The real-time photocurrent response of the self-powered UV detector at 0-V bias is shown in Figure  5 under an incident UV light with a wavelength of 385 nm, corresponding to the bandgap of ZnO nanoneedle arrays. The incident radiation is switched with an on/off interval of 10 s. Six repeated cycles are displayed in Figure  5a, in which the photocurrent is observed to be consistent and repeatable with no degenerate effect found during the detection process. From

the magnified rising and decaying edges of photocurrent shown in Figure  5b,c, respectively, Palbociclib mw a fast photoresponse can be seen clearly. The rising time (defined as the time to increase from 10% to 90% of the maximum photocurrent) and the decaying time (defined as the time to recover from 90% to 10% of the maximum photocurrent) are both approximately 0.1 s, indicating rapid photoresponse characteristics. Figure 5 The real-time photocurrent response of the ZnO nanoneedle array/water UV detector. (a) Photocurrent response under on/off UV light radiation with the illumination wavelength of 385 nm. Enlarged (b) rising edge and (c) decaying edge of the photocurrent

response. In order to clearly clarify the working principle of this self-powered UV detector, a simple energy band diagram is schematically shown in Figure  6. Since the Fermi level of the n-type semiconductor (ZnO) is higher than the redox potential of the aqueous electrolyte (deionized water), when a semiconductor is placed in contact mafosfamide with an electrolyte, electric current Ceritinib datasheet initially flows across the junction until electric equilibrium is reached [28–30]. In this case, electrons will transfer from the semiconductor (ZnO) into the electrolyte (deionized water), which will produce a region on each side of the heterojunction where the charge distribution differs from the bulk material, known as the space charge layer. Electron depletion from solid into the solution results in a positive excess

charge by immobile ionized donor states. Hence, an electric potential difference across the solid-liquid interface is set up, which works in a Schottky barrier mode, as is reflected by the upward bending of the bandgaps of the n-type semiconductor. Figure 6 Energy band diagram and working principle for the UV photodetector under 0-V bias and illumination. When incident light travels through FTO glass and reaches the active layer of ZnO nanoneedle arrays, photons with energy exceeding that of the ZnO bandgap will be absorbed and electron-hole pairs will be generated thereafter. The built-in potential across the interface works as the driving force to separate the electron-hole pairs. Negative charge moves along the ZnO nanoneedle and gets collected by the FTO electrode and poured into the external circuit easily since the work function of FTO matches with the conduction band of ZnO. The positive holes are driven to the surface and got captured by the reduced form of the redox molecule (h+ + OH- → OH·).