**2. Materials and Methods**

#### *2.1. Materials*

## 2.1.1. Materials for Comparison with Copper Nitride

Copper(I) oxide (powder; particle size, ≤7 μm) and copper(II) oxide (nanopowder; particle size, <50 nm, as assessed by transmission electron microscopy) were purchased from Sigma-Aldrich (Tokyo, Japan). Copper nanoparticles (particle size, <100 nm) were obtained from a copper nanoparticle paste (Daiken Chemical, Osaka, Japan) by washing with acetone and then centrifuging three times.

## 2.1.2. Materials for Preparation of Copper Nitride Nanoparticles

Copper acetate monohydrate (special grade) and 1-nonanol (special grade) were purchased from Kanto Chemical (Tokyo, Japan). Urea (special grade) and hexane (special grade) were purchased from Wako Pure Chemical Industries (Osaka, Japan). These reagents were used without further purification.

#### 2.1.3. Materials for Preparation of Vehicles for Liquid Inks

Ethanol (special grade) and ethylene glycol (special grade) were purchase from Wako Pure Chemical Industries (Osaka, Japan) and used without further purification.
