1 Introduction

Photoelectrochemical (PEC) water splitting, offers the ability to convert solar energy into chemical energy by producing O₂ and H₂ gases^[1,2]. BiVO₄ has become a promising n-type semiconductors for PEC water splitting since 1999 when it was reported by Kudo and coworkers as a photocatalyst for water oxidation^[3,4]. Monoclinic scheelite BiVO₄ has high light absorption (bandgap, Eg=2.4 eV), ample abundance, low toxicity, and appropriate valence band position for O₂ evolution^[3], but it suffers from poor electron transfer and collection in the photoelectrode, and sluggish water oxidation kinetics^[3-5]. Efforts having been made to alleviate these restrictions include doping with nitrogen^[6], introducing catalysts such as Co-Pi^[7], p-type Co₃O₄^[8], NiOOH^[9], and constructing nanostructures or heterojunctions^[10-12]. The use of passivation layers has been an effective way to improve charge separation and transfer at semiconductor-liquid interfaces^[13], for example, using Al₂O₃^[14] or TiO₂^[15,16] and other nanomaterials^[17-19] to passivate hematite, and NiO or CoO_x to passivate BiVO₄^[20,21]. We developed a new method to passivate BiVO₄, in which atomic layer deposition (ALD) was utilized to deposit an ultrathin Al₂O₃ overlayer on BiVO₄ photoanodes, which perform much better in PEC water splitting.

2 Experimental

3 Results and Discussion

The BiVO₄ photoanodes were prepared by dropping mixed precursor on FTO substrates, BiVO₄ nanoparticles were formed after crystallization. Ultrathin Al₂O₃ films were coated on BiVO₄ photoanodes via ALD process (Fig. 1a). SEM images in Figs. 1(b) and 1(c) show similar morphologies of BiVO₄ photoanodes before and after ALD process (5 cycles), indicating that ALD coating Al₂O₃ had little effect on the morphology of BiVO₄ photoanode., TEM images in Figs. 1(d) and 1(e) show that the BiVO₄ and BiVO₄·Al₂O₃ particles have an interlunar spacing of 0.309 nm, being well consistent with the JCPDS 75-2480, and the pristine and coated nanostructures do not differ from each other. According to Ref.[14], the 5-cycle Al₂O₃ is 0.75 nm thick (by spectroscopic ellipsometry on Si), and that is probably why the ALD treatment has little influence on the morphology and nanostructure of BiVO₄.

Fig. 1.Full-screen View

Schematic illustration of prepairing Al₂O₃-coated BiVO₄ particles (a), top-view SEM images of the pristine and Al₂O₃-coated BiVO₄ photoanode (5 cycles) (b & c), andTEM images of the pristine and Al₂O₃-coated BiVO₄ (5 cycles) (d & e).

The electronic structure of BiVO₄ was checked by XPS (Figs. 2a–2c). The pure BiVO₄ sample exhibited typical spin-orbit split of Bi 4f_5/2 and Bi 4f_7/2 signals, and V 2p_1/2 and V 2p_3/2 signals, in agreement with the literature^[22,23]. This confirms that the BiVO₄ sample prepared was identically the monoclinic scheelite BiVO₄. For the Al₂O₃-coated BiVO₄ sample, as shown in Fig. 2(d), Al was detected, and a significant shift of O 1s peak can be seen in Fig. 2(e), indicating two crystal structures in the sample. All these were consistent with the fact that the Al₂O₃ layer had covered the samples through ALD deposition.

Fig. 2.Full-screen View

X-ray photoelectron spectra of the pristine BiVO₄ sample (a–c) and the Al₂O₃-coated BiVO₄ sample: (d and e).

PEC performance of the BiVO₄ photoanodes before and after the ALD process was investigated. As shown in Fig. 3(a), the photocurrent densities of Al₂O₃-coated BiVO₄ photoanodes were significantly higher than the pristine. In particular, the photocurrent density of BiVO₄·Al₂O₃ photoanode by 5 ALD cycles reached over 1.0 mA/cm² at 1.23 V vs. RHE under the standard illumination of AM 1.5 G (100 mW/cm²), which was twice of the pristine one’s. It confirms that Al₂O₃ overlayer deposition can strongly enhance the PEC performance of BiVO₄ photoanodes. Yet, when by further increasing the ALD cycles to 10 ALD cycles, the photocurrent density decreased, partly because of the large increased resistance in the Al₂O₃ overlayer. In fact, the Al₂O₃-coating was optimized at 5 ALD cycles..

Fig. 3.Full-screen View

Photocurrent densities of Al₂O₃-coated BiVO₄ photoanodes under simulated solar illumination (a) and, light chopping photocurrent densities at 0.5 (b) and 0.9 (c) V vs. RHE, for BiVO₄ photoanodes before and after 5-cycle Al₂O₃ deposition.

Photocurrent transient measurement was performed to assess the charge recombination at the sample/electrolyte interface. The measurement was taken under curtain bias potential, and the time-resolved photocurrent was recorded while the illumination was turned on and off, with the BiVO₄·Al₂O₃ sample of 5 ALD cycles. As shown in Figs. 3(b) and 3(c), the BiVO₄·Al₂O₃ sample at 0.5 and 0.9 V vs. RHE has much smaller current spikes than those of the pristine BiVO₄ photoanode. According to Ref.[14], the appearance of sharp spike is mainly because that photogenerated holes accumulate at the interface or oxidize the trap states on the semiconductor surface and in the bulk. A larger bias potential allows the holes to cross over the interface more easily to oxidize water. And the motion of holes reduces the anodic current. In our experiment, the anodic current spike at 0.5 V vs. RHE is much higher than that at 0.9 V vs. RHE. After the ALD treatment, the current spikes became smaller and much smoother. This suggests that the charge recombination on the surface of BiVO₄ photoanode may be passivated after depositing the ultrathin Al₂O₃ overlayer.

To better support this hypothesis, photoluminescence (PL) of the Al₂O₃-coated BiVO₄ sample (5 cycles) was studied. PL spectroscopy is an effective technique to study separation efficiency of the photogenerated carriers. The higher PL intensity indicates the bigger probability of charge recombination^[24]. Fig. 4 shows the PL quantum yield of the Al₂O₃-coated BiVO₄ photoanode was smaller than the pristine one, indicating that the charge recombination in BiVO₄ photoanode was effectively inhibited after the Al₂O₃ coating ^[8]. Therefore, the ultrathin Al₂O₃ overlayer can effectively passivate the charge recombination on the surface of BiVO₄ photoanodes.

Fig. 4.Full-screen View

The PL emission spectra of BiVO₄ photoanode before and after Al₂O₃ deposition (5 cycles).

Also, electrochemical impedance spectroscopy (EIS) was performed with the samples covered with 5 ALD cycles Al₂O₃ and the control (without Al₂O₃ overlayer). Here we used an equivalent circuit consisted of 2 RC elements in series, accounting for semiconductor behavior and surface processes, as shown in Fig. 5(a)^[14,25].

Fig. 5.Full-screen View

Electronic equivalent circuit (a) signing the photoanode/electrolyte system in our EIS measurement before and after 5 ALD cycles of Al₂O₃,the space charge and charge transfer resistances (b & c),and the space charge and Helmholtz capacitances (d & e).

In Fig. 5(b), space charge resistances (R_SC) of the samples before and after ALD treatment are all stable around 10³ Ω, but a remarkable decrease in charge transfer resistances (R_CT) can be seen after ALD treatment (Fig. 5c). The R_CT decrease could lead to a reduction of overpotential, hence the increase of photocurrent we observed. Yet it will be much clearer to reveal the effect of Al₂O₃ layer when we look into the change of Helmholtz capacitance (C_H) or space charge capacitance (C_SC) after ALD treatment (Figs. 6d and 6e). The C_SC increased and C_H decreased as a result of Al₂O₃-coating. As capacitance is the charge over voltage, so the C_H decrease after the ALD treatment may be related to the change in voltage or charge distribution at the sample/electrolyte interface. In this experiment, the C_H decrease is due to the decreased free charge density in surface trap states because of the passivation of surface states. Another reason is that the relatively high dielectric of Al₂O₃ layer may improve the charge screening of anions in Helmholts layer^[14]. On the other hand, as shown in Fig. 5(d), after ALD treatment, the space charge capacitance (C_SC) increased significantly in the entire range of potential bias applied. According to Ref.[14], this can be explained as follows: in order to balance the applied voltage, charges are extract more from the space-charge layer than before due to the decrease of surface states, leading to charge increase in the space charge region^[14], i.e. the significant increase of C_SC. Therefore, we can conclude that the Al₂O₃ overlayer brought about a significant decrease in the charge density on the surface (surface states), and better performance of the BiVO₄ photoanode.

4 Conclusion

In summary, ALD coating ultrathin Al₂O₃ overlayer can effectively enhance the PEC water splitting performance of BiVO₄ photoanodes. Particularly, 5 ALD cycles Al₂O₃ deposition increased the photocurrent density of BiVO₄ photoanode from ~0.5 mA/cm² to ~1.0 mA/cm², at 1.23 V vs. RHE under the standard illumination (100 mW/cm²). In transient photocurrent measurement, the anodic current spikes of the sample became much smoother after depositing Al₂O₃ overlayer. In photoluminescence spectra, the photoluminescence quantum yield of the Al₂O₃ coated BiVO₄ photoanode was smaller than the pristine one. And in electrochemical impedance measurement, the charge transfer resistance and capacitance of Al₂O₃ coated sample differed significantly from the pristine. All of these provide convincing evidence that Al₂O₃ overlayer successfully passivates the surface trap states of BiVO₄, thus inhibits charge recombination and benifits the performance of BiVO₄. Though the stability of Al₂O₃ coated BiVO₄ in electrolyte (PBS) was not as good as expected, it still provides a promising approach to passivate BiVO₄, and thus improve the performance of BiVO₄ for efficient photoelectrochemical water splitting.