Photo‐Driven Ion Transport for a Photodetector Based on an Asymmetric Carbon Nitride Nanotube Membrane

Abstract Conventional photosensing devices work mainly by electron processing and transport, while visual systems in intelligence work by integrative ion/electron signals. To realize smarter photodetectors, some photoionic device or the combination of ionic and electronic devices are necessary. Now, an ion‐transport‐based self‐powered photodetector is presented based on an asymmetric carbon nitride nanotube membrane, which can realize fast, selective, and stable light detection while being self‐powered. Local charges are continuously generated at the irradiated side of the membrane, and none (fewer) at the non‐irradiated side. The resulting surface charge gradient in carbon nitride nanotube will drive ion transport in the cavity, thus realizing the function of ionic photodetector. With advantages of low cost and easy fabrication process, the concept of ionic photodetectors based on carbon nitride anticipates wide applications for semiconductor biointerfaces.


Experimental section:
General: Unless otherwise noted, all of the commercial reagents were used as received. Melamine (purity >98.0%) was purchased from Sigma-Aldrich. Target 60-m-thick AAO membrane with pore width 84 ± 15 nm were purchased from Heifei Puyuan Nano, China. Glass test tube for vapor-deposition polymerization (VDP) were purchased from Merck Millipore. Blue, green, and yellow LED light were used for light irradiation, respectively. In this work, unless otherwise noted, all the light illumination were provided by blue LED light. I-V curves and constant voltage ionic current were adjusted to zero current at zero voltage to remove small offsets experienced between runs. All measurements were carried out at room temperature. The main transmembrane potential used in this work was stepped at 0.05 V/step for 1 s/step (0.05 V/s) from -0.5 to +0.5 V, with its period of 21 s.
Fabrication of ACNNM. The carbon nitride nanotube membrane was fabricated by a vapor-deposition polymerization (VDP) method described before. 1 Firstly, the commercial AAO membrane (Diameter: 5 mm) were cleaned by ethanol and deionized water, then dried by nitrogen. Then, the cleaned AAO were put into the bottom of the glass test tube in a certain direction (Supporting Figure1) to get an asymmetric structure. The samples were placed in the oven to heat to 773 K with a heating rate of 10 K/min, and then keep for 4h to insure the sufficient polymerization. After the temperature naturally cooled dawn to room temperature, the AAO membrane tuned from transparent white to brown, and yellowish carbon nitride power at the bottom of the test tube can be obtained. To get a pure carbon nitride nanotube for TEM or SEM, the carbon nitride nanotube membrane was immersed in 1M acid for chemical etching (72 h), then cleaned by deionized water and dried in 60 °C oven.
Ionic photodetector properties. The ions transport and ionic photodetector properties were studied by measuring the current-voltage curves and constant voltage ionic current through the ACNNM with and without light illumination. ACNNM membrane was mounted between two chambers of a home-made H cell, which are full of electrolytes. The cell has a transparent glass window for light irradiation. Ag/AgCl electrodes were used to collect the current and voltage signals. Open circuit voltage (photovoltage) was measured by electrochemical work station (Gamry interface 1000). Ionic current (photocurrent) was measured by a Keithley 6430 picoammeter (Keithley Instruments, Cleveland, OH). To the light density dependent measurements, the light density can be controlled by controlling the distance between light source and ACNNM. And light power intensity was measure by portable light intensity meter.
Calculation. The trans-nanotube potential was systematically analyzed by a theoretical model based on Poisson and Nernst-Planck (PNP) equations with proper boundary conditions, 2 Where Ф, ci, Di, ji, zi are, respectively, the electrical potential, ion concentration, diffusion constant, ionic flux, and charge of species i. ɛ is the dielectric constant of the electrolyte solution. The diffusion coefficients for cations and anions are 2.0×10 -9 m 2 /s (we use KCl electrolyte for simplicity). The boundary condition for potential Ф on the nanotube wall is, where σ is the surface charge density. And the surface charge density is various along with our experiment condition. The ion flux has the zero normal components at boundaries, The geometric parameters are in Fig. S13. The stationary solver was generally used. But when it fails, the parametric solver was applied. For all the calculations, the accuracy is set to be less than 10 -6 .

Characterizations
The released CNNs was transferred to a quartz glass substrate and analyzed. The scanning electron microscope (SEM) JSM-7500F (JEOL) at an accelerating voltage of 3 kV was used to get the top view and cross section of the CNNs. X-ray diffraction (XRD) patterns were recorded with a Bruker D8 Advance instrument with Cu Kα radiation.
Shimadzu UV 2600 was used to reveal the optical absorbance spectra of CNNs and powders. Surface photovoltaic spectroscopy was measured by a surface photovoltaic spectrometer (CEL-TPV1000). The membrane Zeta Potential was measured with SurPASS 3, Anton Paar. Figure S1. The fabrication of asymmetric carbon nitride nanotube membrane.              . The fabrication of asymmetric carbon nitride nanotube membrane. a, Vapor-deposition polymerization chemical process. b, In fabrication, the AAO membrane was used as a template, which can separate the reaction chamber to two different areas. The area with precursor will has a high carbon nitride concentration (CH) while another area has a low concentration (CL). In the polymerization and condensation process, asymmetric carbon nitride was generated. c, Schematic form of C3N4 asymmetric nanotube membrane in AAO template.