• 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.