Solar hydrogen is known to be a promising energy source to substitute the conventional fossil fuels, as it is sustainable and environmentally friendly. Solar hydrogen can be produced in a number of ways. Photoelectrochemical (PEC) water splitting is one of the most attractive methods for converting solar energy into H2 fuel. A substantial research is being performed in PEC water splitting to improve the materials solar conversion efficiency. In this thesis, Cu2O was investigated for use as photocathode in PEC water splitting for solar hydrogen production. Two major bottlenecks of the application of cuprous oxide to water splitting are its poor stability in aqueous solution and its low photocatalytic efficiency caused by fast electron−hole recombination. The mismatch of the electron diffusion length (20–100 nm) with the light absorption depth near the band gap (~10 μm) also limits the application of Cu2O photoelectrode. The existing literatures dealing with these issues are highly limited by the complexity of the electrode fabrication, low performance, cost of noble metal (Pt) and the availability of rare earth oxides or sulfides as a cocatalyst. Therefore, this thesis focuses on nanostructuring Cu2O, thorough characterization and designing solutions to enhance the photostability and photocurrent density of Cu2O for solar hydrogen production.
In the first part of this study, we fabricate a one dimensional (1D) Cu2O straddled with graphene with simple and facile electrodeposition and dip coating, followed by thermal treatment methods. We found that the Cu2O nanowire arrays modified with optimized concentration of graphene lead to significantly enhanced photocurrent density 4.8 mA cm–2, (which is two times that of bare 1D Cu2O, 2.3 mA cm–2), at 0 V vs RHE under AM 1.5 illumination (100 mW cm–2) and a fivefold increased in photostability at the end of 20 minutes measurement (83%). Our detailed characterizations of the photocathode’s after PEC measurement revealed that graphene played a role as a protection layer and is suitable for high-performance and long-term application for PEC water splitting. That is, the high PEC performance of graphene/Cu2O nanocomposite is attributed to the improved crystallinity and the synergetic effect of graphene in absorbing the visible light, suppressing the charge recombination and suppressing photocorrosion of the photoelectrode by preventing direct contact with the electrolyte.
In the second part of our work, we report the design, synthesis and characterization of a novel Cu2O/CuO heterojunction decorated with nickel cocatalyst as a highly efficient photocathode for solar hydrogen production. The heterojunction structure was shown and examined by X-ray absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman Spectroscopy and Tip-enhanced Raman spectroscopy (TERS). Due to the heterojunction synergistic effect, the Cu2O/CuO gave a remarkably improved photocurrent density (–2.1 mA/cm2), i.e. 3.1 times higher than a Cu2O photoelectrode. Additionally, the Cu2O/CuO heterojunction, when decorated with nickel cocatalyst, showed six-fold and two-fold increases in photocurrent density (–4.3 mA/cm2) respectively when compared to Cu2O and bare Cu2O/CuO at 0 V vs. RHE under AM 1.5 illumination (100 mW/cm2). Interestingly, the Ni decorated Cu2O/CuO photocathode showed an impressive solar conversion efficiency of 2.71% at –0.4 V vs. Pt, i.e. 467% higher compared to bare Cu2O/CuO. After 20 minutes of standard solar illumination, 87.7% initial photocurrent density was remained for the nickel decorated Cu2O/CuO heterojunction. This is more than 1.5 times that of bare Cu2O/CuO (53.6%), suggesting surface modification with Ni not only effectively promotes water splitting but also stabilizes the photoelectrode. The enhanced photoelectrochemical performance is attributable to the efficient charge transfer and protective role of Ni, the improved crystallinity and the synergistic effect of the heterojunction in light absorption and charge separation. This inexpensive photocathode with increased photocurrent density and photostability offers a higher promise for solar hydrogen production
In the final part of this dissertation, we further enhance photocurrent density and photostability of the Cu2O/CuO heterostructure by modifying the surface with CuS as a cocatalyst using a facile SILAR method. The interfacial reaction between CuS and Cu2O/CuO was shown by many experimental evidences, including Raman, XANES/EXFS and XPS. The optimized Cu2O/CuO/CuS photocathode provided remarkably enhanced photocurrent density of – 5.4 mA /cm2 (i.e. > 2.5 times than that of the bare Cu2O/CuO) at 0 V vs. RHE under standard AM 1.5 light illumination. Due to the bicatalytic effects in suppressing the electron- hole recombination, a further increase in photocurrent density, – 5.7 mA/cm2 was noticed after decorating the Cu2O/CuO surface with both CuS and Pt. To the best of our knowledge, this is the highest performance yet reported for a cocatalyst modified Cu2O/CuO photoelectrode for solar water splitting. More importantly, the Cu2O/CuO heterostructure modified with optimum CuS afforded an impressive solar conversion efficiency of ABPE% = 3.6% which is greater than fourfold increase compared with the bare Cu2O/CuO. The stability of the bare Cu2O/CuO photocathode showed about a 44% decrease in initial photocurrent density within 1 h, but the stability was significantly improved i.e. 85% and 92% of the initial photocurrent is maintained after 1 h when the photocathode was modified with CuS and with both CuS and Pt respectively. This highly enhance photoeclrochemical property is due to the fast transfer of photogenerated electrons resulting suppressed electron-hole recombination and synergistic effects of heterojunction in light absorption and charge separation.

Solar hydrogen is known to be a promising energy source to substitute the conventional fossil fuels, as it is sustainable and environmentally friendly. Solar hydrogen can be produced in a number of ways. Photoelectrochemical (PEC) water splitting is one of the most attractive methods for converting solar energy into H2 fuel. A substantial research is being performed in PEC water splitting to improve the materials solar conversion efficiency. In this thesis, Cu2O was investigated for use as photocathode in PEC water splitting for solar hydrogen production. Two major bottlenecks of the application of cuprous oxide to water splitting are its poor stability in aqueous solution and its low photocatalytic efficiency caused by fast electron−hole recombination. The mismatch of the electron diffusion length (20–100 nm) with the light absorption depth near the band gap (~10 μm) also limits the application of Cu2O photoelectrode. The existing literatures dealing with these issues are highly limited by the complexity of the electrode fabrication, low performance, cost of noble metal (Pt) and the availability of rare earth oxides or sulfides as a cocatalyst. Therefore, this thesis focuses on nanostructuring Cu2O, thorough characterization and designing solutions to enhance the photostability and photocurrent density of Cu2O for solar hydrogen production.
In the first part of this study, we fabricate a one dimensional (1D) Cu2O straddled with graphene with simple and facile electrodeposition and dip coating, followed by thermal treatment methods. We found that the Cu2O nanowire arrays modified with optimized concentration of graphene lead to significantly enhanced photocurrent density 4.8 mA cm–2, (which is two times that of bare 1D Cu2O, 2.3 mA cm–2), at 0 V vs RHE under AM 1.5 illumination (100 mW cm–2) and a fivefold increased in photostability at the end of 20 minutes measurement (83%). Our detailed characterizations of the photocathode’s after PEC measurement revealed that graphene played a role as a protection layer and is suitable for high-performance and long-term application for PEC water splitting. That is, the high PEC performance of graphene/Cu2O nanocomposite is attributed to the improved crystallinity and the synergetic effect of graphene in absorbing the visible light, suppressing the charge recombination and suppressing photocorrosion of the photoelectrode by preventing direct contact with the electrolyte.
In the second part of our work, we report the design, synthesis and characterization of a novel Cu2O/CuO heterojunction decorated with nickel cocatalyst as a highly efficient photocathode for solar hydrogen production. The heterojunction structure was shown and examined by X-ray absorption spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman Spectroscopy and Tip-enhanced Raman spectroscopy (TERS). Due to the heterojunction synergistic effect, the Cu2O/CuO gave a remarkably improved photocurrent density (–2.1 mA/cm2), i.e. 3.1 times higher than a Cu2O photoelectrode. Additionally, the Cu2O/CuO heterojunction, when decorated with nickel cocatalyst, showed six-fold and two-fold increases in photocurrent density (–4.3 mA/cm2) respectively when compared to Cu2O and bare Cu2O/CuO at 0 V vs. RHE under AM 1.5 illumination (100 mW/cm2). Interestingly, the Ni decorated Cu2O/CuO photocathode showed an impressive solar conversion efficiency of 2.71% at –0.4 V vs. Pt, i.e. 467% higher compared to bare Cu2O/CuO. After 20 minutes of standard solar illumination, 87.7% initial photocurrent density was remained for the nickel decorated Cu2O/CuO heterojunction. This is more than 1.5 times that of bare Cu2O/CuO (53.6%), suggesting surface modification with Ni not only effectively promotes water splitting but also stabilizes the photoelectrode. The enhanced photoelectrochemical performance is attributable to the efficient charge transfer and protective role of Ni, the improved crystallinity and the synergistic effect of the heterojunction in light absorption and charge separation. This inexpensive photocathode with increased photocurrent density and photostability offers a higher promise for solar hydrogen production
In the final part of this dissertation, we further enhance photocurrent density and photostability of the Cu2O/CuO heterostructure by modifying the surface with CuS as a cocatalyst using a facile SILAR method. The interfacial reaction between CuS and Cu2O/CuO was shown by many experimental evidences, including Raman, XANES/EXFS and XPS. The optimized Cu2O/CuO/CuS photocathode provided remarkably enhanced photocurrent density of – 5.4 mA /cm2 (i.e. > 2.5 times than that of the bare Cu2O/CuO) at 0 V vs. RHE under standard AM 1.5 light illumination. Due to the bicatalytic effects in suppressing the electron- hole recombination, a further increase in photocurrent density, – 5.7 mA/cm2 was noticed after decorating the Cu2O/CuO surface with both CuS and Pt. To the best of our knowledge, this is the highest performance yet reported for a cocatalyst modified Cu2O/CuO photoelectrode for solar water splitting. More importantly, the Cu2O/CuO heterostructure modified with optimum CuS afforded an impressive solar conversion efficiency of ABPE% = 3.6% which is greater than fourfold increase compared with the bare Cu2O/CuO. The stability of the bare Cu2O/CuO photocathode showed about a 44% decrease in initial photocurrent density within 1 h, but the stability was significantly improved i.e. 85% and 92% of the initial photocurrent is maintained after 1 h when the photocathode was modified with CuS and with both CuS and Pt respectively. This highly enhance photoeclrochemical property is due to the fast transfer of photogenerated electrons resulting suppressed electron-hole recombination and synergistic effects of heterojunction in light absorption and charge separation.

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