Copper quantum dots on TiO2: A high-performance, low-cost, and nontoxic photovoltaic material

The surface decoration of TiO2 with Cu quantum dots (QDs) was carried out through a simple chemical redox deposition method. The QDs in the form of Cu(I)/(II)-O-Ti(IV) network were attached tightly and highly dispersed onto the pre-sintered TiO2 surface, and no obvious change could be detected from the lattice and surface morphology of TiO2 after the modification. Quantum size effect was evidenced by diffuse reflectance spectra (DRS), from which the absorption spectrum extended from 380 nm to 440 nm. The concentration of Cu measured by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) was higher than its theoretical value, a result which suggested that the QDs resided on the TiO2 surface and that the QD sensitization was primarily a surface process. In comparison to pure TiO2, the introduction of 1.0 wt. % Cu QDs increased the photocurrent density from 3.1 to 17.0 μA/cm2. Even a trace amount of Cu (only about 0.25 wt. %) can strongly enhance the photoelectric activity of TiO2. When excessive Cu was coated onto the TiO2 surface, the recombination of the photo-induced charges would be aggravated by the aggregation of QDs, and the growth of Cu grains made the bandgap of the QDs and TiO2 unmatched so that the photovoltaic conversion was restrained. Besides Cu QDs, the photoelectrical properties of TiO2 nanoparticles can be enhanced by the incorporation of other copper-based QDs (e.g., CuO, Cu2O, CuS, and Cu2S); the elementary Cu QDs demonstrated the best characteristics among them.