Three-Dimensional CuO Branched Nanowires for Efficient Photoelectrochemical Water Splitting
Authors: Lim Kwan Woo , Lee Jong-Lam 
Published: February 2020

1. Objective
Recently photoelectrochemical (PEC) water splitting has considered as one of the most promising method for clean and renewable energy. In an effort to find proper materials for water splitting, many metal oxides and semiconductors were studied in recent years. Among various materials, copper oxide (CuO) have attracted a lot of interest due to non-toxicity, abundance, facile, low cost, and scalable production. In particular, CuO could absorb a wide range of the solar light including near IR region due to its narrow band gap (Eg = 1.2 eV). However, its short diffusion length and low conductivity cause the recombination of photo-generated carriers. As a result, it could lead to decrease of PEC performance immediately. To prevent this problems in water splitting, several approaches have been developed including a three-dimensional (3D) branched nanowires (b-NWs) design [1], [2] or 3D networks structure design [3] or TiO2 coating on the NWs surfaces [4], or a TMD materials used as co-catalyst [5]. In this work, we report the fabrication of 3D CuO b-NWs for efficient water splitting. Highly dense b-NWs are grown on copper substrates and it used as photocathodes directly. Using these 3D CuO b-NWs, we demonstrated improved PEC performance compared to conventional CuO NWs.
2. Experiments
To improve the PEC performances, three-dimensional (3D) CuO branched nanowires (b-NWs) were fabricated by combining several solution-based process such as wet chemical oxidation method [6] and chemical precipitation methods [7]. Then, highly dense 3D CuO b-NWs were grown on copper mesh substrates and it used as photocathodes directly.
PEC performance was measured in a 3-electrode system under 100 mA/cm2 (1 sun) light intensity. Furthermore, Ag/AgCl electrode in saturated KCl was used as a reference electrode, Pt mesh was used as a counter electrode, and 0.25 M Na2SO4 at pH 5.68 was used as electrolytes. The current density-potential (J-V) characteristics of the electrodes were measured with a potentiostat (Iviumstat, Ivium Technologies) to apply to external potential and scan rate for measurement was 10 mV/s.
3. Results and Discussion
Fig. PEC performance of conventional CuO NWs and 3D CuO b-NWs photocathodes.
Diameters ranging from 50 to 500 nm and over 10 µm in lengths CuO NWs were observed. In particular, bending of CuO NWs were observed due to H2O molecule released during annealing process. The transformation of Cu(OH)2 nanowires to CuO nanowires as follows: Cu(OH)2 → CuO + H2O.

Highly dense branches were attached to CuO NWs backbone. Branches ranging from 100 to 500 nm in length and ranging from 10 to 200 nm in diameter 3D CuO b-NWs are observed.
Figure 2 shows PEC measurement, dark current and photo current could be observed simultaneously. The b-NWs provide a higher photocathodic current density compared to conventional CuO NWs due to effective charge transport and increased surface area. In particular, maximum current density 1.37 mA/cm2 is observed at ~0.05 V under 100 mW/cm2 (1 sun) illumination when 3D CuO b-NWs grown on copper gauze substrates. In case of conventional CuO NWs on copper gauze substrates, maximum current density 0.77 mA/cm2 was observed at ~0.05 V under 100 mW/cm2 (1 sun) illumination.
4. Conclusion
We fabricated the three-dimensional (3D) CuO branched nanowires (b-NWs) by simple process. This fabrication process required short growth time and low growth temperature. Copper mesh substrates with highly dense NWs were used as photocathode directly. In particular, the highest PEC performance showed when CuO b-NWs grown on copper gauze substrates. This report presents a facile, low cost, and scalable 3D CuO b-NWs fabrication route that is widely applicable to photo-electrochemical devices.
5. Reference
[1] A. Kargar, et al. ACS NANO. 2013, 7, 11112.
[2] A. Kargar, et al. ACS NANO. 2013, 7, 9407.
[3] A. Kargar, et al. Nanotechnology. 2014, 25, 205401.
[4] Savio J. A. et al. ChemCatChem. 2015, 7, 1659.
[5] Z. Yin. et al. Small. 2014, 10, 3537.
[6] Zhang. W, Adv. Mater. 2003, 15, 822.
[7] Q. Zhang, Progress in Materials Science. 2014, 60, 208.

Figure 1