Synthesis of copper sulfide (CuxS) films by one-step thermal reduction of copper formate-amine-sulfur complex pastes with low sulfur ratios

A one-step thermal-reduction method was used to synthesize copper sulfide (CuxS, x = 1-2) films using copper formate-sulfur-amine pastes with low sulfur ratios (sulfur/copper formate = 0.5-1.0). The synthesized films were mixtures of Cu + Cu2S, Cu2S + Cu1.8S, or Cu1.8S + CuS, depending on the sulfur ratio. Cu1.8S alone films were synthesized from the paste with a sulfur ratio of 0.75 at 200 degrees C. The films comprised single-crystal nanoparticles with a size of 10-20 nm, which were covered with excess sulfur and agglomerated to form pillar-like structures. The amount of excess sulfur increased with an increasing sulfur ratio in the paste. The formation of pillar-like structures was due to the individual nanoparticles being connected by excess sulfur and the evaporation of CO2, H2O, and amine during the thermal reduction of copper formate.