Highly efficient hamburger-like nanostructure of a triadic Ag/Co₃O₄/BiVO₄ photoanode for enhanced photoelectrochemical water oxidation

Abstract

The combination of a semiconductor heterojunction and oxygen evolution cocatalyst (OEC) is an important strategy to improve photoelectrochemical (PEC) water oxidation. Herein, a novel hamburger-like nanostructure of a triadic photoanode composed of BiVO₄ nanobulks, Co₃O₄ nanosheets and Ag nanoparticles (NPs), that is, Ag/Co₃O₄/BiVO₄, was designed. In our study, an interlaced 2D ultrathin p-type Co₃O₄ OEC layer was introduced onto n-type BiVO₄ to form a p–n Co₃O₄/BiVO₄ heterojunction with an internal electric field (IEF) in order to facilitate charge transport. Then the modification with Ag NPs can significantly facilitate the separation and transport of photogenerated carriers through the surface plasma resonance (SPR) effect, inhibiting the electron–hole recombination. The resulting Ag/Co₃O₄/BiVO₄ photoanodes exhibit largely enhanced PEC water oxidation performance: the photocurrent density of the ternary photoanode reaches up to 1.84 mA cm⁻² at 1.23 V vs. RHE, which is 4.60 times higher than that of the pristine BiVO₄ photoanode. The IPCE value is 2.83 times higher than that of the pristine BiVO₄ at 400 nm and the onset potential has a significant cathodic shift of 550 mV for the ternary well-constructed photoanode.

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1. Introduction

Photoelectrochemical (PEC) water splitting is of great significance to mitigate global environmental problems by converting abundant solar energy into chemical fuels in the form of H₂ or hydrocarbon compounds.¹ The photoanodes have low efficiency compared to the photocathodes since the water oxidation half-reaction occurring in the photoanode is recognized as a speed-limiting step in the overall water splitting reaction. Therefore, it is important to develop an efficient and practical anode system for construction of high-performance PEC cells.^2–4 BiVO₄ is a highly promising photoanode, which exhibits proper band gap (∼2.4 eV) for solar light absorption and favorable valence band very near the thermodynamic oxygen evolution potential.^5–7 However, poor charge transport, excessive electron–hole recombination, slow hole kinetics of oxygen evolution reaction and limited quantum efficiency are major challenges with the BiVO₄ photoanodes.^8,9 To overcome these challenges, various strategies such as elemental doping,^10,11 construction of heterojunctions^12,13 and coupling with various oxygen evolution cocatalysts (OECs)^14–16 have been employed to improve the photoactivity of BiVO₄.

Formation of BiVO₄-based nanostructured heterojunction photoanodes was recognized as an attractive route to improve PEC water splitting performances because it enlarges the interfacial area, promotes efficient charge separation, and improves the optical absorption.^17–19 Among the rest, staggered (type-II) band structure alignment and complementary solar light harvesting contribute to further improve the PEC property of BiVO₄. Especially, p-Co₃O₄ is not only expected to form a p–n junction with n-BiVO₄, but also acted as an effective OEC. Thus among the various BiVO₄-based heterojunction photoanodes, p–n Co₃O₄/BiVO₄ photoanode exhibits the synergetic enhancement of surface reaction kinetics and bulk charge separation. For instance, Wang et al. synthesized powdered composite of BiVO₄/Co₃O₄ with O₂ evolution of 11 mmol g⁻¹ h⁻¹, which is 17 times greater than for unmodified BiVO₄.²⁰ Hou et al. also reported Co₃O₄@C/BiVO₄ photoanode derived from MOF-template with the enhanced photocurrent density, which is approximately tenfold higher with respect to bare BiVO₄.²¹

Although it has been well recognized the surface catalytic effect of Co₃O₄, the modification of Co₃O₄ may introduce more surface recombination centres that hamper the PEC performance inversely, in consideration of the junction formed at the photoanode/OEC interface. In this work, firstly, BiVO₄ photoanodes with a nanoporous morphology were synthesized to reduce the travel distance of photogenerated holes, decreasing the chance of recombination. Secondly, p-Co₃O₄ OECs with interlaced nanosheets were dispersed on the surface of n-BiVO₄ for constructing heterojunctions,⁸ enhancing charge separation by an internal electric field (IEF) and accelerate the kinetics of oxygen evolution reaction by the active site of cobalt. Lastly, the small particle size and high dispersion of Ag NPs accelerated interfacial electrons transport due to the formation of Schottky barrier, effectively improving the charge separation and greatly reducing the recombination of photogenerated electron hole pairs. Thus, highly efficient hamburger-like nanostructure of triadic Ag/Co₃O₄/BiVO₄ photoanode was fabricated, as illustrated in Fig. 1. Considering that the loading amount would change the size or morphology of Co₃O₄ OECs, the effects of Co₃O₄ loading amount on PEC performance for Co₃O₄/BiVO₄ heterojunction photoanode are discussed in detail. A working mechanism was also proposed to understand the improved separation of the photogenerated carriers by energy band structure, and the accelerated kinetics of oxygen evolution reaction at the interface of ternary components. In conclusion, the novel hamburger-like nanostructure of triadic Ag/Co₃O₄/BiVO₄ photoanode designed in this work is proven to represent a highly effective pathway to improve the PEC conversion efficiency.

Fig. 1 Schematic diagram, modeling diagram and cross-sectional image of triadic Ag/Co₃O₄/BiVO₄ photoanode.

2. Experimental sections

2.1 Preparation of pristine BiVO₄ photoanode

The pristine BiVO₄ photoanode was prepared from a Bi metal precursor reported previously.¹⁹ In a typical synthesis, a Bi metal film was first electrodeposited in a three-electrode quartz cell system on the electrochemical workstation. Then the Bi metal film was converted into BiVO₄ after VO(acac)₂ DMSO solution were dipped onto the surface of the Bi metal film and heated at 450 °C for 2 h in air. Finally, the residual V₂O₅ was removed by 1 M NaOH solution. Thus, the BiVO₄ photoanode was obtained after washing and drying.

2.2 Preparation of Co₃O₄/BiVO₄ heterojunction photoanode

The Co₃O₄/BiVO₄ photoanode was fabricated after Co₃O₄ was loaded onto the surface of BiVO₄ photoanode by combing hydrothermal with heat treatment process. In order to investigate the effects of Co₃O₄ loading amount on PEC performance, the Co₃O₄/BiVO₄ photoanodes with different Co₃O₄ loading amount were prepared by adjusting the reactant concentration of Co precursor. Taking the optimal reactant concentration as an example, the BiVO₄/FTO substrates were transformed into Teflon-lined stainless steel autoclave containing 3 mM Co(NO₃)₂·6H₂O, 10 mM NH₄F and 25 mM urea. The autoclave was sealed and maintained at 120 °C for 5 h and then cooled down to room temperature. The obtained composite photoanode was then calcined at optimum 350 °C for 3 h, thus the Co₃O₄/BiVO₄ photoanode was prepared accordingly. Similarly, the Co₃O₄/BiVO₄ photoanodes with different Co₃O₄ loading amount were prepared according to the Co(NO₃)₂·6H₂O concentration of 1 mM, 2 mM, 3 mM and 4 mM, keeping the concentration of 10 mM NH₄F and 25 mM urea unchangeably.

2.3 Preparation of Ag/Co₃O₄/BiVO₄ triadic photoanode

The FTO substrate coated with Co₃O₄/BiVO₄ heterojunction was immersed in a mixture solution containing 0.5 mM silver nitrate and 1 mM sodium citrate at 100 °C for 2 h. Afterwards, the Ag/Co₃O₄/BiVO₄ triadic photoanode²² was rinsed with deionized water and dried in air. The complete synthesis process was sketched in Fig. 2. Additional details of chemicals, characterizations, PEC measurements and supporting data are available in ESI.†

Fig. 2 Synthetic procedure of triadic Ag/Co₃O₄/BiVO₄ photoanode.

3. Results and discussion

The phase composition of the samples was detected by XRD technique, and the corresponding results are shown in Fig. 3. In the XRD pattern of pure BiVO₄ photoanode (the red line), all diffraction peaks can be easily indexed to those of a monoclinic BiVO₄ cell (JCPDS No. 75-1866). The peaks at 2θ value of 19.00, 28.97 and 30.55 indexed to (011), (112) and (004) plane respectively, and no other crystalline phase like V₂O₅ or Bi₂O₃ was detected. Furthermore, it can be seen that weak diffraction peaks are consistent with cubic spinel Co₃O₄ phase (cell parameters: a = 8.084 Å; JCPDS No. 74-2120), as shown in the blue line. Even higher calcination temperature cannot improve the crystallinity of Co₃O₄ due to the low loading amount and high dispersion (Fig. S1†). Based on some experimental results and discussion (Fig. S2 and S3†), 350 °C was determined to the optimal calcining temperature for the formation of the Co₃O₄/BiVO₄ photoanode. After Ag NPs loading onto the Co₃O₄/BiVO₄/FTO, all monoclinic BiVO₄, cubic Co₃O₄ and Ag phases appeared simultaneously (the pink line in Fig. 3). In order to differentiate the lapped peaks of FTO and Ag NPs, the XRD patterns of 2θ values from 33 to 47 are enlarged partially. (111) and (200) with 2θ values around 38.11° and 44.30° preferred orientation diffraction peaks appeared, consistent with the crystal planes of metallic silver.

Fig. 3 (a) XRD patterns of FTO, BiVO₄/FTO, Co₃O₄/BiVO₄/FTO and Ag/Co₃O₄/BiVO₄/FTO. (b) Enlarged view of the area marked with the red dots in the (a) XRD patterns.

To further confirm the composition of the composite sample, XPS measurement was carried out as shown in Fig. 4. From the survey XPS spectrum (Fig. 4a), C 1s peak is located at 284.6 eV,²³ and except C, Bi, V, O, Co and Ag, no peaks of other elements could be observed, which indicates the presence of Co and Ag species on BiVO₄, compared with pristine BiVO₄. Fig. 4b and d present the high resolution XPS scan spectra of Bi 4f, V 2p, and O 1s. The Bi 4f XPS spectra in Fig. 4b show that the binding energies of Bi 4f_7/2 and Bi 4f_5/2 occur at 158.7 and 164.05 eV, respectively.²⁴ The V 2p peak in Fig. 4c can be fitted with two peaks at 523.9 and 516.3 eV, which are assignable to V 2p_1/2 and V 2p_3/2 signals, respectively.²⁵ Compared with the pure BiVO₄, an obvious negative shift in V 2p peaks and a slight negative shift in Bi 4f peaks are observed for Ag/Co₃O₄/BiVO₄, confirming the electron transfer from Co₃O₄ to BiVO₄.²⁶ At the same time, the O 1s spectra of pristine BiVO₄ and Ag/Co₃O₄/BiVO₄ shown in Fig. 4d exhibit the binding energies for O 1s (529.1 eV) core levels, while in the O 1s XPS spectrum of Ag/Co₃O₄/BiVO₄, a new peak (531.1 eV) appears as compared to pristine BiVO₄, which is probably caused by the Co loading and also indicates that the loaded Co species is exist in oxides form. The peaks located at 529.1 and 531.1 eV can be assigned to the oxygen species of lattice oxygen of layer-structured Bi₂O₂²⁺ and Co₃O₄, respectively. The high-resolution spectrum for Co 2p (Fig. 4e) shows two major peaks with binding energies at 779.3 and 795.1 eV, corresponding to Co 2p_3/2 and Co 2p_1/2, respectively, which is the characteristic of a Co₃O₄. The spectra in the Co 2p_1/2 region can be further deconvoluted into two peaks. The peaks at 793.6 eV and 795.4 eV are ascribed to Co³⁺ and Co²⁺ species respectively, which indicates the Co²⁺ and Co³⁺ species co-exist in the as-grown Co₃O₄ layer.²⁷ Fig. 4f shows that Ag 3d binding energies are 367.2 and 373.2 eV, confirming the existence of Ag.²⁸ Based on the XPS data, the examined sample was confirmed as Ag/Co₃O₄/BiVO₄ composite, and a strong interface interaction between photocatalyst BiVO₄ and cocatalyst Co₃O₄ would influence the resulting PEC properties, which will be discussed in later sections.

Fig. 4 XPS survey spectra of pristine BiVO₄ and Ag/Co₃O₄/BiVO₄: (a) survey scan; (b) Bi 4f; (c) V 2p; and (d) O 1s. (e) Co 2p and (f) Ag 3d scan spectra of Ag/Co₃O₄/BiVO₄.

The morphology and microstructure of triadic Ag/Co₃O₄/BiVO₄ photoanode was characterized by FESEM and HRTEM in Fig. 5. The triadic Ag/Co₃O₄/BiVO₄ photoanode was fabricated by BiVO₄ nanobulks deposited with Co₃O₄ nanosheets and Ag nanoparticles as shown in Fig. 5a. It can be observed that BiVO₄ nanobulks were covered under the Co₃O₄ nanosheets, and Ag nanoparticles were evenly distributed on the surface of Co₃O₄ in Fig. 5b. The microstructure and elemental distribution of Ag/Co₃O₄/BiVO₄ are further revealed by the TEM and energy dispersive X-ray (EDX) mappings. EDX image of Ag/Co₃O₄/BiVO₄ photoanode indicates the presence of the Bi, V, Co, Ag and O (Fig. 5d). The elemental mappings of Fig. 5c display that the BiVO₄ nanobulks are well-distributed under Co₃O₄ nanosheets and Ag nanoparticles, and the EDX mapping analyses are consistent with the SEM observation. From TEM image in Fig. 5e, three components of the triadic photoanode were uniformly distributed. The HRTEM image in Fig. 5f shows the distinct lattice fringe of Ag/Co₃O₄/BiVO₄ complex. The fringes spacing is measured to be 0.310 nm, which corresponded to the (−121) lattice spacing of BiVO₄. In addition, the Co₃O₄ displays a lattice spacing of 0.241 nm, corresponding to the (311) plane.⁸ A typical lattice spacing of 0.236 nm indexed to (111) plane of Ag, revealing that the Ag nanoparticles were loaded on the Co₃O₄ nanosheets closely.²⁹

Fig. 5 SEM images (a and b) of Ag/Co₃O₄/BiVO₄, EDX mappings (c and d) for Ag/Co₃O₄/BiVO₄, TEM (e) and HRTEM (f) images of Ag/Co₃O₄/BiVO₄.

To give a deep insight into the loading amount for changing the nanostructure of Co₃O₄ layer, the effects of Co₃O₄ loading on BiVO₄ photoanodes were further investigated by controlling the reactant concentration of Co(NO₃)₂·6H₂O. As illustrated in Fig. 6, it is interesting to note that various nanostructured Co₃O₄ cocatalysts were obtained in the reaction solutions with different cobalt precursor concentrations. As shown in Fig. 6a and b, when the cobalt precursor was 1 mM, Co₃O₄ can only be dispersed in particle shape on BiVO₄ nanobulks. When the cobalt precursor concentration was adjusted to 2 mM, the one-dimensional Co₃O₄ nanowires with a diameter of ∼50 nm in grass shape were obtained, as shown by the SEM images in Fig. 6c and d. When the cobalt precursor concentration continued to rise to 3 mM, the two-dimensional Co₃O₄ nanosheets in flake shape were observed in Fig. 6e and f, which are interwoven with each other and laid flat on BiVO₄ nanobulks. As illustrated in Fig. 6g and h, the solvothermal syntheses in the solutions with 4 mM cobalt precursor resulted in the growth of three-dimensional Co₃O₄ nanosheets in ribbon shape. The as-prepared Co₃O₄/BiVO₄ heterojunction photoanodes with different Co₃O₄ loading amount have obvious effects on PEC properties, which will be discussed in detail next.

Fig. 6 SEM images (a and b) of 1 mM Co₃O₄/BiVO₄; (c and d) of 2 mM Co₃O₄/BiVO₄; (e and f) of 3 mM Co₃O₄/BiVO₄; (g and h) of 4 mM Co₃O₄/BiVO₄.

The PEC measurements were achieved to disclose the functional roles of Co₃O₄ when coupled with BiVO₄ (Fig. 7). The current–voltage behaviors of the photoanodes under illumination and in the dark were studied as shown in Fig. 7a. In the dark, the current densities of the four photoanodes are almost zero, until the electrocatalytic oxidation peaks of cobalt appear at 0.95 V vs. Ag/AgCl. It can be observed that with the increase of Co₃O₄ loading amount, the electrocatalytic oxidation of water is stronger, that is, the peak strength increases and the onset potential shifts negatively. However, the difference is that 3 mM Co₃O₄/BiVO₄ heterojunction electrode has no obvious oxidation peak, which may be ascribed to highly dispersed Co²⁺/Co³⁺ species oxidizing more easily to higher valence Co³⁺/Co⁴⁺, expediting the kinetics.³⁰ Under illumination, 3 mM Co₃O₄/BiVO₄ photoanode exhibited the highest photocurrent response, with the increase of Co₃O₄ loading amount. However, with the increase of excessive Co₃O₄ loading on 4 mM Co₃O₄/BiVO₄ photoanode, the corresponding light response is particularly poor, even shows no obvious response compared to the behaviour in the dark. Nevertheless, an in-depth investigation of the current–potential curves measured under chopped light illumination in Fig. 7b reveals that the 4 mM Co₃O₄/BiVO₄ photoanode is not nonresponsive but severely suppressed. When the photoanode was exposed to incident light, an anodic photocurrent spike was observed and then rapidly dropped to nearly zero, which is associated with the sudden generation of charge carriers and the subsequent recombination process.³¹ Electrochemical impedance spectroscopy (EIS) studies were further carried out to investigate the charge transport behaviour at the electrode/electrolyte interface in these photoanodes. The obtained EIS data is fitted to an equivalent circuit shown in Fig. 7c, and the parameters extracted from the Nyquist fit are listed in Table S1.† R_s is defined as the series resistance, R_ct1 in low impedance (high frequency) represents charge transfer resistance across the interface between the semiconductors, R_ct2 in high impedance (low frequency) is the charge transfer resistance across the electrode/electrolyte interface.^32–34 A smaller semidiameter of the semicircle represents a better charge transfer ability (i.e., faster surface reaction kinetics).³⁵ 4 mM Co₃O₄/BiVO₄ photoanode exhibited the largest semidiameter and the highest low-frequency impedance R_ct2 in agreement with its worse PEC activity, which also suggests the severe aggregation and recombination of photogenerated carriers, rather than favourable migration to involve in water oxidation reaction.³² 3 mM Co₃O₄/BiVO₄ photoanode presented the lowest low-frequency impedance R_ct2 and the best PEC behaviour, which may be also attributed to its interlaced 2D nanosheet structure for facilitating the carrier transportation and surface water oxidation reaction. Current–potential curves of the 3 mM Co₃O₄/BiVO₄ photoanode under different illumination conditions are shown in Fig. 7d. The dark and light behaviours are very consistent with current–potential curve under chopped illumination upon turning on or off the light, but the sharp photocurrent spikes can still be clearly observed, which requires further improvement of its PEC performance. The above results indicate that the PEC properties of Co₃O₄/BiVO₄ composite photoanode are affected by the loading amount and nanostructure of Co₃O₄ layer.

Fig. 7 (a) Current–voltage curves of the photoanodes under illumination (dashed line) and in the dark (solid line). (b) Current–potential curves of the photoanodes under chopped AM1.5G illumination. (c) Nyquist plots of the photoanodes under illumination. Inset: the corresponding equivalent circuit. (d) Current–potential curves of the 3 mM Co₃O₄/BiVO₄ photoanode under different illumination conditions. All measurements were taken in 0.1 M PBS solution.

Diffuse reflectance UV-visible spectra (DRS) were collected for BiVO₄, Co₃O₄ and Co₃O₄/BiVO₄ photoanodes to understand the influence of the heterostructure on light harvesting capability and to discuss the mechanism of charge separation for p-Co₃O₄/n-BiVO₄ heterojunction. As demonstrated in Fig. S4a,† pristine BiVO₄ photoelectrode shows a typical semiconductor absorption, whose absorption edge is located approximately at 500 nm. After loading the Co₃O₄ layer, the absorption edge of Co₃O₄/BiVO₄ obviously red-shifts and the absorption can be significantly enhanced in the whole visible region, which implies that the Co₃O₄/BiVO₄ composite photoanode has better visible light absorption capacity. Moreover, it is not difficult to find that the trend of light absorption enhancement for p-Co₃O₄/n-BiVO₄ photoanode is very consistent with the light absorption characteristics of Co₃O₄ film, which is attributed to Co₃O₄ as a p-type semiconductor with narrower bandgap width. The direct band gap energy (E_g) of BiVO₄ and Co₃O₄ are evaluated to be 2.50 and 2.06 eV respectively, as shown in Fig. S4b.† The E_g of the composite photoanode is further narrowed to 2.0 eV, which suggests the formation of heterojunction with the strong interaction between n-BiVO₄ and p-Co₃O₄, consistent with the aforementioned XPS results. The band positions of pure BiVO₄ and Co₃O₄ before contact were predicted theoretically from the absolute electronegativity using eqn (1) and (2):

E_V = X − E_e + 0.5E_g (1)

E_C = E_V − E_g (2)

where E_C and E_V are conduction band (CB) and valence band (VB) positions, E_e is the energy of free electrons on the hydrogen scale (∼4.5 eV), E_g is the bandgap and X is the absolute electronegativity of the semiconductor.¹⁹ The X values for Co₃O₄ and BiVO₄ are 5.903 eV and 6.035 eV, respectively.^8,20 According to the estimated conduction, valence band edges, and E_g values of BiVO₄ and Co₃O₄, a mechanism of charge separation for p-Co₃O₄/n-BiVO₄ heterojunction is proposed as displayed in Fig. S4c.† In the heterostructure formed with p-Co₃O₄ and n-type semiconductors, there is a space charge region caused by the opposite direction diffusion of electrons in n-type semiconductors and holes in p-type semiconductors, leading to a p–n junction at the interface of semiconductors and an IEF from n-type semiconductors to p-type semiconductors. Then Fermi levels of p-type and n-type semiconductors must be identical at thermal equilibrium, forming a favorable band bending between BiVO₄ and Co₃O₄. On illumination as demonstrated, the photoexcited holes on the VB of BiVO₄ can transfer to Co₃O₄, while the electrons on the CB of Co₃O₄ transfer to BiVO₄ driven by an IEF at the Co₃O₄/BiVO₄ interface after contact. As a consequence, the photoexcited electrons and holes in the Co₃O₄/BiVO₄ heterostructure can be separated efficiently by the p–n junction due to the contrary direction of carrier migration. Furthermore, the sluggish surface reaction kinetics is overcome by the Co₃O₄ OECs, which further lowers the overpotential for water oxidation.

Although the as-prepared Co₃O₄/BiVO₄ heterojunction photoanode has played a good role in charge separation, carrier transport is still not ideal, as can be seen from the transient current curve in Fig. 7d. Considering the excellent electron conductivity and catalytic performance of silver nanoparticles, we tried to further improve the PEC performance of composite photoanode by loading Ag NPs. As shown in Fig. 8a, pristine BiVO₄ photoanode displays a relatively low photoresponse with a photocurrent density of 0.40 mA cm⁻² at 1.23 V vs. RHE. A largely enhanced photocurrent density (1.08 mA cm⁻²) is observed for the Co₃O₄/BiVO₄ heterojunction photoanode. In the case of triadic Ag/Co₃O₄/BiVO₄ photoanode, the photocurrent density further reaches to 1.84 mA cm⁻² at 1.23 V vs. RHE, 4.60 and 1.70 times higher than that of the pristine BiVO₄ and Co₃O₄/BiVO₄ samples, respectively. Moreover, the photoconversion efficiency (PCE) of each photoanode was calculated based on the photocurrent response as shown in Fig. S5.† The PCE of triadic Ag/Co₃O₄/BiVO₄ photoanode reached 0.483% at 0.7 V vs. RHE, 28.4 times higher than the pristine BiVO₄ (0.017%) and 1.8 times higher than Co₃O₄/BiVO₄ photoanode (0.267%). In addition, Fig. 8b shows the transient photocurrent response of the photoanodes under chopped light illumination. The instantaneous current when the light is switched ON is a measure of the flux of holes into the surface. The subsequent decay towards the steady state current results from the capture of build-up holes in surface states inducing an electron flux associated with recombination.³⁶ In particular, the cathodic current overshoot observed when the light is switched OFF is due to the continuing flux of electrons from Co₃O₄ into the surface, correspondingly, no cathodic current was observed in the pristine BiVO₄ photoanode. When Ag NPs were loaded onto the surface of Co₃O₄, and interlayers between Co₃O₄ and BiVO₄, the surface plasma resonance (SPR) effect and excellent electrical conductivity of Ag NPs inhibited the recombination of surface electrons and holes, which further improved the photocurrent response.³⁷ The effect of Ag NPs was further confirmed by the chronoamperometry measurement under chopped light illumination in Fig. 8c. Amperometric I–t curves also depict the fast response toward irradiation stimulation in five cycles of switching light ON and OFF. The current density of these electrodes under illumination follows a descending order as: Ag/Co₃O₄/BiVO₄ > Co₃O₄/BiVO₄ > pristine BiVO₄. This is in high accordance with the LSV results (Fig. 8a) and further demonstrates the improvement in PEC performance provided by Co₃O₄ and Ag NPs. Moreover, the square profiles and steady photocurrent density of transient photocurrent response in Ag/Co₃O₄/BiVO₄ indicate a fast surface oxidation kinetics, suppressed electron–hole recombination and excellent photochemical stability.³⁸ Fig. 8d shows an enlarged image of the current–voltage performance of the photoanodes under illumination based on Fig. 8a. Compared to pristine BiVO₄, the onset potential of the Co₃O₄/BiVO₄ heterojunction significantly reduces from 0.86 V to 0.35 V with a cathodic shift of 510 mV. This cathodic shift value is the highest amongst all reported ones, in which BiVO₄ photoanodes were composited with a p-type semiconductor Co₃O₄, NiO or WO₃.^8,39–41 Furthermore, the onset potential is further negatively shifted 40 mV after modification of Ag NPs. The above results indicate that the triadic Ag/Co₃O₄/BiVO₄ photoanode further inhibited the electron–hole recombination and enhanced the photoresponse, after Ag NPs were introduced into the p-Co₃O₄/n-BiVO₄ heterojunction.

Fig. 8 (a) Current–voltage curves of the photoanodes under illumination (dashed line) and in the dark (solid line). (b) Current–potential curves of the photoanodes under chopped AM1.5G illumination. (c) Photocurrent–time curves at 1.23 V vs. RHE. (d) Enlarged image of current–voltage curves of the photoanodes under illumination (solid line). (e) EIS measured at 1.23 V vs. RHE under illumination. (f) Mott–Schottky plots collected at a frequency of 1 kHz in the dark. (g) Intensity-modulated photocurrent spectroscopy (IMPS) responses of the photoanodes. (h) The IPCE measured at 1.23 V vs. RHE under monochromatic light irradiation. (i) A schematic illustration for the PEC water oxidation process over the triadic Ag/Co₃O₄/BiVO₄ photoanode. All measurements were taken in 0.1 M PBS solution.

To gain further insight into the mechanism of the triadic Ag/Co₃O₄/BiVO₄ photoanode, the charge transfer at the photoanodes and water oxidation kinetics at the photoanode/electrolyte interface are evaluated using EIS plots. As shown in Fig. 8e, the obtained EIS data is fitted to an equivalent circuit,^32–34 and the parameters are listed in Table S2.† As reported, the first arc (high frequency) represents the charge transfer step and the second arc (low frequency) corresponds to the surface water oxidation step.²⁶ Compared to the pristine BiVO₄ photoanode, Co₃O₄/BiVO₄ photoanode exhibits smaller diameter of both two arcs, indicating that Co₃O₄ can boost the charge separation/transfer and surface reaction kinetics simultaneously, which can be attributed to the formation of Co₃O₄/BiVO₄ p–n junction and great catalytic activity of Co₃O₄ cocatalysts toward water oxidation. After introducing Ag NPs, the semicircle in the high-frequency region is eliminated and the semicircle in the low-frequency region is further reduced, indicating that the triadic Ag/Co₃O₄/BiVO₄ photoanode has better charge separation and transportation performance. Moreover, the flat band potential is obtained by extrapolating the Mott–Schottky plot to 1/C² = 0 (Fig. 8f). Compared to the pristine BiVO₄, an evidently positive shift is observed for the Co₃O₄/BiVO₄ photoanode. The positive shift of the flat band potential for Co₃O₄/BiVO₄ suggests a decrease in the band bending, thus facilitating electrode/electrolyte interface charge transfer.

Intensity modulated photocurrent spectroscopy (IMPS) is a useful method to study semiconductor charge transport properties and is widely used in electron transport characterization of PEC devices.⁴² Fig. 8g shows typical IMPS responses of pristine BiVO₄, Co₃O₄/BiVO₄ and Ag/Co₃O₄/BiVO₄ photoanodes in the complex plane. The semicircle observed in the fourth quadrant provides information on the combination of charge transfer and relaxation inside the photoanode.⁴³ Accordingly, the frequency of the maximum imaginary corresponds to the sum of the charge transfer (k_trans) and recombination (k_rec) rate constants (k_trans + k_rec = 2πf_max). The transient time of the electron (τ_d) was calculated according to the characteristic frequency apex of IMPS imaginary components. According to Fig. 8g, τ_d values of BiVO₄, Co₃O₄/BiVO₄ and Ag/Co₃O₄/BiVO₄ were calculated to be 2.52, 1.42 and 0.90 ms, respectively. This result indicates that the Co₃O₄/BiVO₄ photoanode has a higher electron transfer rate than that of pristine BiVO₄ photoanode, which results from band bending at the interface in heterojunction, leading to a directional flow of electron. The time constant further decrease to 0.90 ms for Ag NPs modified Co₃O₄/BiVO₄ photoanode, indicating the enhanced carries transport and better surface reaction kinetics for water oxidation, which supports the EIS and Mott–Schottky results very well. The incident photon-to-current efficiency (IPCE) value is a critical index to study solar energy conversion efficiency. The Co₃O₄/BiVO₄ photoanode presents a higher IPCE value than BiVO₄ from UV extending to the visible region (Fig. 8h). Moreover, the Ag/Co₃O₄/BiVO₄ has higher IPCE profile than that of Co₃O₄/BiVO₄ until 500 nm, which is attributed to their corresponding SPR absorption peaks in the visible region.³⁸ This SPR wavelength dependent IPCE feature signifies that excitation of the Ag SPR is also responsible for the improved PEC performance of the triadic Ag/Co₃O₄/BiVO₄ photoanode. As a consequence, the IPCE value of ternary photoanode at 400 nm is up to 45.3%, which is 2.83 and 2.25 times higher than that of BiVO₄ and Co₃O₄/BiVO₄ respectively. The largely enhanced IPCE of the triadic Ag/Co₃O₄/BiVO₄ photoanode further confirms the integrated contribution of Co₃O₄ layer and Ag NPs to the PEC performance.

Given the discussion above, a mechanism for the enhanced PEC water oxidation performance in this triadic Ag/Co₃O₄/BiVO₄ photoanode is proposed and shown in Fig. 8i. Firstly, BiVO₄ with a small bandgap can utilize a significant portion of visible light to generate electrons and holes, and its valence band (VB) edge is positive enough to provide the holes with sufficient overpotential for the water oxidation reaction. When p-Co₃O₄ and n-BiVO₄ come into contact, a p–n junction is formed at their interface. Then the photoexcited holes on the VB of BiVO₄ can transfer to Co₃O₄, while the electrons on the CB of Co₃O₄ transfer to BiVO₄ that is driven by an IEF at the Co₃O₄/BiVO₄ interface. Furthermore, the unique interwoven nanostructure composed of 2D Co₃O₄ nanosheets could shorten the hole diffusion distance and facilitate the transport of holes to the surface. Besides, the highly dispersed Co²⁺/Co³⁺ species within the Co₃O₄ ultrathin nanosheets can be oxidized to higher valence Co³⁺/Co⁴⁺, which are believed to catalyse water oxidation and expedite the kinetics. Finally, the interlaminar Ag induced surface potential at Co₃O₄/BiVO₄ interface facilitates rapid carrier separation and transport.⁴⁴ Meanwhile, under the irradiation of simulated solar light, the hot electrons generated by the SPR effect of Ag NPs can transfer to the conduction band (CB) of Co₃O₄.^37,45 Then the oscillation of the Ag NPs facilitates electron movement from Co₃O₄ to BiVO₄ and improves the photocurrent response.³⁷ Besides, the Schottky barrier existing at the interface of Ag and Co₃O₄ can efficiently prevent the hot electrons accumulated in Co₃O₄ from returning back to Ag nanoparticles, which is confirmed by the XPS and IPCE results.^45,46 Consequently, the significant enhancement in PEC performance of the triadic Ag/Co₃O₄/BiVO₄ photoanode can be ascribed to the 2D structure/p–n junction/SPR effect triple functionality.

4. Conclusions

The work demonstrates that the novel hamburger-like nanostructure of triadic Ag/Co₃O₄/BiVO₄ photoanode based on ingenious design is successfully prepared, where the Co₃O₄ plays a synergistic role of OECs and p–n heterojunction, meanwhile Ag NPs enhance carrier transport and inhibit electron–hole recombination due to the SPR effect. The resulting ternary photoanode exhibit significantly enhanced efficiency in PEC water splitting: the photocurrent density enhances 4.60 times than pristine BiVO₄ and higher than other BiVO₄ composites reported in literatures. The IPCE of Ag/Co₃O₄/BiVO₄ reaches 2.83 times higher than BiVO₄ due to the charge transit time shorten trebly than BiVO₄ from IMPS spectra. The onset potential of the ternary well-constructed photoanode has an astonishing cathodic shift of 550 mV compared to the pristine BiVO₄ photoanode. Besides, the positive shift of flat band potential, the enhancement of carrier density and the decrease in energy gap are also driven by synergistic action of p–n junction and SPR effect. It is expected that this strategy can be extended to other multilevel heterostructures for advanced performance in the fields of energy conversion and storage.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Central University Basic Scientific Research Operating Expenses Project and National College Student Innovation and Entrepreneurship Training Program from Beijing University of Chemical Technology.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra08102g

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