Nanoengineered Advanced Materials for Enabling Hydrogen Economy: Functionalized Graphene-Incorporated Cupric Oxide Catalyst for Efficient Solar Hydrogen Production

Abstract

Cupric oxide (CuO) is a promising candidate as a photocathode for visible-light-driven photo-electrochemical (PEC) water splitting. However, the stability of the CuO photocathode against photo-corrosion is crucial for developing CuO-based PEC cells. This study demonstrates a stable and efficient photocathode through the introduction of graphene into CuO film (CuO:G). The CuO:G composite electrodes are prepared using graphene-incorporated CuO sol-gel solution via spin-coating techniques. The graphene is modified with two different types of functional groups, such as amine (-NH₂) and carboxylic acid (-COOH). The -COOH-functionalized graphene incorporation into CuO photocathode exhibits better stability and also improves the photocurrent generation compare to control CuO electrode. In addition, -COOH-functionalized graphene reduces the conversion of CuO phase into cuprous oxide (Cu₂O) during photo-electrochemical reaction due to effective charge transfer and leads to a more stable photocathode. The reduction of CuO to Cu₂O phase is significantly lesser in CuO:G-COOH as compared to CuO and CuO:G-NH₂ photocathodes. The photocatalytic degradation of methylene blue (MB) by CuO, CuO:G-NH₂ and CuO:G-COOH is also investigated. By integrating CuO:G-COOH photocathode with a sol-gel-deposited TiO₂ protecting layer and Au-Pd nanostructure, stable and efficient photocathode are developed for solar hydrogen generation.

Keywords: Raman spectroscopy; photocatalytic degradation; photocorrosion stability; solar hydrogen.

Figure 1

Absorbance spectra of CuO and functionalized graphene incorporated CuO films after thermal treatment at 600 °C for 10 min in furnace.

Figure 2

a) Current–voltage characteristics of CuO and functionalized graphene incorporated CuO (CuO:G‐NH₂, CuO:G‐COOH) photocathode. b) Stability of CuO:G‐COOH significantly improved compared with bare CuO photocathode.

Figure 3

a) Current–voltage characteristics of CuO and –COOH‐functionalized graphene incorporated CuO (CuO:G‐COOH) photocathode. b) Stability of CuO:G‐COOH photocathode with different amount of graphene.

Figure 4

XPS spectra of Cu 2p core level spectra from a) CuO photocathode, b) CuO:G‐NH₂ photocathode, and c) CuO:G‐COOH photocathode.

Figure 5

XRD spectra of CuO (100 nm), CuO:G‐NH₂ (100 nm), and CuO:G‐COOH (100 nm) photocathode a) before PEC test and b) after PEC test. For CuO and CuOG‐NH₂, mixed phases of CuO and Cu₂O observed after PEC test. XRD spectra of CuO (500 nm) and CuO:G‐COOH (500 nm) photocathode c) before PEC test and d) after PEC test.

Figure 6

Surface morphology of spin‐coated a) CuO, b) CuO:G‐NH₂, and c) CuO:G‐COOH thin films before photocurrent measurement, respectively. Surface morphology of d) CuO, e) CuO:G‐NH₂, and f) CuO:G‐COOH thin films after photocurrent measurement, respectively. The formation of Cu₂O significantly reduced for CuO:G‐COOH photocathode.

Figure 7

Raman spectra recorded from the samples before and after PEC studies. a,b) Spectra with 488 nm visible Raman excitation. c,d) Spectra with 325 nm UV Raman excitation from three sets of samples (CuO, CuO:G‐NH₂, and CuO:G‐COOH).

Figure 8

Nyquist plot of CuO, CuO:G‐NH₂, and CuO:G‐COOH photocathode.

Figure 9

Comparison of MB. C/Co versus time of CuO, CuO:G‐NH₂, and CuO:G‐COOH.

Figure 10

Current–voltage characteristics of CuO and CuO:G‐COOH photocathode with a) TiO₂ surface protecting layer and b) TiO₂‐Au‐Pd nanostructure. Photocorrosion stability of c) CuO‐TiO₂ and (CuO:G‐COOH)‐TiO₂ photocathodes and d) CuO‐TiO₂ and (CuO:G‐COOH)‐TiO₂ photocathodes with Au‐Pd decoration.

Figure 11

Hydrogen evolution of CuO and (CuO:G‐COOH) photocathodes with a) TiO₂ passivation and b) TiO₂‐Au‐Pd nanostructure under standard light illumination.