Improved Photoelectrical Performance of Single-Crystal TiO2 Nanorod Arrays by Surface Sensitization with Copper Quantum Dots
Through the redox reaction between Cu(NH3)42+ and H2O2, copper quantum dots (QDs) were deposited onto the surface of single-crystal rutile TiO2 nanorod arrays that were grown directly on transparent, conductive fluorine-doped tin oxide substrates by a facile hydrothermal process. Compared with pristine TiO2 nanorods, the top facets of TiO2 nanorods decorated with Cu QDs became flattened and adherent to each other, and the lateral facets were rough and covered with vast amounts of extremely small particles. The QDs were tightly attached on the surface of the nanorods, and the nanoparticle size measured from high resolution transmission electron microscopy images was around 6 nm, which is comparable with the Bohr exciton radius. X-ray photoelectron spectroscopy measurements showed that the QDs existed in the form of Cu(II)O and Cu(I)2O after the deposition process, and the Cu(0) QDs were unstable on the TiO2 surface. Furthermore, under the irradiation of a solar simulator, the photocurrent response of the QD sensitized TiO2 nanorods was improved dramatically with a small amount of QDs, and the optimal photocurrent density (98 μA/cm2) was much greater than that of the undecorated sample (16 μA/cm2). Likewise, external quantum efficiency (EQE) characterization demonstrated the superiority of the surface modification with Cu QDs, by which the highest EQE value of the photoanode was enhanced nearly ten times. In addition, a red shift of the peak in EQE measurement was found from the Cu QD sensitized samples, suggesting a quantum size effect caused by small QD particles.