One-step direct fabrication of phase-pure Cu2O films via the spin-spray technique using a mixed alkaline solution

The spin-spray technique was used to fabricate phase-pure Cu2O films at 70 °C with a high deposition rate of 0.3 μm/min. The source solution, containing a mixture of CuSO4･5H2O and l-ascorbic acid, and the reaction solution, containing NH3 and NaOH, were sprayed on the rotating substrates. It was possible to control the grain size, the crystalline orientation of the films, the surface morphology, and the optical band gap by adjusting the concentrations of NH3 and NaOH in the reaction solution. The films fabricated with a low NH3 aq. concentration were composed of nanosized grains and highly [111] oriented. On the other hand, films fabricated with a high NH3 aq. concentration were composed of submicron grains and highly [100] oriented. The surface morphologies of the films varied according to the concentration of NaOH. The surface of the films fabricated with a low NaOH concentration were almost flat and had a {100} orientation on their surface. On the other hand, films fabricated with a high NaOH concentration were composed of pyramidal grains. A smaller grain size in the films corresponded with a higher optical band gap. [Display omitted] •The spin-spray technique enabled the fabrication of phase-pure Cu2O films at 70 °C.•Cu2O films were fabricated with a high deposition rate of 0.3 μm/min.•The fabricated Cu2O films had strong adhesion to the substrates.•It was possible to control the morphology of Cu2O films via solution conditions.•The smaller grain size in the films corresponded to a higher optical band gap. A novel solution processing technique referred to as the “spin-spray technique” enabled the fabrication of phase-pure Cu2O films at 70 °C with a high deposition rate of 0.3 μm/min on glass substrates. In this technique, two solutions are sprayed on to the substrates placed on a rotating disk heated to a temperature below 100 °C. Two kinds of mixed aqueous solutions were used in this study: one was an acid aqueous solution of CuSO4･5H2O and l-ascorbic acid, referred to as the source solution; the second was a strongly alkaline aqueous solution of NaOH and NH3, referred to as the reaction solution. Because Cu+ is less stable than Cu2+ in aqueous solutions, the preparation of phase-pure Cu2O is very difficult via solution processing. Since l-ascorbic acid can efficiently reduce copper ions in the acidic source solution, unstable Cu+ could be supplied to the reaction site stably. Furthermore, the sprayed reaction solution provided an alkaline environme