Photocatalytic Activeties of Copper Oxides/Carbon Nanotube Composites
Authors: Matsunaga Mariko , Takahashi Kensei , Saito Yuki 
Published: February 2020

Cu2O has the advantage of absorbing visible light unlike the practical photocatalyst, TiO2. To use most of Cu2O as a photo catalyst, increase of the surface area and electro-conductivity are considered to be promising approach. For this purpose, our studies have focused on the composition of Cu2O with Multi-Walled Carbon Nano Tube (MW-CNT). In our recent studies, we demonstrated that the various morphologies of copper oxides can be electrochemically deposited on a MW-CNT film-modified electrode(*). In this paper, the photocatalytic activities of these samples are compared.
CNT thin film that thickness was less than 17.5 μm was prepared on a transparent electrode by electrophoretic electrodeposition with 17.5 V at 35 ℃.　And Cu2O was deposited by electrochemical deposition method, using a three electrode cell: an ITO substrate as anode electrode, Pt wire as counter electrode, and Ag/AgCl as reference electrode, at various potentials and deposition charge. The plating bath was 0.4 M CuSO4, and NaOH is used to adjust the pH to 11.　Scanning Electron Microscope (SEM) and Energy Dispersive x-ray Spectroscopy (EDS) was used to observe the morphology and composition of the samples, respectively. The crystallinity and crystal structure were examined by XRD method. Photocatalytic activity was tested by photocurrent response and charge transfer resistance evaluated by electrochemical impedance spectroscopy.
The results of SEM and EDS demonstrate that copper oxide was deposited over the entire surface of CNT when the deposition charge and potential is 250 mC and -0.4 V, respectively. At the lower potential, copper oxide becomes aggregated and CNT is partially exposed. In addition, CNT was also partially exposed with less and more electric charge. The copper oxide is considered to peal off by its excess deposition. For all samples, the diffraction peaks in the XRD patterns match well with those of Cu2O and Cu crystals. With 250 mC, the ratio of Cu is appeared to increase at the more negative potential than -0.4 V. At -0.4 V, the ratio of Cu2O increase with increasing the deposition charge up to 250 mC. Charge transfer resistance of the hydrogen evolution reaction was evaluated by EIS measurement at 0 V vs SHE. As a result, the sample prepared at -0.4 V with both 250 mC and 500 mC, charge transfer resistance become minimal. In addition, the charge transfer resistance also increases at the more negative potential than -0.4 V when the deposition charge is 250 mC. Based on the above-mentioned results, the ratio of Cu2O versus Cu is considered to be important to decrease the charge transfer resistance. Photocatalyc acitivity was evaluated using the cell illuminated by 50 W Xe arc lamp with a calibrated AM 1.5 solar light simulator (class A). Upon the illumination of 100 mW/cm2 light, the largest photocurrent was observed under the condition with 250 mC at -0.4 V. As discussed above, the difference between 250 mC and 500 mC is mainly the morphology of Cu2O. Thus, based on the fact that the carbon materials are known to adsorb light, the condition Cu2O was deposited over the entire surface of CNT is important to show the better performance. The details will be discussed on the day.
*“Preparation of Cu2O/ CNT Composite Photocatalysts using Electrodeposition Methods,” Yuki Saito, Kensei Takahashi, Mariko Matsunaga, The 83th Annual Meeting of Japan Electrochemical Society, Osaka, Japan, Mar.2016.