1. Introduction

The use of photoelectrochemcial (PEC) water splitting to harvest intermittent solar sources in the form of hydrogen is an attractive potential method to address the energy and environmental issues. Since Honda and Fujishima in 1972 demonstrated titanium dioxide (TiO₂) for PEC water splitting [1], extensive efforts have been devoted to the development of photoelectrodes with good stability and high solar-to-hydrogen efficiency. Metal oxides (e.g., TiO₂, Fe₂O₃, BiVO₄, and SrTiO₂) have been extensively studied but their wide band gap and sluggish charge transfer kineties typically limited their solar-to-hydrogen conversion efficiency [1–4]. III-V semiconductor materials have attracted for PEC water splitting due to their high-efficiency, optimal band gap, and excellent optical properties but are readily susceptible to corrosion in strongly acidic or basic aqueous solutions during PEC process [5–11]. Of III-V semiconductor materials, gallium phosphide (GaP) has an appropriate band gap of 2.26 eV for harvesting a large portion of solar energy and has a valence-band edge more negative than the water oxidation potential [5,12]. Although GaP-based photocathodes has been extensively studied [9,10,13,14], only relatively few studies demonstrated water oxidation using GaP photoanodes due to the challenges presented, which also include a requirement for high overpotential due to slow hole-transfer kinetics [5]. Recently, amorphous TiO₂ has been widely used to protect III-V photoelectrodes in combination with OER catalysts during water oxidation. For example, a leaky TiO₂ (~143 nm) coupled with Ni catalyst to stabilize the GaP photoanode and was achieved up to 6 h stability in 1M KOH [5]. In contrast, the earth-abundant CoO_x is effective for both functions, enhancing the oxygen evolution and stabilizing photoanodes, because it possesses efficient hole-conductivity and high resistance to photocorrosion under highly alkaline media [15]. Many studies have demonstrated highly active CoO_x as a co-catalyst for photoanodes. For instance, a silicon photoanode coated with ultra-thin CoO_x via atomic layer deposition (ALD) exhibited high stability for 24 h in 1 M NaOH [16], and a BiVO₄ photoanode coated with 2 nm CoO_x has shown stable photocurrent density in KOH solution [17]. By using amorphous TiO₂ as a protection layer and a layer made of highly active oxygen evolution reaction (OER) catalysts could further improve the stability of photoelectrodes under extreme pH conditions. Photocarriers is transferred to the solution via mid-gap states of a leaky amorphous TiO₂ layer and facilitated by the Cobalt Oxide (CoO_x) co-catalyst for water oxidation [18]. In this work, a 20 nm amorphous TiO₂ thin film was deposited as protection layer followed by ~2 nm cobalt oxide (CoO_x) as a co-catalyst both deposited $via$ ALD on GaP surface. The GaP/TiO₂/CoO_x photoanode exhibited good stability for 24 hours in 1 M NaOH (pH 13.7) electrolyte under simulated one sun illumination. The GaP/TiO₂/CoO_x photoanode provides increased photocurrent density and improved onset potential compared to the reference GaP photoanode, ascribed to the high catalytic activity of cobalt oxide which is in agreement with previous reports [16,17,19].

2. Experiments

A GaP photoanode modified CoO_x was fabricated as follows: firstly, a single-crystalline n-GaP substrate was immersed in HCl solution (50% v/v) for 45 s, and then washed with deionized water followed by drying with N₂ gas. The dried sample was then transferred to an ALD chamber. An amorphous layer of TiO₂ with ~20 nm thickness was deposited directly onto the GaP electrode at 150 °C using a home-built ALD reactor by using titanium isopropoxide (TTIP) and water as metal and oxygen precursors respectively [20]. TTIP was kept at room temperature (25 °C) while water was kept at 5 °C. The deposition temperature was maintained at 150 °C. The growth rate of ALD process was approximately 0.4 Å/cycle. Following this, ALD of CoO_x layer (∼2 nm) was deposited onto the TiO₂ layer at 250 °C [21,22]. X-ray Photoelectron Spectroscopy (XPS) was performed on a Thermo monochromated aluminium K-alfa photoelectron spectrometer, using monochromic Al-Kα radiation (1486.7 eV). Low resolution survey scans were measured between 0 and 1300 eV and high-resolution scans were used to measure O 1s, Ti 2p and Co 2p spectra to determine oxidation and elemental concentration. All spectra were calibrated to adventitious carbon at 284.5 eV and data was analyzed using CasaXPS software. To produce simulated sunlight, a 200W Xe arc lamp was used as a light source with AM 1.5 G filter. A calibrated silicon solar cell was used to calibrate the illumination intensity. The PEC performances were conducted in a three-electrode system using the photoanode as a working electrode, Ag/AgCl reference electrode and platinum coil as a counter electrode in 1 M NaOH solution (pH 13.7). The linear sweep voltammetry measurements were conducted by using a potentiostat (Ivium CompactStat). The incident-photon-to-current conversion efficiency (IPCE) and the applied bias photon-to-current efficiency (ABPE) was measured using a published procedure (19).

The AFM images were acquired with a Veeco Dimension V Scanning Probe Microscope with tapping mode at atmospheric pressure with a Si cantilever with 10 nm of radius. Gas evolution measurements were conducted in a leak-tight glass reactor with a quartz window, in a three-electrode PEC cell configuration where the GaP/TiO₂/CoO_x electrode acted as the photoanode with a Pt coil counter electrode and a $\text{Ag}/\text{AgCl}$ reference electrode in 1 M NaOH (pH 13.7) electrolyte. The reactor was connected to a high purity nitrogen gas circulation system to purge the reactor before illumination. The oxygen gas produced during the water splitting experiment was collected and quantified using SRI 310c gas chromatograph (GC) with a molecular sieve (5A) column and helium as the carrier gas.

3. Results and discussion

A schematic illustration of the GaP electrode modified by amorphous TiO₂ and cobalt oxide CoO_x are shown in Fig. 1(a). XPS measurements on the GaP photoanode was used to confirm the deposition of TiO₂ and CoO_x films, as shown in Figs. 1(b)-1(c). Figure 1(b) shows the Ti 2p ionisation with the Ti 2p_3/2 peak centered at 458.1 eV, corresponding to TiO₂ [23]. The Co 2p_3/2 spectrum was deconvoluted to five fitting peaks using the parameters given by Biesinger et al [24]. The principle peak at 780.0 eV matches well with that expected for a Co₃O₄/CoO layer [24]. It should also be noted that no Ga and no P were observable, suggesting the material is pinhole free.

 

Fig. 1 (a) Schematic illustration of the GaP photoanode modified by amorphous TiO₂ and cobalt oxide CoO_x. 1(b) XPS spectra showing the Ti 2p and Co 2p spectra measured on the surface of the GaP/TiO₂/CoO_x photoanode pre-PEC analysis.

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Atomic Force Microscopy (AFM) has been used to assess the surface morphology for the GaP/TiO₂/CoO_x photoanode. Figures 2(a)-2(c) shows AFM images of a planar view of the GaP/TiO₂/CoO_x photoanode surface and a 3D representation of the surface morphology. This measurement shows that the top CoO_x layer is a smooth thin film. The root-mean-square (rms) surface roughness of the GaP/TiO₂/CoO_x photoanode has been measured at 0.2 nm confirming high quality of the top layer.

 

Fig. 2 Atomic force microscopy images morphology for the photoanode surface. Figures 2(a) and 2(b) Surface morphology of GaP-TiO₂/CoO_x photoanode. 2(c) 3D surface morphology of GaP-TiO₂/CoO_x. The z-scale is 2 nm and 784.5 pm for the AFM images in 2(a) and 2(b) respectively.

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The current–voltage ($J–V$) characteristics for both GaP/TiO₂/CoO_x and bare GaP photoanodes are shown in Fig. 3(a). The bare GaP photoanode here was used a reference and exhibited an onset potential of −1 V and a photocurrent density of 0.19 mA/cm² at 0.5 V versus reference electrode ($\text{Ag}/\text{AgCl}$). By comparison, GaP/TiO₂/CoO_x photoelectrode shows a great improvement in onset potential and photocurrent density, with the photocurrent was increased to around 0.788 mA/cm² at 0.5 V versus reference electrode $\text{Ag}/\text{AgCl}$ in 1 M NaOH solution (pH 13.7) and the onset potential observed to be approximately −1.12 V. The improvement in photocurrent and onset potential could be attributed to enhance the charge transfer to semiconductor/electrolyte interface and the cobalt oxide activities. The PEC performance of the GaP photoanodes with respect to the applied potential was examined; the applied bias photon-to-current efficiency (ABPE) are shown in Fig. 3(b), with the peak ABPE values, measured at 0.5 V, for bare GaP and GaP/TiO₂/CoO_x photoanodes being 0.14% and 0.57% versus reference electrode ($\text{Ag}/\text{AgCl}$), respectively. The higher ABPE value for the GaP/TiO₂/CoO_x photoanode indicates the enhancement of hole collection and water oxidation likely due to the CoO_x-based catalysts. Similar to NiO_x, CoO_x is a highly active OER catalyst in a strong alkaline solution due to its relatively high CB edge and hence excellent hole-transfer property [15]. Moreover, the Ni and Co OER catalysts can result in oxyhydroxides on their surface, e.g. NiOOH and CoOOH, which can reduce the overpotential for OER activity [25,26].

 

Fig. 3 Photoelectrochemical properties of GaP photoanodes. 3(a) The current-potential curves under AM 1.5G-simulated sunlight using three-electrode configuration in 1 M NaOH electrolyte (pH 13.7). 3(b) Applied bias photon-to-current efficiency (ABPE) for GaP photoanodes.

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To further investigate the wavelength-dependent photoelectrochemical contribution of GaP photoanode to the photocurrent, the incident-photon-to-current conversion efficiency (IPCE) was measured. As shown in Fig. 4(a), the maximum IPCE achieved at wavelength 400 nm for the GaP/TiO₂/CoO_x photoanode was 44.6%, which is higher than the value of the bare GaP photoanode 34.2%. Moreover, the IPCE up to 36% between (350-450 nm) for the GaP/TiO₂/CoO_x photoanode whereas the IPCE value of the bare GaP photoanode was 28.3% which indicates the improved efficiencies of carrier separation and collection while using TiO₂ protection layer and CoO_x as a co-catalyst. The IPCE value decreases below 26.7% for GaP/TiO₂/CoO_x and 19.8% for bare GaP at the wavelength of 450 nm and further decreases towards longer wavelengths (> 500 nm). Marginal photoresponse was measured for the wavelength range from 500 to 550 nm while the absorption spectrum of the GaP/TiO₂/CoO_x extended to 550 nm. This is attributed to the indirect nature of GaP bandgap [9,13,27].

 

Fig. 4 (a) Incident photon-to-current conversion efficiency (IPCE) of GaP/TiO₂/CoO_x photoanode in 1M NaOH electrolyte at 1 V versus Ag/AgCI. 4(b) Gas chromatography of the oxygen evolved from GaP/TiO₂/CoO_x photoanode. The photoanode was biased at zero V versus reference electrode Ag/AgCl under AM 1.5G-simulated sunlight in 1 M NaOH solution.

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Oxygen production was evaluated for the GaP/TiO₂/CoO_x photoanode at zero V vs a reference electrode ($\text{Ag}/\text{AgCl}$) under AM 1.5G-simulated sunlight as shown in Fig. 4(b). The amount of gas produced during approximately 2 hours was collected and analysed using a gas chromatography system. The GaP/TiO₂/CoO_x photoanode showed increased oxygen evolution with time. An amount of approximately 0.15 mmol/cm² of oxygen was produced after 2 hours of continuous irradiation, indicating an efficient water oxidation process. The advantage of the CoO_x-based catalyst can be clearly seen from the improved PEC water splitting stability and the improved $J–V$ curve of the the GaP/TiO₂/CoO_x photoanode.

The stability of the photoanode was studied by performing current density versus time ($J–t$) measurements. The photocurrent density of the GaP/TiO₂/CoO_x photoanode was evaluated for 24 h using chronoamperometry at a fixed potential of zero bias V vs ($\text{Ag}/\text{AgCl}$) in 1 M NaOH solution. As shown in Fig. 5, the photocurrent is stable for 24 hours under continuous illumination for GaP/TiO₂/CoO_x photoanode with marginal degradation. Clearly, the cobalt oxide catalyst along with the TiO₂ layer shows good stability because the TiO₂ layer has excellent hole-conducting and is chemical resistant under strongly basic electrolyte while cobalt oxides effectively transfer holes for water oxidation [15,28].

 

Fig. 5 Stability and Chemical properties of GaP/TiO₂/CoO_x photoanodes. 5(a) Steady-state photocurrent of the GaP photoanode held at zero V versus reference electrode (Ag/AgCl) under simulated one sunlight. 5(b) XPS spectra showing the Ti 2p (b) and Co 2p. 5(c) transitions from the surface of the photoanode after the reliability test for 24 hours.

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XPS analysis of the GaP/TiO₂/CoO_x photoanodes was used to probe the surface chemistry after the long stability test as shown in Figs. 5(b)-5(c). Before PEC analysis, only peaks for Co, and Ti were observed on the surface as shown in Figs. 1(b)-1(c). After PEC, the intensity of the Co 2p peaks are reduced (as evidenced by the low signal to noise ratio) but still visible therefore indicating the presence of the CoO_x layer even after exposure to the harsh electrolyte. Again, similar to CoO_x, the TiO₂ layer was still intact after 24 hours of exposure to the strongly basic solution as a Ti 2p signal was still seen with the Ti2p_3/2 peak at 457.9 eV, matching Ti in the 4 + oxidation state. Despite the CoO_x and TiO₂ layers being present, a spectrum for P 2p was measured, this was most likely due to possible diffusion of P through the TiO₂ and CoO_x layers or formation of pinholes after PEC experiment.

4. Conclusion

In conclusion, we have deposited a TiO₂-protection layer along with an ultra-thin layer of cobalt oxides as an OER catalyst on a GaP photoanode by ALD. The TiO₂/CoO_x improved the activity and increased the stability of the GaP photoanode for 24 h in 1 M NaOH (pH 13.7) solution under the AM 1.5G simulated solar light irradiation. Moreover, the GaP/TiO₂/CoO_x photoanode provides increased photocurrent density, improved onset potential, and incident-photon-to-current conversion efficiency (IPCE) compared to a bare GaP photoanode due to the high catalytic activity and corrosion resistant of cobalt oxide.