Two-Step Electrochemical Route to Synthesize Nanostructured Cu2O Thin Films

Abstract

The conversion of solar energy to chemical fuels via a photoelectrochemical (PEC) cell is one promising approach to produce clean energy. A PEC cell consists of two light absorbing electrodes with appropriate optoelectronic properties. Depending upon the band alignment, one of two electrodes is a photoanode producing O2, and the other one is a photocathode which produces H2. One such suitable photocathode for a PEC cell is Cu2O; an intrinsic p-type semiconductor with a direct bandgap of 2.0 eV and a high absorption coefficient of 104 cm-1 in the visible range. Its conduction band minimum (CBM) lies 0.7 V negative of water reduction potential and therefore, can act as a photocathode. However, one major challenge associated with Cu2O is its poor minority carrier diffusion length resulting in a large recombination rate in bulk and lower overall cell efficiency. In this aspect, nanostructured materials are of great use owing to their inherent properties. Orthogonal charge separation, enhanced light absorption, and large electrode-electrolyte are few advantages of the nanostructured materials which can be used to overcome the issue of minority carrier diffusion length. However, the synthesis of nanostructured Cu2O thin films is quite challenging as it involves multiple steps and/or require high-temperature processing.In this work, we present a low temperature, and straightforward two-step synthesis method to obtain nanostructured Cu2O with 10-30 nm thick nano-walls on FTO coated glass substrate. First, the Cu2O thin films are electrodeposited at a potential of -0.35 V vs. Ag/AgCl in a pH 12 lactate stabilized copper sulfate electrolyte at 55 °C. The electrodeposited Cu2O films are 1.3 µm thick and show pyramidal grains of size up to 2 µm on the surface after a deposition period of 15 mins. Later, these films are subjected to voltage cycling in a potential range of -0.6 V to 0.6 V (vs. Ag/AgCl) in a pH 5 electrolyte comprised of 0.5 M Na2SO4 and 0.1 M K2HPO4 at RT. This causes top 0.5 µm of deposited films to reconstruct to 10-30 nm thick nano-walled porous morphology after 20 cycles of voltage cycling. The voltammogram shows that reconstruction is a concomitant etching of Cu2O as Cu2+ ions along with oxidation to CuO. In fact, a potentiostatic hold at anodic potentials leads to porous CuO. This porous CuO is reduced back to Cu2O during the cathodic part of the voltage cycling. Interestingly, the number of cycles performed during voltage cycling is found to affect the phase purity of the material. The XRD scans reveal that Cu2O films are phase pure only up to 50 cycles. However, phase purity is no longer preserved as Cu impurity is detected in Cu2O films post 50 cycles. A comparison of linear sweep voltammetry (LSV) scans of pristine Cu2O, and CuO thin films show that reduction of Cu2O to Cu is a slow process in this potential range, and hence, Cu-impurity is not observed up to 50 cycles. The bare nanostructured Cu2O film show a photocurrent of -2.0 mA-cm-2 at a potential of 0.3 vs. RHE in a pH 5 media, which is almost 1.5 times of the dense Cu2O. This enhancement in photocurrent is attributed to efficient separation of photogenerated charge carriers in the orthogonal direction, increased electrode-electrolyte contact area, and enhanced light trapping. The photocurrent can further be improved via coating with wide bandgap metal oxide layers such as AZO/TiO2. Such a facile fabrication technique can be utilized to prepare nanostructures of various metal oxides on a large scale for their application in PEC cells and other optoelectronic devices. Figure 1