The ZnO seed layer was formed by spin coating the colloid solution at 3,000 rpm followed by annealing in a furnace at 400°C for 1 h. The following hydrothermal growth was carried out at 90°C for 6 h in a Teflon bottle by placing the seeded substrates vertically in aqueous growth solutions, which contain 20 mM zinc nitrate, 20 mM hexamethylenetetramine, and 125 mM 1,3-diaminopropane. Then the FTO glass with ZnO nanoneedle arrays was rinsed with deionized water

thoroughly and annealed at 500°C for 1 h to remove any residual organics and to improve the crystalline structure. Assembly of the solid-liquid heterojunction-based UV detector The solid-liquid heterojunction-based UV detector was assembled in the same structure as that of a dye-sensitized solar cell, except that no dye molecules were adsorbed and the electrolyte used in this case was deionized C646 manufacturer water, as discussed in our previous work URMC-099 [32]. Figure  1 shows the schematic structure of the nanocrystalline ZnO/H2O solid-liquid heterojunction-based UV detector. For device manipulation, FTO glass with vertically aligned ZnO nanoneedle arrays was used as the active electrode. A 20-nm-thick Pt film deposited on FTO glass by magnetron sputtering formed the counter electrode.

Afterwards, the work electrode (ZnO/FTO) and the counter electrode (Pt/FTO) were adhered together face to face with a 60-μm-thick sealing material (SX-1170-60, Solaronix SA, Aubonne, Switzerland). Finally, deionized water was injected into the space between the top and counter electrode. A ZnO/H2O solid-liquid heterojunction-based UV detector was fabricated with an active

area for UV irradiation of about 0.196 cm2. Figure 1 Schematic device structure of the ZnO nanoneedle array/water solid-liquid heterojunction-based ultraviolet photodetector. Characterization of ZnO nanoneedle arrays and the UV photodetector The crystal structure of the ZnO nanoneedle arrays Thymidine kinase was analyzed by XRD (XD-3, PG Instruments Ltd., Beijing, China) with Cu Kα line radiation (λ = 0.15406 nm). The surface morphology was characterized using a scanning electron microscope (Hitachi S-4800, Hitachi, Ltd., Chiyoda, Tokyo, Japan). The optical transmittance was measured using a UV-visible dual-beam spectrophotometer (TU-1900, PG Instruments, Ltd., Beijing, China). The photoresponse characteristics of the UV detector under illumination were recorded with a programmable voltage-current sourcemeter (2400, Keithley Instruments Inc., Cleveland, OH, USA). A 500-W xenon lamp (7ILX500, 7Star Optical Instruments Co., Beijing, China) equipped with a monochromator (7ISW30, 7Star Optical Instruments Co.) was used as the light source. For the photoresponse switching behavior measurement, photocurrent was measured by an electrochemical workstation (RST5200, Zhengzhou Shirusi Instrument Technology Co. Ltd, Zhengzhou, China).