PbS Quantum Dots-Decorated BiVO4 Photoanodes for Highly Efficient Photoelectrochemical Hydrogen Production

While metal oxides such as TiO2, Fe2O3, WO3, and BiVO4 have been previously studied for their potential as photoanodes in photoelectrochemical (PEC) hydrogen production, their relatively wide band-gap limits their photocurrent, making them unsuitable for the efficient utilization of incident visible light. To overcome this limitation, we propose a new approach for highly efficient PEC hydrogen production based on a novel photoanode composed of BiVO4/PbS quantum dots (QDs). Crystallized monoclinic BiVO4 films were prepared via a typical electrodeposition process, followed by the deposition of PbS QDs using a successive ionic layer adsorption and reaction (SILAR) method to form a p-n heterojunction. This is the first time that narrow band-gap QDs were applied to sensitize a BiVO4 photoelectrode. The PbS QDs were uniformly coated on the surface of nanoporous BiVO4, and their optical band-gap was reduced by increasing the number of SILAR cycles. However, this did not affect the crystal structure and optical properties of the BiVO4. By decorating the surface of BiVO4 with PbS QDs, the photocurrent was increased from 2.92 to 4.88 mA/cm2 (at 1.23 VRHE) for PEC hydrogen production, resulting from the enhanced light-harvesting capability arising from the narrow band-gap of the PbS QDs. Moreover, the introduction of a ZnS overlayer on the BiVO4/PbS QDs further improved the photocurrent to 5.19 mA/cm2, attributed to the reduction in interfacial charge recombination.


Introduction
Growing concern over air pollution and global warming caused by the extensive burning of fossil fuels has led to an increased focus on producing and utilizing carbonneutral energy sources [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. One promising approach for the generation of clean and renewable energy is photoelectrochemical (PEC) hydrogen production. In a typical PEC device, either a photoanode or a photocathode serves as the working electrode for light harvesting and either the oxygen evolution reaction (in the case of the photoanode) or the hydrogen evolution reaction (in the case of the photocathode) while the other half-reaction takes place at the counter electrode [16]. Under illumination, for example, the photoanode absorbs photons with energies greater than its band-gap energy, generating electron-hole pairs. The photoelectrons are then transported and collected to the conducting substrate through the conduction band (CB) of the photoanode, while the remaining holes participate in the oxidation reaction. The collected photoelectrons then flow to the counter electrode through an external circuit where they participate in the hydrogen evolution reaction. To achieve efficient PEC hydrogen production, the photoanode must have a suitable band-gap for broad light absorption, superior charge transport properties, and good photo-and chemical stabilities.
To this end, various metal oxides, such as TiO 2 [5,6], Fe 2 O 3 [7,8], ZnO [9,10], and BiVO 4 [11][12][13][14][15], have been intensively studied for use in PEC photoelectrodes due to their intrinsic chemical stability in water and low materials cost. BiVO 4 , in particular, has Nanoporous BiVO 4 films were prepared via a typical electrodeposition process, following a previously reported method [11]. Fluorine-doped tin oxide (FTO) glasses (TEC-8, Pilkington) were cleaned using ethyl alcohol in an ultrasonic bath for 30 min and subsequently treated with UV/O 3 (Yuil Ultraviolet System Inc.) for 20 min to remove surface contaminants. For the electrodeposition of BiOI films, a 0.04 M aqueous solution of Bi(NO 3 ) 3 was prepared by dissolving Bi(NO 3 ) 3 ·5H 2 O (Daejung) in a 0.4 M KI (Daejung) aqueous solution with pH adjusted to 1.7 using HNO 3 (Daejung). A 20 mL ethanolic solution of 0.23 M p-benzoquinone (Daejung) was added to this solution and vigorously stirred to ensure complete mixing. The electrodeposition was carried out using a potentiostat (Multi Autolab M204, Metrohm) with a three-electrode configuration consisting of the cleaned FTO glasses as the working electrode and a Pt mesh and an Ag/AgCl electrode as the counter and reference electrodes, respectively, in the prepared Bi(NO 3 ) 3

solution.
Electrodeposition was performed at -0.1 V vs. Ag/AgCl at room temperature (RT) for 4 min, followed by washing the surface of the electrodeposited BiOI films with deionized (DI) water and drying at RT. Subsequently, a 0.2 M VO(acac) 2 (Sigma-Aldrich) solution in dimethyl sulfoxide (DMSO, Kanto) was dropped onto the BiOI films with an amount of 100 µL/cm 2 , and the resulting films were annealed at 450 • C for 2 h in air. To remove excess V 2 O 5 on the BiVO 4 surface, the annealed BiVO 4 films were stirred in a 1 M NaOH (Daejung) aqueous solution for 30 min, followed by washing with DI water and drying at RT.

Deposition of PbS QDs on the Surface of BiVO 4 Films
The deposition of PbS QDs onto the BiVO 4 films was performed using a SILAR method, which was previously reported [17,26]. The FTO/BiVO 4 electrodes were immersed in a 0.02 M methyl alcohol solution of Pb(NO 3 ) 2 (Sigma-Aldrich) for 90 s, followed by immersion in a solution of 0.02 M Na 2 S (Sigma-Aldrich) in methanol/DI water (1:1, v/v) for 90 s. After each dipping, the electrodes were thoroughly washed with methyl alcohol, and the SILAR cycle was repeated 3-7 times. To apply ZnS overlayers on the BiVO 4 /PbS QDs films, the electrodes were alternatively immersed in a 0.06 M ethyl alcohol solution of Zn(NO 3 ) 2 ·6H 2 O (Sigma-Aldrich) and a 0.06 M solution of Na 2 S (Sigma-Aldrich) in methanol/DI water (1:1, v/v) for 50 s each. After each dipping, the electrodes were thoroughly washed with methyl alcohol, and the SILAR cycle was conducted three times.

Characterization
The surface morphology and structure of the electrodes were characterized using various analytical techniques. A field-emission scanning electron microscope (FE-SEM, S-4700, Hitachi) and high-resolution transmission electron microscopy (HR-TEM; JEM-2010, JEOL) were utilized to examine the surface morphology and structure. Elemental mapping was conducted using a SEM (CX-200, COXEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) detector. X-ray diffraction (XRD) analyses were performed using an X-ray diffractometer (SmartLab 9 kW system, Rigaku). The chemical and electronic states of the electrodes were investigated using X-ray photoelectron spectroscopy (XPS, K-alpha+, Thermo Fisher). The UV-vis absorption spectra of the BiVO 4 /PbS QDs films were obtained with UV-vis spectroscopy (OPTIZEN 2120 UV, KLAB). Steady-state photoluminescence (PL) spectra were recorded using a fluorescence spectrophotometer (FlouTime 300, PicoQuant). The PEC performances were measured using a potentiostat (Multi Autolab M204, Metrohm) with a three-electrode configuration consisting of a BiVO 4 /PbS QDs photoanode and a Pt mesh and SCE electrode as the counter and reference electrodes, respectively, in a quartz reactor. The electrolyte consisted of 0.5 M KH 2 PO 4 and 1.0 M Na 2 SO 3 (pH~7) in DI water. Photocurrent density-voltage (J-V) curves were obtained under illumination from a solar simulator (PEC-L01, Peccell) equipped with a 150 W Xe lamp and an AM 1.5G filter. The scan rate was 20 mV/s, and the light intensity of the solar simulator was adjusted to one sun (100 mW/cm 2 ) using a NREL-certified Si reference solar cell. Electrochemical impedance spectroscopy (EIS) data were obtained using a frequency response detector in the potentiostat, applying a sinusoidal perturbation of ±10 mV with the frequency varying from 10 −1 Hz to 10 5 Hz.  (Figure 1a). After the deposition of PbS QDs, the overall surface structure remains similar to that of the bare sample, but the pore size is slightly reduced and the main diameter marginally increased (Figure 1c). The thickness of both films is nearly the same (Figure 1b,d), indicating that PbS QDs were homogeneously coated on the surface of the nanoporous BiVO 4 . Figure 2 shows the EDX spectra and mapping images for the surfaces of both samples. The atomic ratio between Bi and V is nearly 1:1 for both samples, as indicated by the EDX spectra. Additionally, the atomic ratio between Pb and S is almost 1:1 for the QDs. The mapping images confirm that the PbS QDs were uniformly deposited on the surface of the BiVO 4 film. Bi and V is nearly 1:1 for both samples, as indicated by the EDX spectra. Additionally, the atomic ratio between Pb and S is almost 1:1 for the QDs. The mapping images confirm that the PbS QDs were uniformly deposited on the surface of the BiVO4 film.    Bi and V is nearly 1:1 for both samples, as indicated by the EDX spectra. Additionally, the atomic ratio between Pb and S is almost 1:1 for the QDs. The mapping images confirm that the PbS QDs were uniformly deposited on the surface of the BiVO4 film.     To further characterize the crystal structures of the prepared BiVO 4 /PbS QDs films, XRD spectra were obtained and are presented in Figure 4. Both spectra of the bare BiVO 4 and the BiVO 4 /PbS QDs films exhibit crystallized monoclinic BiVO 4 (JCPDS #14-0688), which is consistent with the SAED pattern presented above. In addition, the peak positions of the BiVO 4 /PbS QDs were almost identical to those of the bare BiVO 4 , implying that the PbS QDs were only physically adsorbed on the BiVO 4 surface and did not affect the crystal structure of BiVO 4 . The XRD peaks corresponding to PbS were not detected due to its poor crystallinity compared to that of BiVO 4 [29,30].  To further characterize the crystal structures of the prepared BiVO4/PbS QDs films, XRD spectra were obtained and are presented in Figure 4. Both spectra of the bare BiVO4 and the BiVO4/PbS QDs films exhibit crystallized monoclinic BiVO4 (JCPDS #14-0688), which is consistent with the SAED pattern presented above. In addition, the peak positions of the BiVO4/PbS QDs were almost identical to those of the bare BiVO4, implying that the PbS QDs were only physically adsorbed on the BiVO4 surface and did not affect the crystal structure of BiVO4. The XRD peaks corresponding to PbS were not detected due to its poor crystallinity compared to that of BiVO4 [29,30].   To further characterize the crystal structures of the prepared BiVO4/PbS QDs films, XRD spectra were obtained and are presented in Figure 4. Both spectra of the bare BiVO4 and the BiVO4/PbS QDs films exhibit crystallized monoclinic BiVO4 (JCPDS #14-0688), which is consistent with the SAED pattern presented above. In addition, the peak positions of the BiVO4/PbS QDs were almost identical to those of the bare BiVO4, implying that the PbS QDs were only physically adsorbed on the BiVO4 surface and did not affect the crystal structure of BiVO4. The XRD peaks corresponding to PbS were not detected due to its poor crystallinity compared to that of BiVO4 [29,30].  The chemical states of the BiVO 4 /PbS QDs/ZnS films were investigated by XPS analysis. The spectra of the bare BiVO 4 and BiVO 4 /PbS QDs (prepared via five SILAR cycles)/ZnS films over a wide scan range are shown in Figure S2 in the Supplementary Materials. Both spectra demonstrate the presence of Bi, V, O, and C. In addition, the BiVO 4 /PbS QDs/ZnS film displays extra peaks that correspond to Pb, S, and Zn. Figure 5 displays the high-resolution XPS spectra. In the case of the bare sample, the Bi 4f 7/2 and 4f 5/2 peaks exhibit BEs of 158.6 and 163.9 eV, respectively (Figure 5a), indicating the presence of Bi 3+ in the monoclinic phase of BiVO 4 [31][32][33]. The minor peaks observed at 156.8 (Bi 4f 7/2 ) and 162.1 eV (4f 5/2 ) were attributed to the metal species Bi 0 [33][34][35]. Moreover, the V 2p 3/2 and 2p 1/2 peaks have BEs of 516.6 and 523.9 eV (Figure 5b), respectively, which are typical of V 5+ in BiVO 4 [31][32][33][34][35][36]. No other significant peaks were observed. Based on the XPS data and the XRD results presented earlier, it can be inferred that most of the Bi and V species existed in the form of the monoclinic phase of BiVO 4 .
To examine the effect of PbS QD coating on the optical properties of the BiVO 4 film, the absorption spectra were measured and are presented in Figure 6a. The absorbance of the films was enhanced gradually with the increase in the number of PbS SILAR cycles compared to the bare BiVO 4 film due to the additional light absorption by the deposited PbS QDs. To investigate the absorption property of only the PbS QDs, the difference in absorbance between the bare BiVO 4 and PbS QD films was compared as a function of the number of PbS SILAR cycles (Figure 6b). The optical band-gap energy (E g ) of the PbS QDs was determined by extrapolating the linear part of (αhν) 2 vs. hν plot, where α is the absorption coefficient and hν is the photon energy (Figure 6c) [17,43]. The E g of the PbS QDs decreased gradually as the number of SILAR cycles increased. This means that the size of PbS QDs enlarged gradually as the number of SILAR cycles increased, leading to the reduced E g due to the quantum confinement effect [44][45][46]. The E g of the PbS QDs (1.38~1.56 eV) was much smaller than that of the BiVO 4 (~2.4 eV) [12][13][14], allowing the photoelectrodes to utilize the full range of visible light. Additionally, the steady-state PL spectrum of the BiVO 4 film was not significantly affected by the deposited PbS QDs ( Figure  S3, Supplementary Materials). The spectral peak position (~545 nm) and the PL intensity are nearly the same between the bare BiVO 4 and the BiVO 4 /PbS QD films, indicating that the optical property of BiVO 4 was not influenced by the deposition of PbS QDs. number of PbS SILAR cycles (Figure 6b). The optical band-gap energy (Eg) of the PbS QD was determined by extrapolating the linear part of (αhν) 2 vs. hν plot, where α is the ab sorption coefficient and hν is the photon energy (Figure 6c) [17,43]. The Eg of the PbS QD decreased gradually as the number of SILAR cycles increased. This means that the size o PbS QDs enlarged gradually as the number of SILAR cycles increased, leading to the re duced Eg due to the quantum confinement effect [44][45][46]. The Eg of the PbS QDs (1.38~1.5 eV) was much smaller than that of the BiVO4 (~2.4 eV) [12][13][14], allowing the photoelec trodes to utilize the full range of visible light. Additionally, the steady-state PL spectrum of the BiVO4 film was not significantly affected by the deposited PbS QDs ( Figure S3, Sup plementary Materials). The spectral peak position (~545 nm) and the PL intensity ar nearly the same between the bare BiVO4 and the BiVO4/PbS QD films, indicating that th optical property of BiVO4 was not influenced by the deposition of PbS QDs. The BiVO4/PbS QDs/ZnS films were utilized as the photoanode for PEC hydrogen production and tested under simulated one-sun illumination. The electrolyte used wa Ar-purged 0.5 M KH2PO4 and 1.0 M Na2SO3 aqueous solution with pH ~7, acting as a hol scavenger to prevent severe photo-corrosion [26]. The J-V curves of each photoanode fo PEC hydrogen production according to the number of PbS SILAR cycles are shown in Figure 7a, and the obtained photocurrent densities are summarized in Table 1. It was con firmed that the PbS QDs-sensitized BiVO4 photoanodes exhibited improved photocurren compared to the bare sample, due to the enhanced light-harvesting capability arising from the narrow band-gap of PbS QDs. In particular, the photocurrent was optimized when th SILAR cycles were repeated five times. While the bare BiVO4 photoanode exhibited a pho tocurrent of 2.92 mA/cm 2 at 1.23 VRHE, the BiVO4/PbS(5) photoanode exhibited a photocur rent of 4.88 mA/cm 2 . In general, as the number of SILAR cycles is increased, the size o QDs is enlarged, resulting in a smaller band-gap of the QDs [26,47]. Although the absorp tion range can be extended when the band-gap of the PbS QDs is reduced, the injection efficiency of the photoelectrons from the PbS QDs to the conduction band (CB) of BiVO can be decreased if the CB of the PbS QDs becomes lower than that of BiVO4. This phe nomenon has been reported in previous studies with other narrow band-gap QDs such a Cu-In-Se [48]. As the size of these QDs increases, their CB becomes lower than that of th host semiconductor (such as TiO2) and results in the poor injection efficiency of photoe lectrons in the PEC cells. A similar phenomenon can be expected with the PbS QDs used The BiVO 4 /PbS QDs/ZnS films were utilized as the photoanode for PEC hydrogen production and tested under simulated one-sun illumination. The electrolyte used was Ar-purged 0.5 M KH 2 PO 4 and 1.0 M Na 2 SO 3 aqueous solution with pH~7, acting as a hole scavenger to prevent severe photo-corrosion [26]. The J-V curves of each photoanode for PEC hydrogen production according to the number of PbS SILAR cycles are shown in Figure 7a, and the obtained photocurrent densities are summarized in Table 1. It was confirmed that the PbS QDs-sensitized BiVO 4 photoanodes exhibited improved photocurrent compared to the bare sample, due to the enhanced light-harvesting capability arising from the narrow band-gap of PbS QDs. In particular, the photocurrent was optimized when the SILAR cycles were repeated five times. While the bare BiVO 4 photoanode exhibited a photocurrent of 2.92 mA/cm 2 at 1.23 V RHE , the BiVO 4 /PbS(5) photoanode exhibited a photocurrent of 4.88 mA/cm 2 . In general, as the number of SILAR cycles is increased, the size of QDs is enlarged, resulting in a smaller band-gap of the QDs [26,47]. Although the absorption range can be extended when the band-gap of the PbS QDs is reduced, the injection efficiency of the photoelectrons from the PbS QDs to the conduction band (CB) of BiVO 4 can be decreased if the CB of the PbS QDs becomes lower than that of BiVO 4 . This phenomenon has been reported in previous studies with other narrow band-gap QDs such as Cu-In-Se [48]. As the size of these QDs increases, their CB becomes lower than that of the host semiconductor (such as TiO 2 ) and results in the poor injection efficiency of photoelectrons in the PEC cells. A similar phenomenon can be expected with the PbS QDs used in this study if their size increases beyond a certain level. Because of this trade-off, the PEC performance of BiVO 4 /PbS QDs/ZnS photoanodes was optimized in the condition of five SILAR cycles, while a higher SILAR cycle (seven cycles) decreased the performance. Nanomaterials 2023, 13, x FOR PEER REVIEW 8 of 12 in this study if their size increases beyond a certain level. Because of this trade-off, the PEC performance of BiVO4/PbS QDs/ZnS photoanodes was optimized in the condition of five SILAR cycles, while a higher SILAR cycle (seven cycles) decreased the performance.  Introduction of a passivation layer has been shown to effectively enhance the PEC performance of QD-based electrodes [27]. The most common approach for passivating QD-based electrodes is to deposit ZnS overlayers using the SILAR method [27,28]. As depicted in Figure 7a, the BiVO4/PbS(5) photoanode coated with ZnS overlayers (BiVO4/PbS(5)/ZnS) exhibited a further enhanced photocurrent of 5.19 mA/cm 2 compared to the one without overlayers. According to the literature, this enhancement can be attributed to suppressed nonradiative carrier recombination and interfacial electron recombination at the photoanode surface by the ZnS passivation layer [17,27,49].
To assess the photostability of each photoanode, a chronoamperometry test was performed at 1.23 VRHE for 2 h (Figure 7b). As shown in Table 1, the photocurrent density of the bare BiVO4 photoanode remained almost unchanged within 2 h of one-sun illumination, owing to the exceptional photostability of BiVO4 (the retention rate was ~99.83%). After sensitizing with PbS QDs (via five SILAR cycles), the retention rate of photocurrent density slightly decreased to 91.24%, which can be attributed to the relatively poor photostability of the metal chalcogenide [26,49]. However, after the ZnS overlayer was coated, the retention rate of photocurrent density improved again to 96.20%. This suggests that the ZnS overlayers prevent the photocorrosion of QDs and carrier recombination at the QD surface. To further enhance the photostability, other overlayers such as lead halide ligands and dinickel phosphide (Ni2P), which were suggested in the previous literature [50], can be explored in further research. The theoretical hydrogen production was also calculated based on these chronoamperometric curves, as shown in Figure S4 in the Supplementary Materials. The superior photostability and high photocurrent density of BiVO4/PbS QDs/ZnS films make them a promising material for highly efficient and reliable PEC hydrogen production. As mentioned previously, this is the first time that narrow band-gap QDs were applied to sensitize a BiVO4 photoelectrode, and the performances  Introduction of a passivation layer has been shown to effectively enhance the PEC performance of QD-based electrodes [27]. The most common approach for passivating QD-based electrodes is to deposit ZnS overlayers using the SILAR method [27,28]. As depicted in Figure 7a, the BiVO 4 /PbS(5) photoanode coated with ZnS overlayers (BiVO 4 /PbS(5)/ZnS) exhibited a further enhanced photocurrent of 5.19 mA/cm 2 compared to the one without overlayers. According to the literature, this enhancement can be attributed to suppressed nonradiative carrier recombination and interfacial electron recombination at the photoanode surface by the ZnS passivation layer [17,27,49].
To assess the photostability of each photoanode, a chronoamperometry test was performed at 1.23 V RHE for 2 h (Figure 7b). As shown in Table 1, the photocurrent density of the bare BiVO 4 photoanode remained almost unchanged within 2 h of one-sun illumination, owing to the exceptional photostability of BiVO 4 (the retention rate was~99.83%). After sensitizing with PbS QDs (via five SILAR cycles), the retention rate of photocurrent density slightly decreased to 91.24%, which can be attributed to the relatively poor photostability of the metal chalcogenide [26,49]. However, after the ZnS overlayer was coated, the retention rate of photocurrent density improved again to 96.20%. This suggests that the ZnS overlayers prevent the photocorrosion of QDs and carrier recombination at the QD surface. To further enhance the photostability, other overlayers such as lead halide ligands and dinickel phosphide (Ni 2 P), which were suggested in the previous literature [50], can be explored in further research. The theoretical hydrogen production was also calculated based on these chronoamperometric curves, as shown in Figure S4 in the Supplementary Materials. The superior photostability and high photocurrent density of BiVO 4 /PbS QDs/ZnS films make them a promising material for highly efficient and reliable PEC hydrogen production. As mentioned previously, this is the first time that narrow band-gap QDs were applied to sensitize a BiVO 4 photoelectrode, and the performances recorded in this study are comparable to the recently reported excellent performances of BiVO 4 photoelectrodes (Table S1 in the Supplementary Materials). While several previous studies have reported higher photocurrent values, they employed cocatalysts such as NiFeO x , FeOOH, and NiOOH to enhance performance. Further work is needed to identify proper cocatalysts for BiVO 4 /PbS QDs photoelectrodes for even greater enhancements in performance.
In order to gain a deeper understanding of the effects of PbS QDs and ZnS overlayers on the performance of the PEC system, EIS analysis was conducted under dark conditions and one-sun irradiation (Figure 8). The impedance spectra were analyzed using Z-view software based on an equivalent circuit model, as shown in the insets. This model includes a solution resistance (R S ) and a RC circuit consisting of a charge transfer resistance (R ct ) and a constant phase element (CPE1) related to the charge transfer properties at the interface between the photoanode and electrolyte [49,51]. The R S was similar for all samples, but the R ct of the BiVO 4 /PbS(5) photoanode was smaller than that of the bare BiVO 4 under both conditions (Table 1). This suggests that the poor hole transfer kinetics at the BiVO 4 surface were improved by the deposition of PbS QDs.
In order to gain a deeper understanding of the effects of PbS QDs and ZnS overl on the performance of the PEC system, EIS analysis was conducted under dark cond and one-sun irradiation (Figure 8). The impedance spectra were analyzed using Z software based on an equivalent circuit model, as shown in the insets. This model inc a solution resistance (RS) and a RC circuit consisting of a charge transfer resistanc and a constant phase element (CPE1) related to the charge transfer properties at the face between the photoanode and electrolyte [49,51]. The RS was similar for all sam but the Rct of the BiVO4/PbS(5) photoanode was smaller than that of the bare BiVO4 u both conditions (Table 1). This suggests that the poor hole transfer kinetics at the B surface were improved by the deposition of PbS QDs.
Furthermore, the BiVO4/PbS(5)/ZnS photoanode exhibited significantly reduc values under both dark and illuminated conditions. This indicates that the surface d sites on the QDs were effectively passivated by the ZnS overlayer, resulting in a redu in electron-hole recombination and an enhancement of surface charge transfer [4 Thus, it can be concluded that the improved PEC performance of the BiVO4 photoa was attributed not only to the narrow band-gap of PbS, but also to the improved transfer properties between the photoanode and electrolyte. Additionally, the passiv by ZnS overlayers was highly effective in reducing electron-hole recombination at th surfaces, leading to further enhancement of the PEC performance.

Conclusions
This study aimed to investigate the effects of PbS QD sensitization on the PEC formance of BiVO4 photoanodes. The nanoporous BiVO4 films were prepared thr electrodeposition, followed by PbS QD sensitization via a SILAR method, which fo a p-n heterojunction. This is the first time that narrow band-gap QDs have been ap to sensitize a BiVO4 photoelectrode. The resulting BiVO4/PbS QDs photoanode exh a significantly increased photocurrent of 4.88 mA/cm 2 (at 1.23 VRHE) for PEC hyd production owing to the improved light-harvesting capability from the narrow ban of PbS and the enhanced charge transfer properties. Furthermore, when a ZnS over Furthermore, the BiVO 4 /PbS(5)/ZnS photoanode exhibited significantly reduced R ct values under both dark and illuminated conditions. This indicates that the surface defect sites on the QDs were effectively passivated by the ZnS overlayer, resulting in a reduction in electron-hole recombination and an enhancement of surface charge transfer [49,51]. Thus, it can be concluded that the improved PEC performance of the BiVO 4 photoanode was attributed not only to the narrow band-gap of PbS, but also to the improved hole transfer properties between the photoanode and electrolyte. Additionally, the passivation by ZnS overlayers was highly effective in reducing electron-hole recombination at the QD surfaces, leading to further enhancement of the PEC performance.

Conclusions
This study aimed to investigate the effects of PbS QD sensitization on the PEC performance of BiVO 4 photoanodes. The nanoporous BiVO 4 films were prepared through electrodeposition, followed by PbS QD sensitization via a SILAR method, which formed a p-n heterojunction. This is the first time that narrow band-gap QDs have been applied to sensitize a BiVO 4 photoelectrode. The resulting BiVO 4 /PbS QDs photoanode exhibited a significantly increased photocurrent of 4.88 mA/cm 2 (at 1.23 V RHE ) for PEC hydrogen production owing to the improved light-harvesting capability from the narrow band-gap of PbS and the enhanced charge transfer properties. Furthermore, when a ZnS overlayer was applied to reduce electron-hole recombination at the QD surface, the photocurrent was further improved to 5.19 mA/cm 2 . These findings provide valuable insights for the development of electrode materials for highly efficient PEC hydrogen production.