## 4 Diamond-Based 1D Metal Oxide Heterojunction Classes
### 1 D N-TiO\({}_{2}\)/p-Diamond Related Optoelectronic Devices
Figure 17: (**a**) SEM morphology of the BDD film; (**b**) plan-view SEM morphology of TiO\({}_{2}\) NTs on BDD; (**c**) photocatalytic decomposition of RY15 solution of n-TiO\({}_{2}\) NTs/p-BDD heterojunction, TiO\({}_{2}\) NTs on the undoped diamond and without photocatalyst. (**d**) Recyclability test of photocatalytic decomposition for n-TiO\({}_{2}\) NTs/p-BDD heterojunction [38; 39]. Figure 18: Photocatalytic decomposition of RY15 solution with (**a**) TiO\({}_{2}\) NTs on hemispherical diamond film (TiO\({}_{2}\) NTs/diamond). (**b**) The linear relationship between ln(C/C\({}_{0}\)) and time [104]. TiO\({}_{2}\) NTs/BDD composite electrode (Figure 19a) was fabricated by microwave plasma-enhanced chemical vapor deposition (CVD) by the Sitzdak group, which greatly enhanced the electrochemical properties. Compared with the BDD prepared on a Ti plate (0.11 mF\(\cdot\)cm\({}^{-2}\)), the composite electrode provides high capacitance (Figure 19b). The enhanced capacitive effect in TiO\({}_{2}\) NTs/BDD can be understood as follows: (1) the particular cooperative morphology of TiO\({}_{2}\) NTs and BDD provides a more efficient conduction route for ion diffusion; (2) NTs are partially decomposed and converted to Ti\({}_{2}\)O\({}_{3}\) and TiC fractions. Finally, TiO\({}_{2}\) NTs, which demonstrate high ordering attained by a simple, rapid and controllable anodic oxidation method, can be used as the conductive BDD layer deposited on a substrate and then embedded into the structure of a supercapacitor [70; 105; 106]. Most recently, [PERSON] et al. [41] reported on the development of a unique photoelectrochemical electrode consisting of a nanostructured BDD layer covering an n-type TiO\({}_{2}\) film (Figure 20). The effects of the nanostructure, B doping level and TiO\({}_{2}\) film thickness on the properties of PEC were studied. Using RF magnetron sputtering, the BDD films with two doping levels (gas phase B/C = 1000 ppm and 10,000 ppm) that had already grown and plasma-nanostructured were used as the substrates to deposit TiO\({}_{2}\) layers of 20, 100 and 500 nm. BDD was used as the structure electrode. When the gas phase B/C ratio was 1000 ppm, the surface was covered with a TiO\({}_{2}\) layer 500 nm thick, and the photocurrent was the highest (60 \(\mu\)A/cm\({}^{2}\) and 3.2 mA/cm\({}^{2}\) at 0 and 2 V vs. Ag/AgCl respectively). The favorable influence of the nanostructure type and pn junction on the hole injection was confirmed by experiments. [PERSON] et al. [37] prepared a mesoporous TiO\({}_{2}\) layer on the BDD and found that deep-UV illumination could improve its photocatalytic efficiency. They also used this hybrid electrode in water electrolysis to produce O\({}_{3}\) gas and other reactive oxygen species (ROS). In addition, ROS produced by electrochemistry were tested in water treatment under UV illumination and demonstrated the system's suitability for advanced oxidation processes [27]. This simple water treatment system can be used to break down refractory organic matter in wastewater, a process that is still being studied. Basically, the 1D n-TiO\({}_{2}\)/p-BDD device suggests a new strategy, which is expected to be used for its wide-range implementation in photocatalysis, photoelectric chemical electrode and sensor, etc. A summary of different synthesis routes and applications (properties) of 1D n-TiO\({}_{2}\)/p-diamond heterojunction is presented in Table 2.  
Figure 19: Schematic illustration for composite TiO\({}_{2}\) NTs/BDD electrode. Cyclic voltammetry plots: (**a**) combination electrodes TiO\({}_{2}\) NTs/BDD-10 k, with pure TiO\({}_{2}\) and BDD immersed in 0.1 M NaNO\({}_{3}\) (v = 50 mV/s), (**b**) the curve of peak currents versus the square root of the scan rate for different BDD-covered electrodes [70; 105; 106].