-n junction devices were extensively studied. [PERSON] et al. [69] developed novel tyrosine biosensor with the biological function based on ZnO NRs on the BDD substrates. The ZnO NRs were for the first time deposited on BDD thin film by a low-temperature solution method. Boron-doped ZnO nanorods were fabricated on the BDD by hydrothermal technique and showed high photocatalytic performance [33]. [PERSON] et al. [81] reported on photocatalytic activity of the n-ZnO NRs/p-BDD heterojunction fabricated with the hydrothermal degradation of methyl orange dye, which depended on the density and diameter of ZnO NRs, the distance between NRs and the interface at ZnO-BDD heterojunction (Figure 10). Vertically aligned ZnO NRs were synthesized by [PERSON] et al. [82] for the first time by using a low-temperature solution method on the BDD films. The heterojunction showed typical rectification characteristics with the standard ideality factor approaching theoretical values under both low and high forward-biasing voltages (Figure 11).  
Figure 8: Diamond cell with a face-centered cubic structure [80].  
Figure 9: Schematic illustration atomic arrangement of (**a**) ZnO (0001) and diamond (111) planes and (**b**) ZnO (0001) facet and diamond (100) plane epitaxial relationship [23].  
In recent years, ZnO NRs/BDD have been prepared and used as an anode for photocatalytic oxidation of aniline. The results showed that ZnO NRs/BDD is a promising photoanode for organic degradation [83]. [PERSON] et al. [84] proposed a low-temperature annealing process to improve the optical response of the ZnO NRs/nano-diamond film substrate for the UV photodetector. Furthermore, novel ZnO NRs/ultra-nanocrystalline diamond may be used as a high-sensitivity device for hydrogen gas sensing (Figure 12) [85]. They also reported ZnO NTs, with the diamond NWs enhancing UV detection and field emission properties [86].  
Compared to n-ZnO NRs/p-BDD heterojunction, n-ZnO NWs/p-BDD heterojunction reported by [PERSON] et al. shows better _I-V_ electrical properties and higher electrical transport performance (Figures 13 and 14) [30]. By comparing both dark and UV illuminated _I-V_ characteristics, one may see that the UV illumination reduces the turn-on voltage and ideality factor of the tested heterojunction. At 10 V, the UV positive current is more than 4 times as much as the dark [31]. To sum up, the high UV response rate, long device life and fast switching speeds of the heterojunction make it an ideal material for multifunctional optoelectronic applications.  
Figure 11: (**a**) The _I-V_ curve of ZnO nanorod (NR)/BDD heterojunction. Insert shows the schematic configuration for conducting measurements. (**b**) The forward-biased In _I-V_ curve of ZnO NR/BDD heterojunction. A diode ideality factor \(n_{1}=1.0\) in the range from 0–0.3 V and \(n_{2}=2.1\) in the range from 1.2–3.0 V were simulated [82].  
Figure 10: SEM micrographs with different morphologies (the insets) of the fabricated ZnO NRs based on various reaction concentrations: (**a**) 0.1, (**b**) 0.05, (**c**) 0.025 and (**d**) 0.01 mol/L [82].