Figure 1. Solutions at 0.04M of copper nitrate (A) without reducing agent, (B) with glucose at 80 mM , (C-E) with diethanolamine (DEA) at different concentrations (250, 350 and 450 mM) and (F-H) with L-ascorbic (AA) acid at different concentrations (60, 80, 100 mM).
Figure 1. Solutions at 0.04M of copper nitrate (A) without reducing agent, (B) with glucose at 80 mM , (C-E) with diethanolamine (DEA) at different concentrations (250, 350 and 450 mM) and (F-H) with L-ascorbic (AA) acid at different concentrations (60, 80, 100 mM).
Figure 2. Scanning electron micrograph of AA-based (deposited at 290 C) Cu2O thin film at x 25k. Inset: AA-based thin films on 25 mm x 25 mm glass substrate.
Figure 2. Scanning electron micrograph of AA-based (deposited at 290 C) Cu2O thin film at x 25k. Inset: AA-based thin films on 25 mm x 25 mm glass substrate.

The abundance, cheapness, and nontoxicity of cuprous oxide (Cu2O) makes this p-type semiconductor of great interest for sensors, transistors, photocatalysts, and photovoltaics. Thin films of Cu2O can be prepared in various ways including high/low temperature oxidation, sputtering, electrochemical and chemical vapor deposition, and spray pyrolysis. The latter is a remarkably versatile, simple and low-cost technique. However, spray pyrolysis of Cu2O films relies on a precursor solution containing D-glucose as the reducing agent, which can adversely affect the electrical properties of the final thin film. Now researchers from CINVESTAV-Unidad Querétaro in Mexico and Northwestern University have demonstrated that novel reducing agents in the precursor solution – diethanolamine (DEA) and L-ascorbic acid (AA) – produce better results [Ugalde-Reygadas et al., Materials Today Communications 32 (2022) 103999, https://doi.org/10.1016/j.mtcomm.2022.103999].

Spray pyrolysis involves the ejection of a precursor solution, which contains salts of the material to be deposited, through a nozzle onto a heated substrate. The technique is highly versatile because different substrate shapes and large areas can be easily accommodated.

“[However], during deposition, glucose undergoes temperature degradation and the remaining subproducts can diminish the resulting film’s electrical properties,” explains Rebeca Castanedo-Pérez, who led the effort with Gerardo Torres-Delgado, both of CINVESTAV-Unidad Querétaro. “[Thin] films obtained in this [way] would not be a proper fit for electronic or photovoltaic applications.”

In contrast, DEA and AA, together with Cu(NO3)2 as the copper source, make more suitable precursors for spray pyrolysis because of their highly reductant functional groups, high solubility, and low decomposition temperature (Fig. 1). The two compounds, DEA and AA, form complexes with metal ions that promote reduction reactions in the precursor solution. This lowers the thermal energy required to reduce the precursor species in solution. So not only do the novel precursors enable the synthesis of Cu2O thin films with no or greatly reduced organic impurities but also at lower temperatures.

“Cu2O films are obtained without secondary phases or organic content,” points out Castanedo-Pérez. “The low deposition temperature also allows deposition [of Cu2O films] on some flexible substrates.”

The researchers believe that thin films of Cu2O prepared using AA are good candidates for the active layer in transparent optoelectronic devices (Fig. 2). Using DEA as the reducing agent, by contrast, produces Cu2O thin films suitable for photocatalysts and different types of sensor.

“Our results could place spray pyrolysis as a favored prospect for industrial production, which in contrast to sputtering [is] cheaper and more versatile,” says Castanedo-Pérez.

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