Effectiveness of CuO Nanoparticle-Based p–n Bulk-Heterojunction Electrodes for Photoelectrochemical Hydrogen Generation

Bulk-heterojunction (BHJ) electrodes are expected to play a significant role in electrode designs aimed to increase the efficiency of photoelectrochemical applications owing to their combined, efficient charge extraction and charge transportation. The photoelectrochemical behavior of BHJ nanoparticulate photoelectrodes with CuO (donor, p-type) and TiO2 (acceptor, n-type) semiconductors is examined with a focus on random distribution nature. Electrodes with different donor/acceptor ratios are prepared and characterized using various spectroscopic and electrochemical tools to reveal interconnected, crystalline, randomly distributed CuO (∼150 nm) and TiO2 (∼25 nm) nanoparticles. A photocathodic current of ∼1.5 mA/cm2 is measured at 0 VRHE for CuO photocathodes (FTO/npCuO) under simulated sunlight (AM 1.5G) in 0.5 M Na2SO4 (pH 5.5). We identify the cathodic photocurrent to be due to internal reduction of Cu2+ to Cu+; photocathodic hydrogen evolution was not observed. BHJ electrodes (FTO/npCuO:npTiO2) display dual (p-type and n-type) semiconducting behavior and correspondingly exhibit cathodic and anodic photocurrents, the magnitude of which is dependent on the donor/acceptor ratio present. The addition of TiO2 increases the electrode resistance and also promotes charge recombination in the photoelectrode. Compared to non-homogeneously distributed donor/acceptor BHJ photoelectrodes, homogeneous electrodes obtained via the co-precipitation method exhibit higher photocurrents but still undergo photodegradation, implying the need for a protective layer or electron shuttle. We demonstrate photocurrents of 0.2 mA/cm2 from TiO2- and Pt-protected npCuO:npTiO2 BHJ photoelectrodes with improved stability and hydrogen evolution albeit with a low Faradaic efficiency (<30%).