Abstract
The ability to control electronic states by utilizing quantum confinement of one of the material components in heterojunctions is a promising approach to perform energy-level matching. In this work, we report the possibility to achieve optimum energy alignment in heterojunctions made from size-controlled quantum dots (Q-dots) of ZnO in combination with three copper oxides: Cu2O, Cu4O3, and CuO. Quantum confinement effects on the ZnO nanoparticles in the diameter range 2.6-7.4 nm showed that the direct optical band gap decreased from 3.99 to 3.41 eV, with a dominating shift occurring in the conduction band (CB) edge, and thus the possibility to obtain close to 0.6 eV CB edge shift by controlling the size of ZnO. The effect was utilized to align the electronic bands in the ZnO Q-dot/copper oxide heterojunctions to allow for charge transfer between the materials and to test the ability to improve the photocatalytic performance for the system, evaluated by the transformation of a dye molecule in water. The catalyst materials were investigated by X-ray diffraction, scanning electron microscopy, ultraviolet-visible (UV-vis), photoluminescence, and Raman spectroscopy. The most promising material combination was found to be the Cu2O copper oxide in combination with an energy aligned ZnO Q-dot system with approximately 7 nm diameter, showing strong synergy effects in good agreement with the energy-level analysis, outperforming the added effect of its individual components, ZnO-Q-dots and Cu2O, by about 140%. The results show that utilization of a heterojunction with controllable energy alignment can provide a drastically improved photocatalytic performance. Apart from increased photocatalytic activity, specific surface states of ZnO are quenched when the heterojunction is created. It is anticipated that the same approach can be utilized in several material combinations with the added benefit of a system with controllable overpotential and thus added specificity for the targeted reduction reaction.
Highlights
The importance of water can hardly be exaggerated
We report the ability to control electronic states and energy alignment by utilizing size-controlled quantum dots (Q-dots) of ZnO in combination with three copper oxides: Cu2O, Cu4O3, and CuO
The band gap and energy-level shifting from the quantum confinement effects in the ZnO nanoparticles in the diameter range from 2.6 to 7.4 nm and a change in the optical band gap from 3.99 to 3.41 eV are reported together with the possibility to utilize this to align the electronic bands in heterojunction photocatalysts to provide an improved efficiency
Summary
The importance of water can hardly be exaggerated. It is a prerequisite of life on earth and a resource that is needed in vast amounts for agriculture, industry, and domestic use.[1]. Water pollution from cities or factories together with an increased demand for freshwater due to the growing world population has strongly increased the pressure on available resources of clean water, evidenced in geopolitical competition of water resources.[4,5] According to the World Health Organization (WHO) and UN-water, there are currently over 2.2 billion people lacking access to safely managed water.[6] In many areas, arid climate is combined with economic water scarcity and lack of infrastructure to extract, manage, and distribute water In such locations, demands on technical water purification methods are high. The energy needed for the purification process in photocatalytic water-cleaning technologies can be provided by sunlight, enabling off-grid solutions
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