Promoting the Photoelectrochemical Properties of BiVO4 Photoanode via Dual Modification with CdS Nanoparticles and NiFe-LDH Nanosheets

Bismuth vanadate (BiVO4) has long been considered a promising photoanode material for photoelectrochemical (PEC) water splitting. Despite its potential, significant challenges such as slow surface water evolution reaction (OER) kinetics, poor carrier mobility, and rapid charge recombination limit its application. To address these issues, a triadic photoanode has been fabricated by sequentially depositing CdS nanoparticles and NiFe-layered double hydroxide (NiFe-LDH) nanosheets onto BiVO4, creating a NiFe-LDH/CdS/BiVO4 composite. This newly engineered photoanode demonstrates a photocurrent density of 3.1 mA cm−2 at 1.23 V vs. RHE in 0.1 M KOH under AM 1.5 G illumination, outperforming the singular BiVO4 photoanode by a factor of 5.8 and the binary CdS/BiVO4 and NiFe-LDH/BiVO4 photoanodes by factors of 4.9 and 4.3, respectively. Furthermore, it exhibits significantly higher applied bias photon-to-current efficiency (ABPE) and incident photon-to-current efficiency (ICPE) compared to pristine BiVO4 and its binary counterparts. This enhancement in PEC performance is ascribed to the formation of a CdS/BiVO4 heterojunction and the presence of a NiFe-LDH OER co-catalyst, which synergistically facilitate charge separation and transfer efficiencies. The findings suggest that dual modification of BiVO4 with CdS and NiFe-LDH is a promising approach to enhance the efficiency of photoanodes for PEC water splitting.


Introduction
The increasing depletion of fossil fuels, environmental degradation, and the threat of global warming represent significant challenges that will persist without intervention.Addressing these issues requires the exploration of alternative energy resources such as wind, solar, hydropower, and sea wave energy [1].Among these, hydrogen gas is distinguished by its numerous advantages over traditional fuels and is considered the most promising alternative for next-generation energy resources [2].Hydrogen offers renewable and sustainable energy with the highest known gravimetric energy density, coupled with zero emissions and excellent storability.Therefore, developing environmentally and economically sustainable techniques for hydrogen production is of paramount importance.
Layered double hydroxides (LDHs) represent a versatile family of crystalline materials, notable for their diverse chemical compositions, morphologies, and particle sizes.They are also inexpensive, easy to make, and structurally tunable [37].Furthermore, LDHs are recognized as one of the most efficient classes of OER catalysts.Certain LDHs have been successfully deposited on BiVO 4 photoanodes as OER co-catalysts to improve PEC water splitting performance.For instance, integrating NiCo-or NiFe-LDHs with BiVO 4 photoanodes has been demonstrated to significantly increase the photocurrent density, surpassing that of the unmodified BiVO 4 photoanode by several folds [38][39][40][41].Meanwhile, constructing heterojunctions between CdS and BiVO 4 semiconductors has been effective in improving the separation and transfer efficiencies of photogenerated charge carriers [42][43][44].However, studies on PEC water splitting using a BiVO 4 photoanode co-modified with CdS and NiFe-LDH have not been documented.In this study, a triadic photoanode of NiFe-LDH/CdS/BiVO 4 was facilely fabricated through hydrothermal growth of CdS nanoparticles on the BiVO 4 surface, followed by electrochemical deposition of NiFe-LDH nanosheets.The resultant photoanode shows good PEC performance with a photocurrent density of 3.1 mA cm −2 in 0.1 M KOH at 1.23 V, which is 5.8 times greater than the unmodified BiVO 4 and 4.9 and 4.3 times greater, respectively, than the binary photoanodes CdS/BiVO 4 and NiFe-LDH/BiVO 4 .In addition, it also exhibits good stability in photocurrent density under continuous illumination for 3 h.

Materials
Unless otherwise specified, the reagents used in the experiments were analytically pure and were utilized as received without further purification.Deionized water was used throughout all experiments.

Preparation of Photoanodes
The fluoride-tin oxide (FTO) substrates (30 × 10 × 1.1 mm) were sequentially cleaned via ultrasonication in a 5% glass washing solution, water, acetone, and ethanol for 30 min, followed by air-drying for subsequent utilization.The BiVO 4 film was synthesized through a two-step synthetic procedure, according to the literature [45].Initially, 40 mmol of Bi(NO 3 ) 3 •5H 2 O was dissolved in 50 mL of HNO 3 aqueous solution (pH = 1.7).Subsequently, 400 mmol of KI was added and stirred until fully dissolved.To this solution, 20 mL of an ethanol solution containing 230 mmol of p-benzoquinone was added, followed by continuous stirring for an additional 15 min to generate the BiOI precursor solution.Electrodeposition of the BiOI layer on the FTO substrate was performed potentiostatically at −0.1 V versus Ag/AgCl for 180 s at room temperature using a typical three-electrode system: the FTO substrate as the working electrode, a saturated Ag/AgCl as the reference electrode, and a Pt net as the counter electrode.Following electrodeposition, the BiOI electrode was extensively rinsed with water and air-dried at room temperature.Conversion of BiOI to BiVO 4 involved the application of 70 µL of DMSO solution containing 0.23 mmol of VO(acac) 2 onto the BiOI electrode (1 × 1 cm), followed by annealing at 450 • C for 2 h (heating rate = 2 • C/min).Residual V 2 O 5 on the BiVO 4 surface was removed by immersion in 1 M NaOH solution for 30 min.The final BiVO 4 photoanode was then rinsed with water and dried at room temperature.
A solution comprising Cd(NO 3 ) 2 •4H 2 O (0.16 mmol), trisodium citrate (0.20 mmol), and thiourea (0.13 mmol) was prepared by dissolving these reagents in 50 mL of distilled water, followed by sonication for 10 min.The pH of the solution was adjusted to 11 by the gradual addition of ammonia solution until a yellow precipitate formed.Subsequently, a BiVO 4 photoanode, suspended from an iron wire, was immersed vertically in this suspension.The solution was then heated at 90 • C in an oil bath for 30 min.After that, the BiVO 4 photoanode was removed and washed repeatedly with water to yield the CdS/BiVO 4 photoanode.This photoanode was further processed by immersion in a mixed solution of 0.1 M Ni(NO 3 ) 2 •6H 2 O and 0.1 M FeSO 4 •7H 2 O. Potentiostatic deposition was executed at −1 V for 180 s.Following this, the photoanode was thoroughly rinsed with water and dried at 60 • C for 2 h in an oven to complete the fabrication of the NiFe-LDH/CdS/BiVO 4 photoanode.

Measurements
The surface morphology of the samples was analyzed using a Hitachi SU8000 field emission scanning electron microscope (Hitachi, Japan), equipped with energy dispersive X-ray (EDS) and mapping capabilities.High-resolution imaging was conducted using a JEOL JEM-2100 transmission electron microscope (JEOL, Tokyo, Japan).The crystalline phases of the samples were determined through X-ray powder diffraction (XRD) using a Mini FLEX600 (Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 0.154 nm).Surface chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS) on a SPECS system operated at 150 W utilizing Al Kα radiation as the X-ray source.

Photoelectrochemical Analysis
Photoelectrochemical measurements were performed at room temperature using an electrochemical workstation CHI 660E (CH Instruments, Shanghai, China) in a threeelectrode system.The as-prepared photoanodes, a saturated Ag/AgCl, and a Pt wire mesh were configured as working, reference, and counter electrodes, respectively.A 300 W xenon lamp PLS-FX300HU (PerfectLight, Beijing, China) coupled with an AM 1.5 G filter was used as the light source, and the light intensity was adjusted to 100 mW cm −2 .All as-prepared photoanodes were illuminated from the back (the irradiated area was 1.0 cm 2 ) and were immersed in an aqueous solution of 0.5 M Na 2 SO 4 as the electrolyte.Potentials reported here, unless otherwise specified, were converted to RHE using Equation (1): The photocurrents were measured by linear scanning voltammetry (LSV) with a scanning rate of 10 mV s −1 from -0.4 V to 1.2 V.For electrochemical impedance spectroscopy (EIS) measurement, a sinusoidal voltage pulse of 10 mV amplitude was applied to a bias voltage of 0.67 V with frequencies ranging from 100 kHz to 10 mHz.The charge injection efficiencies (η inj ) and charge separation efficiencies (η sep ) can be calculated from Equations (2) and (3): where J H 2 O and J Na 2 SO 3 are the photocurrent densities without and with Na 2 SO 3 as a hole scavenger, respectively, and J abs is the photocurrent density by assuming 100% absorbed photon-to-current efficiency.The applied bias photon-to-current efficiency (ABPE) was calculated according to Equation (4): where J ph is the photocurrent density, V app is the applied external potential, and P light is the light density of the illumination (100 mW cm −2 ).The incident photon-to-current conversion efficiency (IPCE) was measured from 350 nm to 650 nm using a monochromator (PLS MC150), which is calculated from Equation ( 5): where J represents the current density under light and darkness, P mono refers to the light intensity, and λ is the incident light wavelength.Mott−Schottky (M−S) spectra were measured in a 0.5 M Na 2 SO 4 (pH = 6.1) in the dark from 0 V to 0.4 V.The donor density (N D ) is calculated from Equation ( 6): where C represents the capacitance of the space charge region, ε 0 is the vacuum permittivity, ε is the relative permittivity of the semiconductor, e is the electron charge, E is the applied potential, E fb is the flat band potential, k is the Boltzmann constant, and T is the absolute temperature.
A stability test of the photoanodes was conducted under successive illumination for 3 h at 1.23 V.The evolved H 2 and O 2 gases were collected and tested in a three-electrode system by a gas chromatograph spectrometer (GC9790II) with a thermal conductivity detector (TCD).The electrolyte was purged with Ar for 30 min to eliminate any dissolved oxygen before the measurement.

Characterization of the Photoanode Materials
The synthetic route for the preparation of the NiFe-LDH/CdS/BiVO 4 photoanode is presented in Figure 1a.BiVO 4 was synthesized using a two-step method [45].First, a precursor solution containing the tetraiodo bismuthate complex [BiI 4 ] -was prepared by reacting Bi(NO 3 ) 3 •5H 2 O with excess KI under acidic conditions.This solution was then subjected to electrodeposition.During this process, benzoquinone, pre-added to the precursor solution, was electrochemically reduced to hydroquinone at the FTO working electrode.This reduction consumed protons and generated hydroxyl ions at the electrode, which subsequently reacted with [BiI 4 ] -to form BiOI on the FTO substrate.In the second step, the BiOI film was converted to BiVO 4 by annealing at 450 • C in the presence of VO(acac) 2 , likely involving a solid-state reaction between Bi 2 O 3 and V 2 O 5 .Next, CdS nanoparticles were grown by reacting Cd 2+ ions with thiourea in the presence of trisodium citrate at 90 • C.These nanoparticles were then directly deposited onto the BiVO 4 film to establish the CdS/BiVO 4 photoanode, which was subsequently coated with ultrathin NiFe-LDH nanosheets by electrochemical deposition.For comparison purposes, the binary counterparts, CdS/BiVO 4 and NiFe-LDH/BiVO 4, were also prepared under similar conditions.The surface morphology and elemental distribution of the photoanode materials were thoroughly characterized using SEM and HRTEM, complemented by EDS mapping.The electrodeposited BiOI exhibited a two-dimensional plate structure oriented perpendicular to the FTO substrate (Figure S1a).Subsequent calcination transformed the BiOI into a BiVO 4 film, which displayed an irregular, worm-like porous architecture (Figure 1b).The CdS/BiVO 4 heterojunction maintained a morphology akin to that of BiVO 4 , but with noticeable coverage by small CdS nanoparticles (Figures 1c and S1b).As can be seen from Figure 1d, NiFe-LDH nanosheets were uniformly electrodeposited over the surface of the CdS/BiVO 4 photoanode.HRTEM analysis, detailed in Figure 1e, identified lattice spacings of 0.16 nm and 0.467 nm, corresponding to the (-121) and (011) crystal planes of BiVO 4 , respectively.Additionally, a lattice fringe of 0.31 nm was observed, indicative of the (101) plane of CdS, confirming the successful formation of the CdS/BiVO 4 heterojunction.SEM-EDS mapping verified the homogeneous presence of Bi, V, O, Cd, S, Ni, and Fe elements (Table S1), demonstrating successful loading of CdS and NiFe-LDH on the BiVO 4 photoanode (Figure S2).
1b).The CdS/BiVO4 heterojunction maintained a morphology akin to that of BiVO4, but with noticeable coverage by small CdS nanoparticles (Figures 1c and S1b).As can be seen from Figure 1d, NiFe-LDH nanosheets were uniformly electrodeposited over the surface of the CdS/BiVO4 photoanode.HRTEM analysis, detailed in Figure 1e, identified lattice spacings of 0.16 nm and 0.467 nm, corresponding to the (-121) and (011) crystal planes of BiVO4, respectively.Additionally, a lattice fringe of 0.31 nm was observed, indicative of the (101) plane of CdS, confirming the successful formation of the CdS/BiVO4 heterojunction.SEM-EDS mapping verified the homogeneous presence of Bi, V, O, Cd, S, Ni, and Fe elements (Table S1), demonstrating successful loading of CdS and NiFe-LDH on the BiVO4 photoanode (Figure S2).The crystal phase and structure of the photoanode materials were examined using Xray diffraction (XRD), with results presented in Figure 2. In addition to the distinct diffraction peaks corresponding to SnO2 from the FTO substrate, the XRD spectrum of the as-prepared BiVO4 aligns well with the monoclinic scheelite BiVO4 (JCPDS: 14-0688).The crystal phase and structure of the photoanode materials were examined using X-ray diffraction (XRD), with results presented in Figure 2. In addition to the distinct diffraction peaks corresponding to SnO 2 from the FTO substrate, the XRD spectrum of the as-prepared BiVO 4 aligns well with the monoclinic scheelite BiVO 4 (JCPDS: 14-0688).
However, no characteristic diffraction peaks for CdS and NiFe-LDH were observed in the XRD spectra of the modified photoanodes.This absence is likely due to the low loading of CdS and the amorphous nature of NiFe-LDH, as corroborated by SEM and HRTEM results.The amorphous structure of the NiFe-LDH nanosheets may introduce abundant defects or vacancies, thereby increasing the active sites on the catalyst surface and facilitating the oxygen evolution reaction (OER) [46].
However, no characteristic diffraction peaks for CdS and NiFe-LDH were observed in the XRD spectra of the modified photoanodes.This absence is likely due to the low loading of CdS and the amorphous nature of NiFe-LDH, as corroborated by SEM and HRTEM results.The amorphous structure of the NiFe-LDH nanosheets may introduce abundant defects or vacancies, thereby increasing the active sites on the catalyst surface and facilitating the oxygen evolution reaction (OER) [46].The chemical composition and valence states of the NiFe-LDH/CdS/BiVO4 photoanode were characterized using XPS.The survey spectrum, depicted in Figure 3a, reveals peaks corresponding to Bi, V, O, Cd, S, Ni, and Fe elements, verifying the formation of the CdS and NiFe-LDH composite.The Bi 4f XPS spectrum (Figure 3b) displays two distinct peaks at 158.89 eV for Bi 4f7/2 and 164.2 eV for Bi 4f5/2, consistent with the Bi 3+ state.In Figure 3c, the split peaks of V 2p corresponding to V 2p3/2 and V 2p1/2 of V 5+ are found at 516.52 eV and 523.98 eV [47,48].The O 1s spectrum in Figure 3d features three peaks: lattice oxygen (Oβ) at 529.64 eV, surface adsorbed oxygen species (Oα) at 531.05 eV, and adsorbed molecular water (Oγ) at 532.4 eV.In the Cd 3d XPS spectrum (Figure 3e), there are two narrow peaks at 405.16 eV and 411.92 eV, which can be assigned to the 3d5/2 and 3d3/2 of Cd 2+ .The peaks of the S 2p3/2 and S 2p1/2 orbitals are located at 161.56 eV and 162.76 eV, respectively, indicating the existence of S 2− (Figure 3f) [49].The Ni 2p spectrum is composed of two major peaks located at 856.19 eV and 873.80 eV, with satellite peaks at 862.17 eV and 879.81 eV (corresponding to Ni 2p3/2 and Ni 2p1/2), which confirms the presence of Ni 2+ (Figure 3g).There are two peaks in Figure 3h, at approximately 711.79 eV and 725.48 eV, with satellite peaks at 717.09 eV and 733.69 eV, which are evidence of the presence of Fe 3+ [50].The above XPS results are in agreement with those reported in the literature, which further confirms the coexistence of CdS and NiFe-LDH in the NiFe-LDH/CdS/BiVO4 photoanode.The chemical composition and valence states of the NiFe-LDH/CdS/BiVO 4 photoanode were characterized using XPS.The survey spectrum, depicted in Figure 3a, reveals peaks corresponding to Bi, V, O, Cd, S, Ni, and Fe elements, verifying the formation of the CdS and NiFe-LDH composite.The Bi 4f XPS spectrum (Figure 3b) displays two distinct peaks at 158.89 eV for Bi 4f 7/2 and 164.2 eV for Bi 4f 5/2 , consistent with the Bi 3+ state.In Figure 3c, the split peaks of V 2p corresponding to V 2p 3/2 and V 2p 1/2 of V 5+ are found at 516.52 eV and 523.98 eV [47,48].The O 1s spectrum in Figure 3d features three peaks: lattice oxygen (O β ) at 529.64 eV, surface adsorbed oxygen species (O α ) at 531.05 eV, and adsorbed molecular water (O γ ) at 532.4 eV.In the Cd 3d XPS spectrum (Figure 3e), there are two narrow peaks at 405.16 eV and 411.92 eV, which can be assigned to the 3d 5/2 and 3d 3/2 of Cd 2+ .The peaks of the S 2p 3/2 and S 2p 1/2 orbitals are located at 161.56 eV and 162.76 eV, respectively, indicating the existence of S 2− (Figure 3f) [49].The Ni 2p spectrum is composed of two major peaks located at 856.19 eV and 873.80 eV, with satellite peaks at 862.17 eV and 879.81 eV (corresponding to Ni 2p 3/2 and Ni 2p 1/2 ), which confirms the presence of Ni 2+ (Figure 3g).There are two peaks in Figure 3h, at approximately 711.79 eV and 725.48 eV, with satellite peaks at 717.09 eV and 733.69 eV, which are evidence of the presence of Fe 3+ [50].The above XPS results are in agreement with those reported in the literature, which further confirms the coexistence of CdS and NiFe-LDH in the NiFe-LDH/CdS/BiVO 4 photoanode.

PEC Properties of the Photoanodes
The PEC performances of the photoanodes were evaluated by measuring the photocurrent density versus applied potential curves under AM 1.5 G solar illumination from a xenon lamp in a 0.5 M Na2SO4 electrolyte at pH 6.1.As demonstrated in Figure 4a, at 1.23 V, the photocurrent density of bare BiVO4 is 0.53 mA cm −2 .In comparison, the CdS/BiVO4 and NiFe-LDH/BiVO4 photoanodes yield marginally higher values, attaining 0.63 mA

PEC Properties of the Photoanodes
The PEC performances of the photoanodes were evaluated by measuring the photocurrent density versus applied potential curves under AM 1.5 G solar illumination from a xenon lamp in a 0.5 M Na 2 SO 4 electrolyte at pH 6.1.As demonstrated in Figure 4a, at 1.23 V, the photocurrent density of bare BiVO 4 is 0.53 mA cm −2 .In comparison, the CdS/BiVO 4 and NiFe-LDH/BiVO 4 photoanodes yield marginally higher values, attaining 0.63 mA cm −2 and 0.71 mA cm −2 , respectively.Strikingly, the NiFe-LDH/CdS/BiVO 4 photoanode delivers a photocurrent density of 3.1 mA cm −2 , surpassing CdS/BiVO 4 by 4.9-fold, NiFe-LDH/BiVO 4 by 4.3-fold, and bare BiVO 4 by 5.8-fold.Moreover, the onset potentials for CdS/BiVO 4 , NiFe-LDH/BiVO 4 , and NiFe-LDH/CdS/BiVO 4 photoanodes exhibit cathodic shifts of 140 mV, 430 mV, and 540 mV, respectively, relative to BiVO 4 .These findings imply that a single-component modification of BiVO 4 only with CdS or NiFe-LDH is barely adequate to improve the PEC performance.However, a significant enhancement of PEC performance could be achieved through dual modification with CdS and NiFe-LDH.The light/dark photocurrent response, depicted in Figure 4b, confirms the excellent optical switching behavior and high visible light sensitivity of all photoanodes.Among them, the NiFe-LDH/CdS/BiVO 4 photoanode demonstrates the most pronounced photoresponse across the entire voltage range.Additionally, the current densities of the photoanodes display minimal decay, eventually stabilizing, as shown in Figure 4c, indicative of their good stability throughout the chronoamperometric tests.
cm −2 and 0.71 mA cm −2 , respectively.Strikingly, the NiFe-LDH/CdS/BiVO4 photoanode delivers a photocurrent density of 3.1 mA cm −2 , surpassing CdS/BiVO4 by 4.9-fold, NiFe-LDH/BiVO4 by 4.3-fold, and bare BiVO4 by 5.8-fold.Moreover, the onset potentials for CdS/BiVO4, NiFe-LDH/BiVO4, and NiFe-LDH/CdS/BiVO4 photoanodes exhibit cathodic shifts of 140 mV, 430 mV, and 540 mV, respectively, relative to BiVO4.These findings imply that a single-component modification of BiVO4 only with CdS or NiFe-LDH is barely adequate to improve the PEC performance.However, a significant enhancement of PEC performance could be achieved through dual modification with CdS and NiFe-LDH.The light/dark photocurrent response, depicted in Figure 4b, confirms the excellent optical switching behavior and high visible light sensitivity of all photoanodes.Among them, the NiFe-LDH/CdS/BiVO4 photoanode demonstrates the most pronounced photoresponse across the entire voltage range.Additionally, the current densities of the photoanodes display minimal decay, eventually stabilizing, as shown in Figure 4c, indicative of their good stability throughout the chronoamperometric tests.The charge transfer capabilities of the photoanodes were assessed using electrochemical impedance spectroscopy (EIS) across a frequency range of 0.1 Hz to 100 kHz under illumination.The resulting Nyquist data were analyzed and fitted by an equivalent circuit model to interpret the impedance characteristics.In general, the semicircle diameter in an EIS Nyquist plot is indicative of the charge transfer resistance (Rct) at the photoanode/electrolyte interface; a smaller diameter corresponds to a reduced Rct, denoting enhanced charge transfer kinetics [51].The NiFe-LDH/CdS/BiVO4 photoanode demonstrated a significantly lower Rct of 420 Ω, as compared to 950 Ω for bare BiVO4, 740 Ω for CdS/BiVO4, and 760 Ω for NiFe-LDH/BiVO4, as detailed in Figure 4d and Table S2.This reduction in Rct for the NiFe-LDH/CdS/BiVO4 photoanode suggests that the dual modification with CdS and NiFe-LDH synergistically diminishes charge transfer resistance, thereby facilitating OER at the interface [52].The charge transfer capabilities of the photoanodes were assessed using electrochemical impedance spectroscopy (EIS) across a frequency range of 0.1 Hz to 100 kHz under illumination.The resulting Nyquist data were analyzed and fitted by an equivalent circuit model to interpret the impedance characteristics.In general, the semicircle diameter in an EIS Nyquist plot is indicative of the charge transfer resistance (R ct ) at the photoanode/electrolyte interface; a smaller diameter corresponds to a reduced R ct , denoting enhanced charge transfer kinetics [51].The NiFe-LDH/CdS/BiVO 4 photoanode demonstrated a significantly lower R ct of 420 Ω, as compared to 950 Ω for bare BiVO 4 , 740 Ω for CdS/BiVO 4 , and 760 Ω for NiFe-LDH/BiVO 4 , as detailed in Figure 4d and Table S2.This reduction in R ct for the NiFe-LDH/CdS/BiVO 4 photoanode suggests that the dual modification with CdS and NiFe-LDH synergistically diminishes charge transfer resistance, thereby facilitating OER at the interface [52].
As the oxidation of sodium sulfite is more kinetically facile than the oxidation of water, the bulk properties of the prepared photoanodes were investigated in the presence of sodium sulfite to exclude the influence of the slow water oxidation kinetics [53].Figure 5a illustrates the LSV curves obtained in a 0.5 M Na 2 SO 4 (pH 6.1) electrolyte, both with and without the addition of 0.5 M sodium sulfite.Indeed, for all photoanodes, the photocurrent generated for sulfite oxidation was noticeably higher than that generated for water oxidation, in particular for bare BiVO 4 , whose photocurrent at 1.23 V is significantly higher with Na 2 SO 3 compared to without, indicating that the photoconversion efficiency is severely hindered by slow OER kinetics.The charge injection efficiency (η inj ), which describes the fraction of injected holes from the electrode surface into the electrolyte, was then calculated according to Equation (2), and the result is plotted in Figure 5b.The NiFe-LDH/CdS/BiVO 4 photoanode has a η inj of 85% at 1.23 V, which is much larger than that of BiVO 4 (11%), CdS/BiVO 4 (18%), and NiFe-LDH/BiVO 4 (44%).Figure 5c illustrates the charge separation efficiency (η sep ) of the photoanodes.An enhancement was also observed for the NiFe-LDH/CdS/BiVO 4 photoanode, but it was less pronounced.
Nanomaterials 2024, 14, 1100 9 of 14 As the oxidation of sodium sulfite is more kinetically facile than the oxidation of water, the bulk properties of the prepared photoanodes were investigated in the presence of sodium sulfite to exclude the influence of the slow water oxidation kinetics [53].Figure 5a illustrates the LSV curves obtained in a 0.5 M Na2SO4 (pH 6.1) electrolyte, both with and without the addition of 0.5 M sodium sulfite.Indeed, for all photoanodes, the photocurrent generated for sulfite oxidation was noticeably higher than that generated for water oxidation, in particular for bare BiVO4, whose photocurrent at 1.23 V is significantly higher with Na2SO3 compared to without, indicating that the photoconversion efficiency is severely hindered by slow OER kinetics.The charge injection efficiency (ηinj), which describes the fraction of injected holes from the electrode surface into the electrolyte, was then calculated according to Equation (2), and the result is plotted in Figure 5b.The NiFe-LDH/CdS/BiVO4 photoanode has a ηinj of 85% at 1.23 V, which is much larger than that of BiVO4 (11%), CdS/BiVO4 (18%), and NiFe-LDH/BiVO4 (44%).Figure 5c illustrates the charge separation efficiency (ηsep) of the photoanodes.An enhancement was also observed for the NiFe-LDH/CdS/BiVO4 photoanode, but it was less pronounced.The photoconversion efficiencies of the photoanodes were evaluated by the applied bias photon-to-current conversion efficiency (ABPE), calculated from the J−V curves according to Equation (3).As revealed in Figure 4d, the NiFe-LDH/CdS/BiVO4 photoanode possesses the highest ABPE of 0.93% at a lower potential (0.75 V), a significant improvement compared to BiVO4 (0.048% at 1.01 V), CdS/BiVO4 (0.073% at 0.96 V), and NiFe-LDH/BiVO4 (0.14% at 0.83 V).The incident photon-to-current efficiency (IPCE), another important index for evaluating photoconversion efficiencies, was further examined.The NiFe-LDH/CdS/BiVO4 photoanode achieved a superior IPCE of 78.6% at 450 nm, outperforming BiVO4 (20.3%),CdS/BiVO4 (14.3%), and NiFe-LDH/BiVO4 (29.3%) (Figure 6a).The photoconversion efficiencies of the photoanodes were evaluated by the applied bias photon-to-current conversion efficiency (ABPE), calculated from the J − V curves according to Equation (3).As revealed in Figure 4d, the NiFe-LDH/CdS/BiVO 4 photoanode possesses the highest ABPE of 0.93% at a lower potential (0.75 V), a significant improvement compared to BiVO 4 (0.048% at 1.01 V), CdS/BiVO 4 (0.073% at 0.96 V), and NiFe-LDH/BiVO 4 (0.14% at 0.83 V).The incident photon-to-current efficiency (IPCE), another important index for evaluating photoconversion efficiencies, was further examined.The NiFe-LDH/CdS/BiVO 4 photoanode achieved a superior IPCE of 78.6% at 450 nm, outperforming BiVO 4 (20.3%),CdS/BiVO 4 (14.3%), and NiFe-LDH/BiVO 4 (29.3%)(Figure 6a).To illustrate the important role of NiFe-LDH in the composite photoanodes, the OER electrocatalytic performance of the photoanodes was also investigated by polarization curves measured under dark conditions (Figure 6b).In contrast to BiVO4 and CdS/BiVO4, which show minimal OER activity, the NiFe-LDH/BiVO4 and NiFe-LDH/CdS/BiVO4 photoanodes exhibit markedly increased water oxidation currents, confirming the OER enhancement by NiFe-LDH.All the above outcomes demonstrate that the integration of CdS and NiFe-LDH advances photogenerated charge separation and transfer while also accelerating water oxidation kinetics, synergistically boosting the PEC water splitting performance.Notably, the PEC parameters of the NiFe-LDH/CdS/BiVO4 photoanode are comparable with, or surpass, several recently documented BiVO4-based photoanodes, as compiled in Table S3.
To examine the influence of carrier density on the photoelectrochemical performance, Mott−Schottky (M−S) analysis was conducted for BiVO4, CdS, NiFe-LDH, and their composites at a frequency of 1 kHz in dark conditions, as presented in Figure S3.The M−S plots consistently exhibited positive slopes, confirming the n-type semiconductor nature of the materials.The flat band potentials (Efb) were determined for BiVO4, CdS, NiFe-LDH, CdS/BiVO4, NiFe-LDH/BiVO4, and NiFe-LDH/CdS/BiVO4 by extrapolation of their M−S plots, and the values are tabulated in Table S4.The more negative Efb of CdS, relative to BiVO4, indicates that the photoexcited electrons in CdS can transfer to the conducting band of BiVO4, while the holes of BiVO4 transfer to the valence band of CdS, thus suppressing the recombination of photogenerated electron-hole pairs [43].Carrier densities (ND) of the photoanodes were also deduced from the slop of the M−S curves and are also listed in Table S4.The NiFe-LDH/CdS/BiVO4 photoanode exhibited the highest ND value, which is about twice compared to the CdS/BiVO4 or NiFe-LDH/BiVO4 photoanode, again verifying more efficient charge separation and hole injection after dual modification.
The photostability of the photoanodes was tested by subjecting them to continuous illumination at 1.23 V for 3 h in a 0.5 M Na2SO4 solution.In contrast to CdS/BiVO4, whose photocurrent density drastically fell to a mere 14% of its initial value over this period, NiFe-LDH/CdS/BiVO4 maintained nearly 50% of its initial photocurrent density under identical conditions (Figure S4).This endurance suggests that the NiFe-LDH nanosheet layer functions effectively as a hole shuttle to transfer holes accumulated at the CdS/BiVO4 interface under the applied potential and, at the same time, protects CdS from photocorrosion.Simultaneously, Ni 2+ ions within NiFe-LDH undergo oxidation by holes from the CdS valence band to higher valence species, which oxidize hydroxide ions (OH − ) to produce O2 gas and then return to their original valence state.Meanwhile, electrons are transferred to the counter electrode (Pt) via the external circuit and reduce protons (H + ) into H2 gas (Figure S5).The generation of H2 and O2 gases by the NiFe-LDH/CdS/BiVO4 photoanode was verified and quantified using gas chromatography (GC) integration (Figure S6).After 3 h of operation, the measured volumes of H2 and O2 evolution were 131 μmol and 65 μmol, respectively.To illustrate the important role of NiFe-LDH in the composite photoanodes, the OER electrocatalytic performance of the photoanodes was also investigated by polarization curves measured under dark conditions (Figure 6b).In contrast to BiVO 4 and CdS/BiVO 4 , which show minimal OER activity, the NiFe-LDH/BiVO 4 and NiFe-LDH/CdS/BiVO 4 photoanodes exhibit markedly increased water oxidation currents, confirming the OER enhancement by NiFe-LDH.All the above outcomes demonstrate that the integration of CdS and NiFe-LDH advances photogenerated charge separation and transfer while also accelerating water oxidation kinetics, synergistically boosting the PEC water splitting performance.Notably, the PEC parameters of the NiFe-LDH/CdS/BiVO 4 photoanode are comparable with, or surpass, several recently documented BiVO 4 -based photoanodes, as compiled in Table S3.
To examine the influence of carrier density on the photoelectrochemical performance, Mott−Schottky (M−S) analysis was conducted for BiVO 4 , CdS, NiFe-LDH, and their composites at a frequency of 1 kHz in dark conditions, as presented in Figure S3.S4.The more negative E fb of CdS, relative to BiVO 4 , indicates that the photoexcited electrons in CdS can transfer to the conducting band of BiVO 4 , while the holes of BiVO 4 transfer to the valence band of CdS, thus suppressing the recombination of photogenerated electron-hole pairs [43].Carrier densities (N D ) of the photoanodes were also deduced from the slop of the M−S curves and are also listed in Table S4.The NiFe-LDH/CdS/BiVO 4 photoanode exhibited the highest N D value, which is about twice compared to the CdS/BiVO 4 or NiFe-LDH/BiVO 4 photoanode, again verifying more efficient charge separation and hole injection after dual modification.
The photostability of the photoanodes was tested by subjecting them to continuous illumination at 1.23 V for 3 h in a 0.5 M Na 2 SO 4 solution.In contrast to CdS/BiVO 4 , whose photocurrent density drastically fell to a mere 14% of its initial value over this period, NiFe-LDH/CdS/BiVO 4 maintained nearly 50% of its initial photocurrent density under identical conditions (Figure S4).This endurance suggests that the NiFe-LDH nanosheet layer functions effectively as a hole shuttle to transfer holes accumulated at the CdS/BiVO 4 interface under the applied potential and, at the same time, protects CdS from photocorrosion.Simultaneously, Ni 2+ ions within NiFe-LDH undergo oxidation by holes from the CdS valence band to higher valence species, which oxidize hydroxide ions (OH − ) to produce O 2 gas and then return to their original valence state.Meanwhile, electrons are transferred to the counter electrode (Pt) via the external circuit and reduce protons (H + ) into H 2 gas (Figure S5).The generation of H 2 and O 2 gases by the NiFe-LDH/CdS/BiVO 4 photoanode was verified and quantified using gas chromatography (GC) integration (Figure S6).After 3 h of operation, the measured volumes of H 2 and O 2 evolution were 131 µmol and 65 µmol, respectively.

Figure 4 .
Figure 4. (a) Linear sweep voltammetry curves (LSV), (b) transient photocurrent curves measured under chopped light (on or off cycle: 8 s), (c) photocurrent-time curves measured at 1.23 V under chopped illumination, and (d) electrochemical impedance spectroscopy in a 0.5 M Na 2 SO 4 electrolyte (pH = 6.1) at 1.23 V under illumination of the photoanodes.

Figure 6 .
Figure 6.(a) IPCE curves obtained at 1.23 V under illumination; (b) polarization curves of the photoanodes measured in a 0.5 M Na2SO4 (pH = 6.1) solution in the dark.

Figure 6 .
Figure 6.(a) IPCE curves obtained at 1.23 V under illumination; (b) polarization curves of the photoanodes measured in a 0.5 M Na 2 SO 4 (pH = 6.1) solution in the dark.
The M−S plots consistently exhibited positive slopes, confirming the n-type semiconductor nature of the materials.The flat band potentials (E fb ) were determined for BiVO 4 , CdS, NiFe-LDH, CdS/BiVO 4 , NiFe-LDH/BiVO 4 , and NiFe-LDH/CdS/BiVO 4 by extrapolation of their M−S plots, and the values are tabulated in Table