Revealing the relationship between photoelectrochemical performance and interface hole trapping in CuBi2O4 heterojunction photoelectrodes†

p-Type CuBi2O4 is considered a promising metal oxide semiconductor for large-scale, economic solar water splitting due to the optimal band structure and low-cost fabrication. The main challenge in utilizing CuBi2O4 as a photoelectrode for water splitting, is that it must be protected from photo-corrosion in aqueous solutions, an inherent problem for Cu-based metal oxide photoelectrodes. In this work, several buffer layers (CdS, BiVO4, and Ga2O3) were tested between CuBi2O4 and conformal TiO2 as the protection layer. RuOx was used as the co-catalyst for hydrogen evolution. Factors that limit the photoelectrochemical performance of the CuBi2O4/TiO2/RuOx, CuBi2O4/CdS/TiO2/RuOx, CuBi2O4/BiVO4/TiO2/RuOx and CuBi2O4/Ga2O3/TiO2/RuOx heterojunction photoelectrodes were revealed by comparing photocurrents, band offsets, and directed charge transfer measured by modulated surface photovoltage spectroscopy. For CuBi2O4/Ga2O3/TiO2/RuOx photoelectrodes, barriers for charge transfer strongly limited the performance. In CuBi2O4/CdS/TiO2/RuOx, the absence of hole traps resulted in a relatively high photocurrent density and faradaic efficiency for hydrogen evolution despite the presence of pronounced deep defect states at the CuBi2O4/CdS interface. Hole trapping limited the performance moderately in CuBi2O4/BiVO4/TiO2/RuOx and strongly in CuBi2O4/TiO2/RuOx photoelectrodes. For the first time, our results show that hole trapping is a key factor that must be addressed to optimize the performance of CuBi2O4-based heterojunction photoelectrodes.


Fabrication of CuBi2O4 photocathodes
The typical synthesis procedure for a CuBi2O4 photocathode is based on our previous work. [1,2] Briefly, the Bi(NO3)3 precursor was first sprayed onto the FTO substrate at a deposition temperature of 450 °C to form a bismuth oxide layer. Then the Cu(NO3)2 precursor was sprayed successively on top of the bismuth oxide layer at 450 °C and a gradient self-doped CuBi2O4 thin film was formed by diffusion of copper into the film.
The thickness of the CuBi2O4 thin film was approximately 280 nm.

Deposition of CdS buffer layer
A CdS buffer layer was deposited on top of the CuBi2O4 film using chemical bath deposition (CBD).
In a typical synthesis procedure, a beaker containing 150 mL of stirred ultrapure water was heated inside a water bath. When the temperature of the solution reached 65 °C, 22 mL of 15 mM CdSO4 solution was added to the bath. Then 22 mL of NH4OH solution was added to the chemical bath followed by immersion of the CuBi2O4 films into the solution for 15 min. The CdS-coated CuBi2O4 films were then rinsed thoroughly with water and dried in an oven at 120 °C. The thickness of the CdS film was approximately 100 nm.

Deposition of BiVO4 buffer layer
BiVO4 buffer layers were prepared using spray pyrolysis. The precursor solution was made by dissolving 4 mM Bi(NO3)3·5H2O (98%, Alfa Aesar) in acetic acid (98%, Sigma-Aldrich) and adding an equimolar amount of vanadium in the form of VO(AcAc)2 (99%, Alfa Aesar) dissolved in absolute ethanol (Sigma-Aldrich). Each spray cycle consisted of 5 s of spray time and 55 s of delay time to allow for solvent evaporation, and a total of 100 cycles were used to deposit the BiVO4 films. More details can be found in previous reports. [3,4] The thickness of the resulting BiVO4 film was approximately 100 nm.

Deposition of Ga2O3 buffer layer
Ga2O3 was deposited by atomic layer deposition (ALD). Before being placed inside the ALD reaction chamber, the CuBi2O4 samples were rinsed thoroughly with deionized water and dried under a stream of N2. The deposition was carried out at 170 °C using sequential pulses of Tris (dimethylamido) gallium (III) 98% (precursor temperature: 130 °C), followed by a purge, O2-plasma treatment, and another purge. O2plasma treatments were done using an RF power of 2800 W with 40 sccm Ar and 100 sccm O2. The thickness of the CuBi2O4 thin film was approximately 280 nm. The thickness of amorphous Ga2O3 thin film was about 25 nm, which was determined from ellipsometric measurements on a piece of silicon witness wafer.

Deposition of TiO2 protection layer
TiO2 was deposited by ALD. Before deposition, the CuBi2O4 sample was rinsed thoroughly with deionized water and dried under a stream of N2 before placing it in the ALD reaction chamber. The deposition was carried out at 120 °C using sequential pulses of tetrakis (dimethylamino) titanium (precursor temperature: 85 °C) and H2O (precursor temperature: 25 °C). The thickness of amorphous TiO2 thin film was about 20 nm, which was determined from ellipsometric measurements on a piece of silicon witness wafer.

Deposition of Co-catalysts
The ruthenium oxide (RuOx) co-catalyst was photo-electrodeposited onto the CuBi2O4/Ga2O3/TiO2, CuBi2O4/BiVO4/TiO2, CuBi2O4/CdS/TiO2 and CuBi2O4/TiO2 samples from an aqueous solution of 1.17 mM KRuO4 in 25 mL deionized water, using a constant current of −0.03 mA/cm 2 for 10 min with constant illumination from the solar simulator (100 mW/cm 2 ). The photo-electrodeposition was carried out in three-electrode configuration with a platinum counter electrode and an Ag/AgCl electrode (saturated KCl) as the reference electrode.

Material Characterization
The morphology of the films was analyzed using a LEO GEMINI 1530 field emission scanning electron microscope (FESEM) operated at an acceleration voltage of 7 kV. Elemental analysis using X-ray photoelectron spectroscopy (XPS) was carried out with a monochromatic Al Kα X-ray source (1486.74 eV, Specs Focus 500 monochromator) and a hemispherical analyzer (Specs Phoibos 100) in an ultrahigh vacuum system (base pressure ~10 −8 mbar). Ultraviolet photoelectron spectroscopy (UPS) was conducted using a He I source (E = 21.21 eV) with the same hemispherical analyzer as in the XPS measurement. In order to remove possible surface contamination all films were cleaned using an oxygen plasma for 5 min prior to the measurement. The plasma was deployed using a radio frequency plasma generator (MANTIS(R)) with an oxygen partial pressure of 4 x 10 -5 mbar (gas purity 99.999 %) and a workload of 200 W.     ~1.6 [1] ~1.12 [1] 5.8 [1] 0.19 [1] CdS 2.4-2.5 [6,7] 4.1-4.3 [6] [10,11] 4.49-4.60 [12,13] [this work]